get start of sysnoise

SYSNOISE Rev 5.6 RELEASE NOTES & GETTING STARTED MANUAL

by

LMS INTERNATIONAL N.V.

Interleuvenlaan 68, B-3001 LEUVEN, Belgium

First Edition: March 2003

COPYRIGHT NOTICE

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the written permission of LMS International N.V., Interleuvenlaan 68-70, B-3001 Leuven, Belgium.

REGISTERED TRADEMARKS

LMS SYSNOISE is a registered trademark of LMS International LMS Virtual.Lab is a trademark of LMS International LMS RAYNOISE is a trademark of LMS International LMS VIOLINS is a trademark of LMS International LMS OPTIMUS is a trademark of LMS International All other product names mentioned in this manual are trademarks or registered trademarks of their respective manufacturers.

SYSNOISE Rev 5.6 Getting Started Manual - 1

CONGRATULATIONS

You have made a great choice by selecting SYSNOISE, for more than 14 years the world’s leading software package for Computational Vibro-Acoustics. Worldwide, the state-of-the-art technology of SYSNOISE is being applied by major corporations in many different industries for the acoustic design and analysis of their products. You too will find SYSNOISE the right tool to analyze, troubleshoot and optimize the noise and vibration characteristics of your product design.

We, at LMS International, wish to thank you for the choice you have made, and would like to express our commitment to continuously support and improve the SYSNOISE code.

2 - SYSNOISE Rev 5.6 Getting Started Manual

Table of contents

1 SYSNOISE Rev 5.6 Release Notes 11

1.1 Introduction and Objectives 11

1.2 What is new in SYSNOISE Rev 5.6? 11 1.2.1 Resource 11 1.2.2 Interfaces 12 1.2.3 Solvers 12 1.2.4 Modeling 13 1.2.5 Aero-acoustic Module 13 1.2.6 High Speed Harmonic BEM 16

1.3 Continuity and stability 19 1.3.1 Database compatibility 19 1.3.2 Command language 19 1.3.3 On-line help 19

2 Overview of application examples 20

2.1 Purpose 20

2.2 Uncoupled BEM analysis 20

2.3 Coupled BEM analysis 20

2.4 Uncoupled FEM analysis 20

2.5 Engine noise radiation, using ATVs and MATVs 21

2.6 Panel acoustic contribution analysis 21

2.7 Inverse numerical acoustics 21

2.8 Aero-acoustics analysis, turbulence noise modeling 21

2.9 Aero-acoustics analysis, loading noise modeling 21

2.10 Aero-acoustics analysis, fan noise modeling 22

3 Getting Started - Uncoupled BEM Analysis: Sound Radiation in a Passenger Car Compartment 23

3.1 Introduction and Problem Description 23

3.2 Start SYSNOISE 24

3.3 Create a Model Database 25


3.4 Define the Analysis Option 26

3.5 Import the Boundary Element Mesh 28

3.6 Check (Show) the Element Normal Vectors 29

3.7 Define Fluid Properties 31

3.8 Identify the Firewall Elements 31

3.9 Create Velocity BCs from FEA Vibration Data 33 3.9.1 Specify the location for velocity boundary conditions 34 3.9.2 Specify FEA vibration data source 35 3.9.3 Specify structural mesh for FEA vibration data 36 3.9.4 Specify Unequal Mesh Interpolation parameters 37 3.9.5 Visual check of velocity boundary conditions 37

3.10 Save the Model Database 39

3.11 Set the Analysis Parameters 39

3.12 Solve over a Range of Analysis Frequencies 40

3.13 Field Point Post-Processing 42

3.14 Graphical Representation of Acoustic Results 44 3.14.1 Mesh postprocessing : color map 44 3.14.2 Response functions : field point pressure 46

3.15 Close the Database and/or End the Session 48

3.16 Uncoupled BEM Analysis: computation alternatives. 48 3.16.1 Uncoupled Indirect BEM 49 3.16.2 Uncoupled High Speed BEM 50 3.16.3 Comparison between IBEM & High Speed BEM 52

4 Getting Started - Coupled BEM Analysis : Vibrating Firewall in a Passenger Car Enclosure 54


4.2 Working with Multiple SYSNOISE Models 55

4.3 Create the Structural Model Database 55


4.5 Import the Structural Finite Element Mesh 57

4.6 Import the Structural Modes 58

4.7 Define Viscous Modal Damping 60

4.8 Define a Point Load Excitation 61

4.9 Save the Structural Model 64

4.10 Create an Acoustic Model Database 64




4.13 Check (Show) the Normal Vectors 66


4.15 Save the Acoustic Model 69

4.16 Link the Acoustic and Structural Models 69

4.17 Define the Field Point Mesh 71

4.18 Set the Analysis Parameters for Both Models 72

4.19 Compute the Coupled Forced Response 73

4.20 Visualization of Structural Results 74

4.21 Visualization of Acoustic Results 77

4.22 Exit the SYSNOISE Session 80

5 Getting Started - Uncoupled FEM Analysis: Sound Radiation in an Engine Manifold 81




5.4 Import the Finite Element Mesh 83


5.6 Identify the Inlet and Outlet Surface 84

5.7 Import the Pressure Tables 86

5.8 Create Manual Boundary Conditions 87 5.8.1 Assign Pressure Boundary Conditions at the Inlets 87 5.8.2 Assign Impedance Boundary Conditions at the Outlet 88

5.9 Set the Analysis Parameters 88

5.10 Solve over a Range of Analysis Frequencies 89

5.11 Graphical Representation of Acoustic Results 90 5.11.1 Mesh post-processing: color map 90 5.11.2 Response functions: node potential 91

5.12 Compute Acoustic Response using Parallel Computing 93 5.12.1 Set-up the analysis model 93 5.12.2 Decompose the Domain. 93 5.12.3 Close the Database and End the Session 95 5.12.4 Run SYSNOISE in Parallel 95 5.12.5 Graphical Representation of Acoustic Results 96


5.13 Close the Databases and/or End the Session 96

6 Getting Started – Numerical Engine Acoustics: Engine Acoustic Signature 97





6.5 Import the ‘Surrogate’ Structural Mesh 103

6.6 Transfer the Structural Modes 104

6.7 Project the Structural Modes on the Element Normals 106

6.8 Import the MRSP’s 107

6.9 Save and Close the Database 108

6.10 Create the “Master” Acoustic Model Database 108

6.11 Define The Analysis Option 108

6.12 Import The Acoustic Mesh 109

6.13 Check the Element Normal Vectors 109


6.15 Define A Symmetry Plane 111

6.16 Create an ‘ISO3744-1994’ Compliant Field Point Mesh 111

6.17 Compute the ATVs 112


6.19 Create a second Acoustic Model Database 113

6.20 Open the Structural Database 114

6.21 Link the Acoustic and Structural Models 115

6.22 Project the ATVs and Compute the MATV 116

6.23 Close the Master Acoustic Model Database 117

6.24 Create Compatible Load Cases 118

6.25 Set the Calculation Parameters 118

6.26 Solve over a range of RPM 120

6.27 Store Results 120

6.28 Acoustic Results Visualization 121

6.29 Waterfall Diagram 121


7 Getting Started - Panel Contribution Analysis (PACA) : Vehicle Interior Noise 123







7.7 Create Panels as Sets of Elements 129

7.8 Create Field points 131

7.9 Assign Impedance Boundary Conditions 133


7.11 Create the seats models 135

7.12 Define fluid Properties 137

7.13 Link the Acoustic Models 138

7.14 Compute the ATVs 140

7.15 Rename Sets 142

7.16 ATV Visualization 143

7.17 Create Velocity BCs from FEA Vibration Data 145 7.17.1 Specify location for velocity boundary conditions 146 7.17.2 Specify FEA vibration data source 147 7.17.3 Specify structural mesh for FEA vibration data 147 7.17.4 Visual check of velocity boundary conditions 148

7.18 Post-processing 149 7.18.1 Element Contribution 149 7.18.2 Panel Contribution 150


8 Panel contributions in exterior analysis 154

9 Getting Started – Inverse Numerical Acoustics : Engine Noise Identification 157




9.3 Define The Analysis Option 159

9.4 Import The Boundary Element Mesh 160

9.5 Check the Element Normal Vectors 161


9.7 Import Field Point Mesh 162

9.8 Compute ATVs 164

9.9 Import the ‘Inverse’ Pressure Data 165

9.10 Apply Zero Velocity on Selected Elements 166

9.11 Regularization Analysis 168

9.12 Inverse Analysis 169

9.13 Display the Velocity Distribution 170

9.14 Optional Extension : Verification Based on Pressures 172 9.14.1 Re-calculate Pressure Field (Optional) 172

9.15 Annex : Forward Calculation of Pressures 173 9.15.1 Create a Model Database 174 9.15.2 Define The Analysis Option 174 9.15.3 Import the Boundary Element Mesh 175 9.15.4 Check Mesh 175 9.15.5 Define Fluid Properties 175 9.15.6 Generate Velocity Boundary Conditions 176 9.15.7 Import Field Point Mesh 178 9.15.8 Solve and Field Point Post-processing 178 9.15.9 Export Pressure Results 178

10 Getting Started – Volume Distributed Quadrupoles: Jet Noise problem 179





10.5 Import the Model Mesh 182


10.7 Import the volume for the quadrupoles distribution 184

10.8 Generate the quadrupoles distribution 185

10.9 Define the Field Point Mesh 187

10.10 Incident Field Analysis 188


10.11 Post-processing 188


11 Getting Started – Surface Dipoles Distributed Sources problem – Multi Domain Analysis 190


11.2 Create an Interior Model Database 191



11.5 Define Symmetry Planes 193


11.7 Define the sets for BC 195


11.9 Define the surface distributed dipole source 198

11.10 Define the coupling nodes and elements 200

11.11 Save the Interior Model Database 201

11.12 Create an Exterior Model Database 201


11.14 Import the Exterior Model Mesh 202

11.15 Define Symmetry Planes 202


11.17 Define the Different Sets for the Analysis 204


11.19 Define the Surface Distributed Dipole Source 204

11.20 Define the coupling nodes and elements 204

11.21 Save the Exterior Model Database 204


11.23 Solve Analysis 205

11.24 Post-processing 206 11.24.1 Field Point Mesh 206

11.25 Close the Databases and/or End the Session 210


12 Getting Started – Fan Source problem – Multi Domain Analysis211

12.1 Introduction and Problem Description 211 12.1.1 Noise from a rotor 211 12.1.2 Rotor/Stator interaction 212 12.1.3 Description of the example 212

12.2 Create an Interior Model Database 214




12.6 Define sets 217


12.8 Define the fan source 218 12.8.1 Discrete Fan Source 220 12.8.2 Generate Fan Source 222


12.10 Create an exterior model database 223




12.14 Define a set 224




12.18 Solve at harmonics of the BPF 226

12.19 Post-processing 226 12.19.1 Model mesh 226 12.19.2 Field Point Mesh 227


References 230


1 SYSNOISE Rev 5.6 Release Notes

1.1 Introduction and Objectives

SYSNOISE Rev 5.6 provides new powerful modeling tools not only for vibro-acoustic analyses but also for aero-acoustic analyses. A new solution procedure based on the aero-acoustic analogy has been implemented. This new module enables the users to tackle a wide range of challenging applications such as wind noise, HVAC, fan noise… Furthermore, following the path of SYSNOISE Revisions 5.4 and 5.5, SYSNOISE Rev 5.6 makes a step even further in the vibro-acoustic analysis performance. A frequency-parameterized version of the uncoupled indirect Boundary Element Module, the High Speed BEM module, is now available. Also, new solvers are introduced in Fluid Finite Element solution sequence such as the iterative solver and the domain decomposition techniques. Finally, the unique ATV concept introduced in SYSNOISE Rev 5.5 is now implemented in Fluid Finite Element Method solution procedure and used in the Panel Acoustic Contribution Analysis procedure.

These Release Notes give a short overview on the new features of SYSNOISE Rev 5.6. More details can be found in the Getting started chapters of this manual. Examples illustrating standard coupled and uncoupled applications together with the use of the ATV features and the aero-acoustics capabilities are provided and explained in details to help the users understand the basics of these powerful tools.

1.2 What is new in SYSNOISE Rev 5.6?

1.2.1 Resource

• Most UNIX versions are now available in 64 bit versions pushing the memory limits of the actual release to 8Gbytes.

• Most UNIX versions use now Native Message Passing Interface (NMPI) for friendly parallel computations. The MPICH versions are nevertheless still available (WinMPI for Windows version).

• SYSNOISE Rev 5.6 is now available on HP Itanium. Please check the Installation Manual to have a complete overview of the available versions.


1.2.2 Interfaces

• The user can now implement his own binary format for I/O access. The user format (ASCII or BINARY) is controlled through the environment variable USERFORMAT in the SETUP section.

• Tables can be transformed in the frequency or time domain directly in the import mechanism. This avoids storing intermediate tables in the database.

• Export of Velocity Boundary Conditions supports now Universal file format and SYSNOISE Free format.

• The Flow Velocity in Fluid FEM can be imported from external files. This enables the user to solve for the mean flow in external software package and to take into account effects such as viscosity… This process supports unequal meshes (Transfer like command) and most formats.

• Distributed aero-acoustic sources (surface dipoles, volume quadrupoles and fan sources) can be generated from time flow data provided in a STAR-CD format file. The generation mechanism includes the transformation from time to frequency domains and supports unequal meshes.

• Distributed aero-acoustic surface dipoles and volume quadrupoles can be generated from time flow data provided in a CFX format file (CFX 5.6 and above). The generation mechanism includes the transformation from time to frequency domains and supports unequal meshes.

• Distributed aero-acoustic surface dipoles can be generated from time flow data in FLUENT format (FLUENT V6.1.18 and above). The generation mechanism includes the transformation from time to frequency domains and supports unequal meshes.

1.2.3 Solvers

• A new powerful FFT/IFFT algorithm has been implemented to speed-up the Fourier transform computations. Unlike the old FFT, the new feature allows for any number of input steps.

• Acoustic FEM analysis sequences can now be solved using an iterative solver. This new linear solver based on an Approximate Factorization Technique (AFT) allows for faster computations with minimal memory requirement (no filling during factorization) [8].

• The NetSolver capabilities are extended with Domain Decomposition in acoustic FEM. Two approaches are available, the fully iterative approached based on the Approximate Factorisation Technique and the so-called Finite Element Tearing and Interconnecting (FETI) approach [9,10].


• Acoustic Transfer Vectors can be computed in acoustic FEM module. Both physical coordinates and modal coordinates can be used in the solution sequence.

• Panel Acoustic Contribution Analysis is now available in acoustic FEM. This enables the user to perform contribution analysis in cavities based on Finite Element Analysis.

• Inverse Numerical Acoustics enables simultaneous multi load case analysis. It also allows for frequency dependent tolerance parameter.

• Regularization L-Curves for the Inverse Numerical Acoustic Module can be computed and visualized. This enables the user to determinate and monitor the optimal tolerance parameters.

• Incident Field Analysis from acoustic discrete sources is now available. This new feature is available in all acoustic FEM and BEM modules.

1.2.4 Modeling

• Models can be decomposed in sub-domains using several algorithms:

• Automatic, based on the Metis Algorithms (information can be found on http://www-users.cs.umn.edu/~karypis/metis/),

• Ellipsoidal, specially designed for models containing infinite elements,

• From file, where the decomposition is imported from an external file,

• Based on material, where each domain correspond to a specific material,

• User defined, using a user implementation.

• Boundary conditions can be searched at specific frequencies using the Search command. The values can then be extracted using the Extract command.

1.2.5 Aero-acoustic Module

1.2.5.1 Theoretical background

The aero-acoustics module of SYSNIOISE is based on the implementation of the aero-acoustic analogy introduced by Lighthill [11] and extended by Curle [12], Ffowcs-Williams and Hawkings [13,14] .


http://www.putherethecorrectaddress.com/


Quadrupole sourcesvolume distribution

Q4(x,t)

Unbounded flowLighthill ('52)

Reflection & diffractionPassive boundary

monopolessurface distribution

Q1(x,t)

thickness effectdisplacement noise

dipolessurface distribution

Q2(x,t)

Loadingnoise

Surface sources:Active boundary

Bounded flowFfowcs Williams - Hawkings ('69)

Acoustic Analogies

Aero-acoustics Analogies

To derive this methodology, the compressible Navier-Stokes equations have been rearranged exactly to yield:

20 j

22 2 iji2

i icTFp p'1 Q

x x t x x xt i j

∂∂∂ ∂ ∂′ − = − +∂ ∂ ∂ ∂ ∂ ∂∂

&

Where (p’) is the acoustic pressure fluctuation, ( Q ) the mass flow rate, (F& i) the external forces acting on the fluid and (Tij) the Lighthill stress tensor containing momentum flux, thermal and viscous terms and c0 the speed of sound. This equation expresses a linear wave problem in a medium at rest with equivalent acoustic sources derived from the unsteady flow phenomena. The main limitation of the acoustic analogy is the implicit assumption of one-way coupling from flow to acoustics. The sources in the right-hand-side (RHS) consist of 3 types:

• The first term in the RHS shows that if mass is added at an unsteady rate, sound will be generated. This type of source is called a monopole. For rotating machinery for instance this type of noise is called thickness noise that is related to the displacement of fluid due to the motion of the blades. If the rotation speed is low or the blades geometry is thin, monopole sources are expected to be of negligible contribution.

• The second term shows that if time varying forces act on the fluid sound will be produced. Such sources are called dipoles. For stationary surfaces the unsteady pressure loading of the surface lead to stationary dipole sources while an interaction of a rotating fan with an unsteady flow produces a “rotating” dipole sound, called also Fan source in SYSNOISE. For stationary or rotating surfaces, the dipole sound is expected to be the predominant one when the flow is subsonic in nature.

• The third term in the RHS shows that the time-dependent stresses, including momentum, viscosity and turbulence, acting on the fluid, also generate sound. Such sources are called quadrupoles. For subsonic rotating machinery, the quadrupole contribution is negligible.


According to the aero-acoustics analogy, flow-generated noise problems can therefore be replaced by equivalent problems where both the flow and the bodies interacting with the flow are replaced by equivalent aero-acoustic sources radiating as if the medium were at rest. The problem of flow-induced noise prediction can be therefore achieved in two steps:

1. Solve the turbulent flow with the CFD methods (STAR-CD, FLUENT, CFX)

2. Based on the flow information provided by the CFD codes, define equivalent sources and take into account the noise radiation with CA methods (SYSNOISE)

1.2.5.2 SYSNOISE AERO-ACOUSTICS FEATURES

Based on the aero-acoustics analogy, new aero-acoustics features have been implemented inside SYSNOISE Rev5.6. These new capabilities are:

Discrete and Distributed quadrupole sources: Discrete and Distributed

Quadrupole sources have been implemented. Considering aero-acoustic

applications, the source strength is defined by the (symmetric) Lighthill

stress tensor that is well defined by the flow velocity components when the

flow is subsonic in nature. The modules added in SYSNOISE compute the

total acoustic field due to quadrupoles including incident and scattered field

from any obstacle. The discrete quadrupole sources are implemented for

DBEM/IBEM and FEM/IFEM models of SYSNOISE. The “generate

distributed quadrupoles” is implemented for DBEM, Multi-DBEM and IBEM.

The source strength, i.e. flow velocity can be read in SYSNOISE Rev5.6

from STAR-CD and CFX5.6 format files. The interface with FLUENT will be

available shortly.

Discrete and Distributed dipole sources: Discrete and Distributed

stationary dipole sources have been implemented. Considering aero-

acoustic applications, the source strength is defined by the compressive

stress tensor containing the pressure fluctuation. The modules added in

SYSNOISE compute the total acoustic field due to dipoles including incident

field and scattered field from any obstacle. The discrete dipole sources are

implemented both for DBEM/IBEM and FEM/IFEM models of SYSNOISE.

The “generate distributed dipoles” is implemented for DBEM and Multi-


DBEM. The source strength, i.e. flow pressure fluctuations can be read in

SYSNOISE Rev5.6 from STAR-CD, CFX5.6 and FLUENT format files.

Discrete and Distributed fan sources: Discrete and Distributed “rotating

dipoles” or fan sources have also been implemented. These sources enable

to handle rotating surfaces. Considering a rotating fan, for instance, the

source strength is defined by the pressure fluctuations or the forces given

on the blades. The modules added in SYSNOISE compute the acoustic

field due to the fan sources including incident field and scattered field from

any obstacle. The discrete and distributed fan sources are implemented for

DBEM/IBEM and FEM/IFEM models of SYSNOISE. For the moment the

source strength i.e. forces can be read in SYSNOISE Rev5.6 from STAR-

CD format files only. The interfaces with CFX5.6 and FLUENT will be

available shortly.

Remark: It is very important to note that flow data written in a standard format file i.e. Universal, Free… can also be read in by SYSNOISE Rev5.6

SYSNOISE reads the time data, transforms it into frequency SYSNOISE reads the time data, transforms it into frequency Domain, maps it and generates the relevant sources on the Domain, maps it and generates the relevant sources on the Acoustic mesh Acoustic mesh

CFDCFDRunRun

SYSNOISESYSNOISERev5.6Rev5.6

Generate commandGenerate command

The data are stored at each time step in binary format filesThe data are stored at each time step in binary format filesPressure, velocity and forces versus time, at each node Pressure, velocity and forces versus time, at each node


ResultsResults

INTERFACESINTERFACES

SYSNOISE reads the time data, transforms it into frequency SYSNOISE reads the time data, transforms it into frequency Domain, maps it and generates the relevant sources on the Domain, maps it and generates the relevant sources on the Acoustic mesh Acoustic mesh

CFDCFDRunRun


Generate commandGenerate command

The data are stored at each time step in binary format filesThe data are stored at each time step in binary format filesPressure, velocity and forces versus time, at each node Pressure, velocity and forces versus time, at each node


ResultsResults

INTERFACESINTERFACES

CFD/SYSNOISE Coupling process for aero-acoustics computation

1.2.6 High Speed Harmonic BEM

The High Speed Harmonic BEM module is based on the Padé expansion [7], which aims at solving the Helmholtz integral equation for a complete frequency


band, using factorization of the matrix at selected frequencies (called master frequencies). For large problems, the computation time is dominated by the factorization time of the matrix. Therefore, avoiding multiple factorizations gives large timesavings.

1.2.6.1 Theoretical Background

In order to solve a standard frequency sweep, a linear system needs to be solved for each frequency : f

)()()( fff bµA =

As the coefficients of the matrix depend on the frequency, must be calculated and factorized for each frequency of interest.

A A

An alternative to this frequency sweep procedure is to approximate the frequency response function by a Padé Approximation which is a rational function:

)()()(fff

QPµ =

The use of Padé Approximation rather than more classical Taylor expansions is justified by the fact that µ can have singular points (poles or eigen frequencies) and is usually not holomorphic but only meromorphic, so that its Taylor series does not converge everywhere. The roots of are the poles or the eigen frequencies of the function µ . The eigen modes are given by . The convergence radius of the Padé Approximation is limited to the distance between the initial frequency (computation point) and the first pole not included in Q . The use of a Padé Approximation makes it possible to compute an accurate approximation of µ even at values of for which the Taylor series of µ diverges.

)( f

)( fQ

)

)( f )( fP

)( f

( f( f f )

The calculation of a Padé Approximation requires the knowledge of the successive derivatives of the acoustic quantities with respect to frequency, evaluated at the central frequency . The first derivative of the surface potential 0f µ is the solution of:

µAbµAfff ∂

∂−

∂∂

=∂∂

The derivative of µ is obtained by solving the same system of equations that was solved to obtain the potentials µ but with a different right-hand side involving the derivative of the matrix , the derivative of the original right-hand side vector b and the potentials .

Aµ


The second- and higher-order derivatives of µ with respect to are the solution of:

f

∑=

−

−

∂

∂

∂

∂−

∂

∂=

∂

∂ n

iin

in

i

iinn

n

n

n

ffC

ff 1

µAbµA

The most important property of this recurrence scheme is that the calculation of the successive derivatives of µ requires the factorization of a single matrix . The right-hand side vectors associated with the higher-order derivatives of

Aµ are

based on the successive derivatives of . A

) i

A A

The coefficients of are explicit functions of the frequency which appears only as a parameter of the Green function G. In a given frequency range, we compute several matrices for different frequencies . The higher-order derivatives of

are computed from these different evaluations of .

A

( if

f

A f

1.2.6.2 SYSNOISE High Speed Harmonic BEM Features

The Padé expansions have been implemented in the Indirect Variational BEM modules and can be used either to compute the potentials on the boundary or the ATVs introduced in SYSNOISE Rev 5.5.

Two approaches are introduced in SYSNOISE Rev 5.6. The first one is more efficient for small and medium size problems whereas the second one is more efficient for large size problem. The choice between the first approach and the second approach is controlled by the environment variable MODE in the PADE section (MODE=1 for the first approach and MODE=2 for the second approach). The following table summarizes the differences between the two approaches:

Mode Assembly Storage on disk Factorization

Mode 1 in-core 4 matrices per master freq.

3 matrices 1 matrix per master freq.

Mode 1 out-of-core 4 matrices per master freq.

4 matrices 1 matrix per master freq.

Mode 2 in-core At each frequency 1 matrix 1 matrix per master freq.

Mode 2 out-of-core At each frequency 2 matrices 1 matrix per master freq.

The choice between the first and the second method depends on the available memory to cash the matrices. For both functioning modes, the master frequencies

and the maximum order derivatives are internally computed to obtain the best balance between accuracy and speed-up. if


1.3 Continuity and stability

1.3.1 Database compatibility

The data handling for SYSNOISE Rev 5.6 remains essentially the same as for Revision 5.5. The changes are thus usually transparent to the user. There is a full upward compatibility of models from Revision 5.5 to Revision 5.6. Conversion to a ‘machine-native’ format is still offered in cases where the database originated from hardware of a different type. The binary compatibility will allow access with Revision 5.6 on other hardware also, but input/output performance will not be optimal.

1.3.2 Command language

There has been no change to previous command language entries, only some extensions to cater for the new features of SYSNOISE Rev 5.6. ‘Obsolete’ keywords from previous Revisions will remain valid, even where the Graphic User Interface now generates a different keyword.

1.3.3 On-line help

The On-line form of the SYSNOISE User’s Manual remains similar to its format with Revision 5.5, with extensions dealing with the new modules and features of Revision 5.6. The Printed form has exactly the same contents as the On-line form, but has page and section numbers and cross-references, rather than hyperlinks; thus, it is more useful as a hard copy document.

You can choose whether to copy the contents of the Documentation CD-ROM onto the disk drive when installing SYSNOISE, or to read the documentation files directly from the CD-ROM (in which case it must be mounted if appropriate). The Help function is context-sensitive and has hyperlinks to other related topics in the SYSNOISE manual, and background information. All command dialogs have a Help button, which takes you to the relevant section of the manual.

The Printed form of the manual is supplied on the CD-ROM in PDF format, which enables easy on-screen browsing or full or selective print-out.


2 Overview of application examples

2.1 Purpose

Eight worked examples are included in this Getting Started manual. The first three are intended to assist new users of SYSNOISE, to learn how to use the standard features of the program together with few of the Rev 5.6 novelties. The fourth, fifth and sixth examples are intended to show the advanced features based on ATV, specifically the Engine Acoustics [1,2], Inverse [3,4,5], and Panel Contribution Analysis [6] capabilities. The last three examples give an overview of the new aero-acoustric capabilities [11,12,13,14,15] for three kind of problems, turbulence noise, loading noise and fan noise. These last six examples assume a degree of familiarity with the basic operating principles in SYSNOISE, for instance from a thorough knowledge of the procedures in the first three examples.

2.2 Uncoupled BEM analysis

The first example describes a typical uncoupled analysis and shows in detail the application of SYSNOISE to an acoustic radiation problem: the computation of the sound field inside a passenger car, based on FEA vibration data for the firewall. Three alternatives are presented, based on direct collocation BEM, indirect variational BEM and High Speed BEM.

2.3 Coupled BEM analysis

The second example describes a typical coupled analysis and shows in detail how SYSNOISE handles the coupling between acoustics and structural dynamics for the computation of the sound field in a passenger car, due to a force excitation on a firewall mount point.

2.4 Uncoupled FEM analysis

The third example describes a typical uncoupled fluid FEM analysis, i.e. the computation of the acoustic field in an engine manifold due to some pressure fluctuation at the inlets. In this example, the new powerful iterative solver embedded in SYSNOISE is presented. Also, as a computation alternative, the new domain decomposition features are presented.


2.5 Engine noise radiation, using ATVs and MATVs

The fourth example describes a Numerical Engine acoustic analysis that calculates the complete acoustic signature of an engine. A particular scenario making advantage of working in the modal space by using so-called MATVs (Modal Acoustic Transfer Vectors) is shown.

2.6 Panel acoustic contribution analysis

The fifth example describes a typical Panel Acoustic Contribution Analysis (PACA) and shows in detail the application of SYSNOISE to vehicle interior noise : the computation of the acoustic contribution of the body panels to the total sound pressure at some target locations (e.g. the driver’s ear).

An extra example follows, without detailed working, to show briefly the concept of panel contribution analysis applied to an exterior radiation case.

2.7 Inverse numerical acoustics

The sixth example describes a typical Inverse Numerical Acoustics analysis and shows how SYSNOISE enables the localization and quantification of sources by using the concept of ATVs in an ‘inverse’ way. This is illustrated on an engine, for which measured acoustic pressure levels are simulated.

2.8 Aero-acoustics analysis, turbulence noise modeling

The seventh example is an illustration of the noise due an unsteady flow. The test case describes the quadrupolar noise due to a subsonic jet and shows how to use the new aero-acoustics features of SYSNOISE Rev 5.6 to model the noise due to the turbulence from a give CFD (Computational Fluid Dynamics) data using a distribution of equivalent quadrupoles.

2.9 Aero-acoustics analysis, loading noise modeling

In the eighth example the noise due to an open sunroof is simulated using the new aero-acoustics sources of SYSNOISE Rev5.6. In this case study, only the loading noise, or dipole noise, due to the fluctuating pressure on the car body is taken into account. This example shows how to define distributed dipole sources from the CFD data to model the pressure loading noise.


2.10 Aero-acoustics analysis, fan noise modeling

The last example is dedicated to the simulation of the noise due to a rotor/stator interaction. This case study is a good example to show how fan sources are generated from the CFD data in a SYSNOISE model to simulate the noise from a rotor/stator interaction.

All data files related to the examples of the Getting Started manual can be found in the /install_path/sysnoise/5.6/Getting_Started* directory of the SYSNOISE installation. To remain consistent with the contents of this manual, you should launch SYSNOISE from this directory or copy the Getting Started files to a local user directory and start SYSNOISE in that directory.

* To obtain the exact pathname, refer to the installation procedure or your system manager


3 Getting Started - Uncoupled BEM Analysis: Sound Radiation in a Passenger Car Compartment

3.1 Introduction and Problem Description

A very common application of SYSNOISE is to compute the sound field radiated by a vibrating structure. SYSNOISE has several ways to characterize structural vibrations : manual input, structural finite element analysis or experimental measurements of the vibrations. For now, we will focus on vibration data which has been computed previously by a structural finite element analysis program.

In summary, the model consists of the interior of a “monovolume” passenger car with a vibrating firewall which creates a sound field in the car interior. The structural vibration vectors due to a point load applied on a firewall mount point have been computed by an FE forced response analysis for a frequency ranging between 10Hz and 100Hz (1 vector per frequency) and this vibration data is stored in an external data file.

SYSNOISE will now be used to compute the acoustic pressure distribution caused by the vibrating firewall within the car compartment. For simplicity, we will consider a car cavity without trim, i.e. without the application of sound absorbing materials.

The complete analysis sequence consists of the following steps :

1. Start SYSNOISE 2. Create an Acoustic Model Database 3. Define the Analysis Option: BEM DIRECT 4. Import the Boundary Element Mesh 5. Check the Mesh 6. Define Fluid Properties 7. Identify Firewall Elements 8. Create Velocity Boundary Conditions from FEA Vibration Data 9. Set the Analysis Parameters 10. Solve over a Frequency Range 11. Field Point Post-Processing 12. Visualization of Acoustic Data Results 13. Save/Close the SYSNOISE session Each of these steps is described in detail on the following pages.


3.2 Start SYSNOISE

Although your system manager will probably have defined an alias to start up SYSNOISE, you can always type the complete command string /path/sysnoise/5.6/bin/sysnoise -m5* to launch the program.

∗ /path refers to the directory tree under which the SYSNOISE software was installed. To have the exact path, refer to the installation procedure or to your system manager

Note: SYSNOISE Rev 5.6 uses dynamic memory allocation such that it is no longer compulsory to define a command option ‘-mX’ to allocate memory at start up. The -mX Option is still valid but now defines an initial maximum memory limit for the session (it can alternatively be set by the MEMORY environment variable)

After start-up, five windows appear:

1. The Start-Up Window allows you to select from a list of typical commands that are used at the beginning of a SYSNOISE session.

2. The Main Window is used for the display of your models and also contains the pull-down menus, the toolbar and the keyboard entry area.

3. The SCL History Window records all the commands you used during your SYSNOISE session and it is possible to create a “Journal” file (in the “Tools” menu) from the contents of this window. This Journal file can be replayed as a command file using the “Read” command (in the “File” menu).


4. The Echo Window gives feedback from the program and informs you about the execution of the commands.

5. The Button Window allows you to customize SYSNOISE and to create your own environment. Pressing a button executes a pre-defined command string, opens a dialog or branches to another button window. A button file is associated with each button window, making its definition very fast and easy.

3.3 Create a Model Database

When creating a model in SYSNOISE Rev 5.5, the first thing you always have to do is to create a new database. This binary database will contain all the information related to your model and is stored in a file with the filename extension “.sdb”, which stands for “SYSNOISE database”.

To create a new database, click with the mouse on the “Create a New Model” button in the Start-Up Window.

Create a database with the name Car_NoTrim. The complete filename of the database file has to be entered as well : Car_NoTrim.sdb.


3.4 Define the Analysis Option

After the creation of the database, the next important thing is to choose the correct analysis option, i.e. what type of numerical method will be used to analyze the problem : FEM (Finite Elements), BEM DIRECT (Boundary Elements - Direct method) or BEM INDIRECT (Boundary Elements - Indirect method). You activate the analysis option form by clicking the “Model” ⇒ “Option” menu entry in the Main Window :

To study sound radiation in a car compartment, SYSNOISE offers a variety of numerical techniques. Since we only have a mesh representing the boundary of the car compartment, and we are only interested in an uncoupled interior acoustic analysis, select the BEM Frequency Direct Uncoupled Interior Element analysis option. Two alternatives are presented in section 3.16.


Note: If you are not sure about which option you have to choose, you can always activate the “Option Wizard’” (see button at the bottom of the “Option” dialog). This wizard sequence assists you in selecting the most appropriate option by requesting answers to a series of questions about the characteristics of your problem, as illustrated below.

1. 2.

3. 4.

5. 6.

Please note that in the present case, several SYSNOISE options are suited to perform the analysis. In some cases however, the physics of the situation forces the choice for a particular analysis option. For example, when the


vibrating structure does not form a closed surface (i.e. contains free edges), only the option BEM Indirect can be used. When the acoustic medium is not homogeneous, only the option FEM Fluid is valid (or multi-domain BEM can be used: see the Panel Acoustic Contribution Analysis example for more details).

3.5 Import the Boundary Element Mesh

The Boundary Element mesh represents the bounding surfaces of the passenger compartment of a “monovolume” vehicle. You can load this mesh by selecting the “File” ⇒ “Import…” menu entry in the Main Window:

The mesh forms a simple closed surface and was generated by an external mesh generator program (e.g. ANSYS, HYPERMESH, I-DEAS Master Series, MSC/PATRAN, Pro/MECHANICA or other). The mesh data is contained in an external Free Format file named monovolume.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.


Note: The Free Format is the SYSNOISE proprietary ASCII format. Please refer to the appendices of the SYSNOISE User’s Manual in order to get a complete description of the content of the free formatted files.

After clicking the “OK” button, the Boundary Element mesh is read in and displayed in the Main Window :

3.6 Check (Show) the Element Normal Vectors

For a Boundary Element analysis, it is absolutely essential that the normal vectors of the elements are pointing in a consistent direction. When the mesh was originally created, the consistency of the normal vectors may have been ignored. Therefore, SYSNOISE offers a “Check Mesh” procedure that will automatically take care of reversing element normal vectors in order to guarantee consistency.

This tool is accessed from the “Geometry” ⇒ “Check Mesh…” menu entry: activate the “Element Normal Vector Correction” toggle button in the form and click “OK” to start the procedure.


During execution of the “Check Mesh” command, information about the mesh geometry (volume, surface area, normal direction) is printed to the Echo Window.

You can visualize the element normal vectors as follows :

First, change your viewing position with respect to the mesh : switch to the predefined XZ view by selecting the “Display” ⇒ “View Point” ⇒ “XZ” menu entry. Then, switch from shaded display to wireframe rendering by clicking the

“Wireframe/Shade/Feature” toolbar button. Finally, display the normal

vectors on the elements by clicking the “Show/Hide Vectors” toolbar button. The normal vectors are represented by blue arrows with the element centroid location as starting point.


You can try other predefined views from the “Display” ⇒ “View Point” dialog to have a better view of the normal vector arrows.

To remove the normal vectors from the display, click the “Show/Hide Vectors”

toolbar button. You can return to the default view by selecting the “Display” ⇒ “View Point” ⇒ “User Default” menu entry.

3.7 Define Fluid Properties

Select the “Model” ⇒ “Fluid Properties…” menu entry and enter the acoustic properties for standard air : speed of sound = 340 m/s ; density = 1.225 kg/m3 . In a Boundary Element analysis, the fluid is always assumed to be homogeneous and hence these properties will apply to the whole model. Therefore, you cannot select a particular set of elements in the Fluid Properties form : this part is inactive or “grayed out”.

Note: The numerical values used in the example refer to the SI International System of units (metre, kilogram, second, pascal,…).

You are allowed to use any combination of units in SYSNOISE, but you must remain consistent in the use of units when defining numerical values. If you have any doubt, do not hesitate to contact your local support representative.

3.8 Identify the Firewall Elements

Before you can apply vibration boundary conditions, you have to identify the boundary elements which make up the firewall panel. For this purpose, you can use the SYSNOISE feature domain set creation functionality, which is found in the “Check Mesh…” dialog. A SYSNOISE feature domain is a collection of elements for which the angle between the normal vectors of two adjacent elements is less than a preset value, called the Feature Angle.


In the form, activate the toggle button “Build Feature Domain Sets” and set the “Feature Angle” parameter to 50 (degrees). Click “OK” to start the procedure, which will create 11 feature domain sets. These sets can be selected for display in the Main Window through the “View” ⇒ “Sets” menu entry.

In the associated form, highlight the first four feature domain sets for display :


Each of the feature domain sets is easily identified in the Main Window by means of a different element shading color for each set (the color legend appears in the upper left part of the window). Together, these four sets represent the firewall and the floorpan of the car.

To have a better view of the sets, you can always select the predefined view “Display” ⇒ “View Point” ⇒ “Z on Top [front]”, as shown above.

3.9 Create Velocity BCs from FEA Vibration Data

The structural response vectors of the firewall due to a mechanical point load (e.g. unit force on a mount point) have been computed by an MSC/NASTRAN Forced Response Analysis over a frequency range from 10 Hz to 100 Hz, with a step of 2 Hz. These response vectors have been stored in an external NASTRAN punch file named firewall_disp.pch.


It is important to note that in acoustics, only the normal component of vibration is used because the “in-plane” vibration component (relative to the panel surface) does not generate any fluid motion (the fluid is inviscid). Hence, we will now automatically convert the NASTRAN vibration vectors into SYSNOISE normal velocity boundary conditions on the acoustic boundary element mesh for the complete frequency range. This conversion process is achieved by the GENERATE wizard, a sequence of dialogs which is started by selecting the “Model” ⇒ “Vibrating Panels” ⇒ “FEA…” menu entry.

3.9.1 Specify the location for velocity boundary conditions

The velocity boundary conditions have to be applied on the elements corresponding to the firewall only; elements which are not assigned any explicit boundary condition get a zero normal velocity by default, i.e. they are considered as rigid and stationary. To achieve this, activate the “Element Selector” dialog by clicking the corresponding button in the “Step 1” wizard dialog. The dialog is automatically set to the “Set Selection” mode : select


Feature-Domain 000001 through 000004 from the pull-down menu and click the “Add Entry” button for each of them to add them to the list of selected elements.

Please note that the present selection of elements also includes the floorpan of the car. We will see later how you can avoid assigning non-zero velocity boundary conditions to the floorpan by an intelligent use of the “Unequal Mesh Interpolation” parameter settings (see section 3.9.4).

Click the “OK” button to return to the “Step 1” wizard dialog and click the “Next” button to proceed to the next dialog.

3.9.2 Specify FEA vibration data source

In the “Step 2” wizard dialog, you have to specify from which data file SYSNOISE has to retrieve the vibration data.

The structural response vectors consist of harmonic displacements (six components, i.e. three X,Y,Z translations and three X,Y,Z rotations) stored in a NASTRAN punch* file, but these vectors could also be stored in the form of structural velocities or accelerations. Therefore, you have to specify the data type as “Displacements”. Activate the “Frequency Selector” button and specify


the frequency range for which velocity boundary conditions will be generated : from 10 to 100 Hz with a linear step of 2 Hz. Finally, enter the name of the punch file and select the format NASTRAN from the pull-down menu.

* If you do not have a license for the SYSNOISE-NASTRAN interface, a Free Format file ‘firewall_disp.fre’ with the same vibration data is also available in the /users/sysnoise/5.6/Getting_Started/ directory.

Click on the “Next” button to proceed to the next dialog.

3.9.3 Specify structural mesh for FEA vibration data

The structural response vectors in the NASTRAN file have been obtained on a structural Finite Element mesh of the firewall. In general, the original FE mesh on which the vibration data has been obtained will be incompatible with the SYSNOISE acoustic Boundary Element mesh because of different node locations, different mesh densities, etc… Therefore, SYSNOISE enables interpolation of the data from the structural FE mesh to the acoustic BE mesh by means of a geometrical interpolation algorithm between each acoustic node and the surrounding structural nodes. Of course, this requires that SYSNOISE also knows the structural FE mesh on which the structural response vectors have been obtained. In the “Step 3” wizard dialog, activate the “Use Other File” button, enter the file name of the original structural FE mesh, called firewall.bdf (NASTRAN bulk data file), and select the format NASTRAN.

*If you do not have a license for the SYSNOISE-NASTRAN interface, a ‘firewall.fre’ Free Format structural mesh file is available in the /users/sysnoise/5.6/Getting_Started/ directory

Click the “Next” button to proceed to the next dialog.


3.9.4 Specify Unequal Mesh Interpolation parameters

Finally, you have to set the interpolation parameters between the structural mesh and the acoustic mesh. In our case, the firewall structural mesh is a subset of the acoustic boundary element mesh, i.e. both meshes have identical mesh densities and node locations. Therefore, there is a one-to-one correspondence between structural and acoustic nodes, except for the nodes located on the floorpan, for which there is no structural equivalent. Therefore, we select “Interpolation Algorithm Number” 1 with as “Number of Nodes Used in Interpolation” 1. In order to prevent the assignment of velocity boundary conditions to the elements of the floorpan, we select a sufficiently small value for the “Maximum Distance to Interpolating Node”, e.g. 0.05 m.

Click the “Finish” button and observe the output in the Echo Window, where the automatic generation of boundary conditions is reported.

3.9.5 Visual check of velocity boundary conditions

SYSNOISE contains tools which enable checks on the normal velocity boundary conditions created by the GENERATE sequence. First, in order to have a better view, you can instruct SYSNOISE to only display the four sets corresponding to the firewall and the floorpan using the “Display” ⇒ “Graphical Options” menu entry : select the “View Port” radio button, toggle the “Display Mesh When Displaying Sets” button to off and click “Apply” and “Close”. This will only show the four sets selected for display in the “View” ⇒ “Sets” menu; the other elements remain invisible.


Then, open the “Postprocess” ⇒ “Color Map…” form. For “Model Mesh [1] “ select the “Velocity BC” entry from the pull-down menu, highlight 50.000 Hz as frequency and set the “Real Part” filter.

Click the “Apply” button to get the following image in the Main Window display :


By clicking the “Inquire” toolbar button and clicking on floorpan nodes, you can double-check that these nodes were assigned a zero normal velocity boundary condition (the corresponding nodal value is printed in the Echo Window).

Click on the “Hide All” toolbar button to “clean” the display and to return to the original display of the complete boundary element mesh.

3.10 Save the Model Database

It is good practice to save the model database at regular intervals, as a protection from accidental corruption of the database. All SYSNOISE operations are recorded in a temporary database _Car_NoTrim.sdb, which is copied to the permanent database Car_NoTrim.sdb when the “Save” command is issued. This command is found in the “File” menu.

You have now completed the pre-processing phase of your SYSNOISE acoustic analysis. You are ready to start a calculation and compute the acoustic response of the system to the excitation vectors.

3.11 Set the Analysis Parameters

Select the “Analysis” ⇒ “Parameter…” menu entry, which opens a form where you can set the parameters which control the analysis.


In the SYSNOISE context, the term Potentials refers to any acoustic data which is obtained on the acoustic mesh (Finite Element or Boundary Element) as a result of the analysis, e.g. nodal pressures in option FEM Fluid or double-layer potentials in option BEM Indirect. The term Results refers to any acoustic data which is obtained on the Field Point Mesh (field points are additional points in which you want to evaluate acoustic quantities and are the numerical equivalent of microphones).

The “Save” command controls the saving of Potentials (for all nodes) or Results (for all field points) in the database, sorted according to increasing frequency. The “Step n” command indicates that data will be saved for every nth analysis frequency. The “Store” command controls the saving of Potentials or Results in the database for a particular selection of nodes or field points, but now for all frequencies.

Here, we will only select the saving of Potentials with a Step 1, i.e. for all analysis frequencies. Click the “Apply” and “Dismiss” buttons to confirm these settings. Observe the corresponding command sequence which is generated in the SCL History Window.

3.12 Solve over a Range of Analysis Frequencies

The “Inquire” ⇒ “Fmaximum…” menu entry allows you to check the maximum frequency for which the discretization of a given mesh is valid. Using the “minimum 6 elements per wavelength” criterion, this maximum frequency will


be computed on the basis of the maximum element edge length, and printed in the Echo Window. Click “OK” on the corresponding form that comes up. For the present BE mesh, the maximum frequency is 215.50 Hz.

Then, select the “Analysis” ⇒ “Solve…” menu entry to set the frequencies for which you want to compute the acoustic response.

Clicking the “Frequency Selector…” button opens a form where the analysis frequencies are specified.

In this case, we will select analysis frequencies corresponding to the frequencies for which we created velocity boundary conditions : from 10 Hz to 100 Hz, with a linear frequency step of 2 Hz . Click the “OK” button on the “Solve” form to start the computations.

Please note that you can always interrupt a SYSNOISE analysis run by

pressing the right-most “Interrupt” toolbar button


3.13 Field Point Post-Processing

As a result of the preceding acoustic response analysis, we have obtained the acoustic pressure and acoustic normal velocity distribution on the boundary element surface.

We will now create a double-plane field point mesh inside the passenger compartment. As was mentioned earlier, field points are additional points in which you want to evaluate acoustic quantities and are the numerical equivalent of microphone positions. At each of these field points, the acoustic pressure and other acoustic quantities will be computed from the acoustic data on the BEM surface by an additional procedure, called Field Point Postprocessing.

To create the Field Point Mesh, select the “Geometry” ⇒ “Field Point…” entry, which gives a list of commonly used geometrical shapes for field point meshes : point, line, plane, sphere, box, cylinder etc…

Select the “Plane” entry and create a first longitudinal plane by entering the X,Y,Z coordinates for three corner points of the plane as shown below. Note that the “Divide” parameters control the number of field points in the two in-plane directions. Then, create a second longitudinal plane by executing the “Geometry” ⇒ “Field Point…” ⇒ “Plane” command again, and replacing the Y coordinate of -0.45 by a Y coordinate of 0.45.


Finally, you will end up with two parallel planes which “cut” approximately through the middle of the four passenger seating positions. Please note that Field Point Mesh creation is an additive operation : you can keep on adding as many field points as you like.

In order to get a better view of the field point planes, you can always rotate the

model in the Main Window display : click the “Rotate” toolbar button click-hold the left mouse button and move the mouse around.

Once the field points have been created, you can compute the acoustic pressure and other acoustic quantities in the field points through the “Analysis” ⇒ “Process Field Points…” form. In the form, the “Frequency Selection” is automatically set to the frequency selection used for the last analysis. Select all field points for the “Points Selection” :


Click the “OK” button to start the field point postprocessing. Allow some time for SYSNOISE to complete these calculations.

3.14 Graphical Representation of Acoustic Results

You have completed the analysis phase of your session. Now it is time to have a look at the results of your analysis, i.e. to start with the post-processing phase. SYSNOISE offers two general categories for acoustic data visualization :

a) Mesh postprocessing, i.e. displaying the acoustic data on the Finite Element or Boundary Element mesh or on the Field Point Mesh

b) Response function plots, i.e. displaying acoustic data at a specific location as a function of frequency in the form of XY-plots

3.14.1 Mesh postprocessing : color map

A typical way to visualize the pressure distribution within the car cavity is to represent this data for a single frequency in the form of colored fringes on the field point planes. Such a Color Map image is controlled by the “Postprocess” ⇒ “Color Map…” form.


Select “Field Point Mesh [1] ” and “Pressure” from the pull-down menus in the “Color Map Display” form, highlight the 70.000 Hz frequency, select the “dB” representation format and click “Apply”.

The pressure distribution ranges from a minimum of 36 dB in the middle of the car compartment (blue areas) to a maximum of 62 dB at the front and the back of the compartment (red areas). Such a pressure distribution is typical for a frequency close to the resonance frequency of the first acoustic cavity mode.


3.14.2 Response functions : field point pressure

Response functions are pressure results or other acoustic quantities at a specific node or field point as a function of frequency.

To build a response function, go to "Analysis ⇒ Store…" and specify the type of response function you want to build (i.e. Results). Then, give the frequency range, i.e. from 10 to 100Hz with a linear step of 2Hz and the point list, i.e. point 152 and 95. Click the "Apply" button to confirm these settings.

Note: You can also graphically select the points for which you want to build a response function. Instead of typing a point number in the top field of the dialog, click on the button in the toolbar and pick the desired points with the mouse directly on the field point mesh.

Once created, the response function can be displayed. Select the “Postprocess” ⇒ “Response Function…” menu entry : in the form, choose field point number 152 (which corresponds more or less to the position of the driver’s head), select “Pressure”, set the representation format to “4: Magnitude/Phase” and click “Apply”. The Curve Window will pop-up and display the response function as a red curve.


It is possible to “overlay” several response function plots on top of each other : e.g. choose point number 95, activate the “Superimpose” toggle button and click “Apply”. The second response function will be plotted as a blue curve on top of the first one. Please note that the Y axis scale has been fixed by the first response curve. You can always set the minimum and maximum bounds of the X and Y axes manually by accessing the “Options” form.

Click the “Exit” button to close the Curve Window.


3.15 Close the Database and/or End the Session

Congratulations ! You have successfully completed a typical SYSNOISE uncoupled analysis sequence.

Complementary analyses after such a first calculation are: • Directivity Analysis • Coupled Fluid-Structure Analysis (in a car compartment) • Acoustic Contribution Analysis • Acoustic Sensitivity Analysis • Acoustic Global Sensitivity Analysis

If you want to go on and try the remaining parts of this Getting Started manual, close the SYSNOISE database using the “File” ⇒ “Close” menu entry. SYSNOISE will prompt you with the pop-up message “Do You Want to Save the Changes of This Model ?” Click the “Yes” button to save the model data and the results of the calculations in the database.

Otherwise, you can end the session using the “File” ⇒ “Exit…” menu entry, which opens the following form :

Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the Car_NoTrim.sdb database and then terminate the SYSNOISE session.

3.16 Uncoupled BEM Analysis: computation alternatives.

As stated in section 3.4, several alternatives for the analysis option can be used for this typical example. The option BEM Direct Collocation Element Interior Uncoupled Frequency has been presented in the preceding sections, we propose in this section to use two alternatives based on the BEM Indirect Variational Uncoupled Unbaffled Frequency option.


3.16.1 Uncoupled Indirect BEM

This section summarizes the analysis of the sound radiation in a passenger car compartment using the Uncoupled Indirect BEM module.

Repeat the thirteen steps described in sections 3.2 to 3.14. When creating a new database (section 3.3), use car_Indirect as database name and car_Indirect.sdb as file name. When defining the analysis option (section 3.4), select BEM Indirect Variational Uncoupled Unbaffled Frequency option.

Before solving over a range of analysis frequencies (section 3.12), you need to set the correct analysis parameters (section 3.11). Select Physical as solution space and save Potentials with a Step 1. Do not save nor store results. Leave the default BEM Quadrature and Post-Processing.

After storing the results (section 3.14.2) you should obtain the following results at the two selected field points:


3.16.2 Uncoupled High Speed BEM

This section summarizes the analysis of the sound radiation in a passenger car compartment using the Indirect High Speed BEM module based on the Padé expansions (see release note for more background on the module).

Repeat the thirteen steps described in sections 3.2 to 3.14. When creating a new database (section 3.3), use car_Pade as database name and car_Pade.sdb as file name. When defining the analysis option (section 3.4), select BEM Indirect Variational Uncoupled Unbaffled Frequency option.

Before solving over a range of analysis frequencies (section 3.12), you need to set the correct analysis parameters (section 3.11). Select Pade Expansion as solution space and save Potentials with a Step 1. Do not save nor store results. Leave the default BEM Quadrature and Post-Processing.


After storing the results (section 3.14.2) you should obtain the following results at the two selected field points:

The solve sequence of the High Speed BEM module should have taken you roughly 50% of the solution time needed for the regular indirect BEM module. Typically, a gain factor of ten is obtained for:

• mid-size problem

• exterior radiation (no resonances)

• wide range of frequencies

with virtually no change in the results! Next section provides a comparison between the regular Indirect BEM module and the High Speed BEM module.


3.16.3 Comparison between IBEM & High Speed BEM

Open the databases through the “Model”⇒”Open” dialog box, first select the car_Indirect.sdb database file and repeat the operation for the car_Pade.sdb database file.

Select the “Postprocess” ⇒ “Response Function…” menu entry : in the form, click “Model…” and select the first model then “Apply” to go back to the Response Function Dialog. Choose field point number 152 (which corresponds more or less to the position of the driver’s head), select “Pressure”, set the representation format to “4: Magnitude/Phase” and click “Apply”. The Curve Window will pop-up and display the response function as a red curve. Then click again on “Model…” and select the second model then “Apply” to go back to the Response Function Dialog. Choose again field point number 152 and click “Apply”.


You can observe that the two curves are perfectly superimposed, proving the accuracy of the High Speed BEM module.


4 Getting Started - Coupled BEM Analysis : Vibrating Firewall in a Passenger Car Enclosure


Another common application of SYSNOISE is the analysis of vibro-acoustic problems, in which there is a strong coupling between the vibration of a flexible structure and the sound field which is radiated by this structure. In other words, the vibration which generates the sound field is influenced itself by the acoustic pressure loading it causes. SYSNOISE is perfectly able to handle such problems by solving the structural dynamics problem and the acoustics problem simultaneously.

In this example, SYSNOISE will be used to compute simultaneously the vibration of a “monovolume” passenger car firewall and the sound field radiated by this firewall into the interior of the car. The excitation creating the firewall vibration and the sound field is a mechanical point force acting on a mount point of the firewall.

The analysis sequence consists of the following steps : 1. Create the Structural Model Database 2. Define the Analysis Option : FEM Structure 3. Import the Structural FE Mesh (Firewall) 4. Import the Structural Modes 5. Define Viscous Modal Damping 6. Define a Point Load Excitation 7. Save the Structural Model 8. Create an Acoustic Model Database 9. Define the Analysis Option : BEM Indirect 10. Import the Boundary Element Mesh (Car Compartment) 11. Check the Normal Vectors 12. Define Fluid Properties 13. Save the Acoustic Model 14. Link the Acoustic & Structural Models 15. Define the Field Point Mesh 16. Set the Analysis Parameters for Both Models 17. Compute the Coupled Forced Response 18. Visualization of Structural Results 19. Visualization of Acoustic Results 20. Exit the SYSNOISE Session

All these steps are described in detail in the following pages.


4.2 Working with Multiple SYSNOISE Models

SYSNOISE Rev 5.5 contains a model manager allowing you to open several models simultaneously. In this multi-model approach, coupled problems are analyzed by connecting two models together with a fluid-structure link. One model is the acoustic model, containing the acoustic mesh, acoustic properties, and acoustic boundary conditions. The other model is the structural model, containing the structural mesh, structural properties and structural boundary conditions.

Several commands are available to work with multiple models : • New : Create a new model database • Open : Open an existing model database • Save : Save the model information (update the database) • Save As : Save the model in a new model database • Activate : Activate a model (only one model can be active at any one

time) • Link : Link two models together

A binary database file (“.sdb” file) is associated with each model, and contains all the information related to the model : analysis option, mesh, boundary conditions, material properties, potentials, results,…

Several operators are available to manage these database files: • Directory : Gives a summary list of all records • Merge Model : Add records of a database to the current database • To File : Export binary database to an ASCII format database • From File : Import an ASCII format database • Compress : Compress the database file • Delete : Delete a selection of records in the database

4.3 Create the Structural Model Database

Create a new model database by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar. This binary SYSNOISE database file will contain all the information related to your structural model : structural mesh, structural properties, structural boundary conditions, structural results (after the analysis), etc…

Create a database with the name Firewall Vibration. The full name of the database file has to be entered as well : Coupled_Firewall.sdb. Since no other database has been opened in this SYSNOISE session, this model database will be assigned the Model Number 1.



In order to identify the present model as a structural model, set the FEM Frequency Structure analysis option by selecting the “Model” ⇒ “Option” menu entry in the Main Window :

Please be careful about the selection of the analysis option : if you change the option in the middle of a SYSNOISE session, the model database will be re-initialized completely and all model data and results are deleted!


If you are not sure about which option to choose, you can always activate the “Option Wizard”. This wizard sequence assists you in defining the most appropriate analysis option by requesting answers to a series of questions as shown in Chapter 2 of this manual.

4.5 Import the Structural Finite Element Mesh

The structural FE mesh is a surface mesh of the firewall panel. The mesh data (nodal coordinates and element connectivities) is contained in an external SYSNOISE Free Format file named firewall.fre. Load this mesh file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form :

After clicking the “OK” button, the mesh is displayed in the Main Window as follows :


4.6 Import the Structural Modes

The structural behavior of the firewall will be represented by a structural modal basis. In other words, we assume that any deformation of the firewall can be written as a linear combination of the deformation shapes of the structural modes of the firewall. This approach is known as modal superposition and is a powerful technique for structural analysis : it reduces the solution of the structural problem to finding the coefficients for the linear combination, the so-called modal participation factors.

These structural modal vectors have been computed previously by a structural FEA code and are stored in the form of a SYSNOISE Free Format data file called struc_modes.fre. This structural modal basis is imported into SYSNOISE using the “File” ⇒ “Import menu entry. After clicking the “OK” button, a total of 12 structural modes is read and imported into the database. Observe the output in the Echo Window, which provides feedback about the import process.


If you want to have an idea of the resonance frequencies of these modes, you can list them in the Echo Window by clicking the “Inquire” ⇒ “Frequency” menu entry. As you will see, these frequencies range up to about 200 Hz. Therefore, the frequency for the coupled analysis should be limited to a maximum of about 100 Hz (good practice is to take 50% of the maximum uncoupled structural resonance frequency as the maximum frequency limit for the coupled analysis).

You can visualize the deformation shapes of the structural modes by entering the “Postprocess” ⇒ “Deformation…” form : for “Model Mesh [1]”, choose “Structural Modes” and double-click for example on the first structural resonance frequency = 50.568 Hz :

This operation results in a deformed display in the Main Window :


You can also animate the deformation by clicking the “Animation…” button in the “Deformation Field Definition” dialog. Choose “Phase” and take a phase step of 30 degrees (instead of the default 10 degrees) which will result in the creation of 12 animation frames in the full animation cycle of 360 degrees.

Click the left-most Play button to start the animation. After the creation of the animation frames, SYSNOISE will continuously “loop” through these frames. You can control the animation speed with the slide button and stop the animation by clicking the right-most Stop button.

4.7 Define Viscous Modal Damping

SYSNOISE can only import undamped structural modes, but allows the definition of viscous or hysteretic modal damping : access the “Structural Modal Damping” form by clicking the “Model” ⇒ ”Structural Properties” ⇒ “Modal Damping” menu entry.


Then, click the “Select All” button of the dialog and select the “Damping Model” Viscous. Enter a value of 0.01 for the KSI parameter (which defines the viscous damping value as 1% of the critical damping value), click the “Set Value” button and finally confirm the settings with the “OK” button :

4.8 Define a Point Load Excitation

Mechanical forces and couples (torques) are applied using the ‘”Model” ⇒ “Load B.C....” ⇒ “Manual” dialog :


You will now apply a normal force of 1 Newton to node number 9175 of the firewall, which could represent a mount point for the firewall. This is the only excitation you will define, i.e. the coupled analysis will compute the acoustic and structural response of the coupled system Firewall – Car Compartment to this unit force. Complete the “Load” form as shown below and click the “Add” button. To close the form, click the “OK” button.


The position of the Load boundary condition is represented on the FE mesh by a small red square at the location of node number 9175. Click the toolbar

button “Wireframe/Shade/Feature” to have a better view. You can also

click the “Show/Hide Vectors” toolbar button to visualize the normal vector direction.

Note that you can also select graphically the node where you want to apply the load. Instead of typing a node number in the node selection box of the dialog, click on the

“Single Pick” toolbar button and pick the desired node with the mouse directly on the mesh.


4.9 Save the Structural Model

You have now completed the structural pre-processing phase of the coupled analysis sequence. Use the “File” ⇒ “Save” command to update the structural model database.

We will now continue with the acoustic pre-processing phase.

4.10 Create an Acoustic Model Database

To create an acoustic model database for the car compartment, select the “File” ⇒ “New…” menu entry. Create a new database named Coupled Cavity Noise. The full name of the database file has to be entered as well : Coupled_Cavity.sdb. Since this is the second database to be opened in this SYSNOISE session, it will be assigned the Model Number 2 automatically. The Model Number is used by SYSNOISE to identify different models when multiple databases are opened in the same session. (The number is not saved in the database).



The car compartment model is a purely acoustic model database that will be linked to the structural model database for a coupled analysis. Therefore, you have to select the BEM Frequency Indirect Uncoupled analysis option. Define the option by clicking the “Model” ⇒ “Option” menu entry in the Main Window :


The Boundary Element mesh is a surface mesh of the walls forming the enclosure of a “monovolume” passenger car compartment. This mesh has been generated by an external mesh generator and is contained in an external SYSNOISE Free Format file named monovolume.fre. Load the mesh by using the “File” ⇒ “Import” menu entry :


4.13 Check (Show) the Normal Vectors

For a BEM Indirect analysis, it is absolutely essential that the normal vectors of the elements are pointing in a consistent direction. The “Check Mesh” procedure of SYSNOISE will automatically take care of reversing element normal vectors in order to guarantee consistency.

This tool is accessed from the “Geometry” ⇒ “Check Mesh…” menu entry. Activate the “Element Normal Vector Correction” toggle button in the dialog and then click “OK”. During execution of the “Check Mesh” command, information about the mesh geometry (volume, surface area, normal direction) is printed to the Echo Window.



First, change your viewing position with respect to the mesh : switch to the predefined XZ view by selecting the “Display” ⇒ “View Point” ⇒ “XZ” menu entry. Then, switch from shaded display to wireframe rendering by clicking the

“Wireframe/Shade/Feature” toolbar button several times. Finally, display

the normal vectors on the elements by clicking the “Show/Hide Vectors” toolbar button several times. The normal vectors are represented by blue arrows with the element centroid location as starting point.


toolbar button again. You can return to the default view by selecting the “Display” ⇒ “View Point” ⇒ “User Default” menu entry.


4.15 Save the Acoustic Model

You have now completed the acoustic pre-processing phase of the coupled analysis sequence. Use the “File” ⇒ “Save” command to update the acoustic model database.

Both acoustic and structural parts of the coupled model are now ready for analysis. The last step before launching the analysis is to link the models together.

4.16 Link the Acoustic and Structural Models

To perform a coupled vibro-acoustic analysis, the acoustic and structural models must be connected by a fluid-structure link. The link defines which parts of the acoustic and structural model are “interfaced” and in this way determines the coupling matrices, which describe the fluid-structure interaction effects. In our case, the link only applies to the firewall surface since this is the contact surface between the firewall and the car compartment.

The definition of a SYSNOISE link is controlled by the Link Wizard, which is started by selecting the “Model” ⇒ “Link” menu entry.

In the first wizard form, select Model Number 1 (the structural model) as the “First Model” and choose All elements as the selection to be linked.


Instead of typing the model number, you can always click the “Model Selector” button which opens a form allowing you to select the model from a list of open databases. In the list, the databases are listed both by model number and model name, which facilitates selection of the correct model by simply highlighting them.

In the second wizard form, select Model Number 2 (the acoustic model) as the “Second Model” and enter the command string “Elements 1 To 174 Internal” (which refers to internal numbering), which corresponds to the part of the car enclosure that coincides with the firewall.


Alternatively, you can click the “Element Selector” button to open a form which facilitates the specification of the element selection. Click the “External/Internal” button to the “Internal” setting, in order to ensure that the internal element numbering is used in the element selection.

Finally, select the “Fluid-Structure” behavior because we want to analyze the fluid-structure interaction between the vibrating firewall and the acoustic cavity encapsulated by the car compartment enclosure. Click the “Finish” button to build the link and observe the output in the Echo Window, which confirms that the link has been built.

IMPORTANT : The link between two SYSNOISE models has a temporary character, as it is only needed for the solution of the coupled problem. As soon as one of the models is Closed, the link will be deleted.

4.17 Define the Field Point Mesh

In the uncoupled analysis example, we have defined a Field Point Mesh after the response analysis and performed a “Process Field Points” operation (see section 2.13). In this example, we will define the Field Point Mesh before starting the coupled response calculation. Select the “Geometry” ⇒ “Field Point…” ⇒ “Plane” menu entry and complete the form as shown below, which creates a “transversal” field point mesh in the front of the passenger compartment.


4.18 Set the Analysis Parameters for Both Models

Before starting the coupled analysis, you have to set the analysis parameters for both the structural and the acoustic models. These parameters control what acoustic and structural data will be saved during the analysis. For this purpose, select the “Analysis” ⇒ “Parameter…” menu entry.

By default, the form is set to the parameters for the last opened model, i.e. the acoustic model. For more information about the meaning of the Save/Store commands and the terms Potentials and Results in the form, consult section 2.11. In this case, we will Save Potentials with a Step 1, i.e. the acoustic potentials will be saved for all analysis frequencies. In contrast, we will Store Results , i.e. retain response functions, at the field points number 37, 41 and 47 only. Click the “Apply” button to confirm these settings. Observe the


corresponding command sequence which is generated in the SCL History Window.

Then, to set the analysis parameters for the structural model, select the model “[1] Firewall Vibration” from the “Solution parameters for model” menu. Activate the Modal radio button for the “Solution Space” in order to force a modal superposition analysis and choose all 12 structural modal vectors. Also, select Save Displacements with a Step 1, i.e. we will save the structural displacements of the firewall for every frequency. Click the “Apply” button to confirm the settings and “Dismiss” to close the form.

4.19 Compute the Coupled Forced Response

Finally, you can start the coupled analysis using the “Analysis” ⇒ “Solve…” menu entry. We will select a frequency range from 10 Hz to 100 Hz with a linear step of 10 Hz , i.e. a total of 10 analysis frequencies.


Coupled results consist of the structural results for the firewall, which are saved in the structural database, and acoustic results for the car compartment, which are saved in the acoustic database. Therefore, after the analysis has finished, it is important to save the computed results in both the structural and acoustic databases.

First, save the acoustic database using the “File” ⇒ “Save” command. Then, activate the structural database by selecting the “File” ⇒ “Activate…” menu entry, highlighting the structural model “Firewall Vibration” in the corresponding form and clicking the “Apply” button.

Then, issue the “File” ⇒ “Save” command again to save the structural database.

4.20 Visualization of Structural Results

First, we will have a look at the structural results of the coupled analysis. In order to have an uncluttered view of the firewall, you can remove the car compartment BE mesh and field point mesh from the display using the “View” ⇒ “Objects” menu :


Highlight only “Model Mesh [1]” for display and click “OK”.

Then, proceed to the “Postprocess” ⇒ “Deformation…” menu. Select “Model Mesh [1]” and “Displacement” in the form and double-click the 80 Hz entry to display the coupled displacement on the firewall structure.

x

You can also animate the deformation pattern by clicking the “Animation” button. Select 30 degrees as phase step (as in section 4.6 above).


In a modal superposition analysis (uncoupled and coupled), it is always interesting to look at the relative contribution of each modal vector to the displacement pattern for a given frequency. In the “Postprocess” menu, select the “Participation Function…” entry, which allows to display the modal participation factor data :

In the form, select the “Frequency” display mode and enter 80 Hz. Click the “Options” button to open and additional form : in this form, select the Type “histogram” (last icon in a pull-down menu of four display types). Click the “Apply” and “Close” buttons to return to the previous from and click "Apply” again to plot the magnitude for each of the 12 modal participation factors.

This plot clearly shows that the structural behaviour of the firewall at 80 Hz is dominated by the third structural mode (with resonance frequency of 85.06 Hz).


4.21 Visualization of Acoustic Results

Now, let’s have a look at the acoustic results of the coupled analysis. First, you can add the field point mesh to the display by highlighting the corresponding entry in the “View” ⇒ “Objects…” form :

After clicking the “OK” button, you will get the following display :

Then, you can try to display the pressure on the field point mesh by selecting the “Postprocess” ⇒ “Color Map…” menu entry :


As you can see, when you select “Field Point Mesh [2]” and “Pressure”, nothing appears in the frequency selection list! This is logical, since we choose not to Save Results in the Parameter settings for the acoustic model (see section 4.18): in this case, the acoustic results on the field point mesh are not retained. Instead, we chose to Store Results at specific field points, i.e. to store only the acoustic response functions at these locations in the database.

This approach is advantageous if you are only interested in the response functions at a few field point locations and the number of analysis frequencies is high : you do not need to retain all of the results data at all the field points and you do not need to perform an additional field point postprocessing operation.

Note : The Save and Store commands are mutually exclusive, i.e. they cannot be combined.

You can visualize the stored response functions using the new “Postprocess” ⇒ “Response Function…” menu entry :


To display the pressure response function at field point number 37 in dB format, first click the “Model” button and highlight model number “[2] Coupled Cavity Noise” to activate the acoustic model. Then, activate the “Results” button, highlight “[2] Result Point 37” in the list (in which the digit between square brackets is the model number), select the “Pressure” and “3: Magnitude(dB)” settings, and click “Apply” :

The rectangular grid on the plot has been obtained by clicking the “Options…” button in the “Response Function” form and activating the “Draw Full Grid” button in the “Options” form.

You can add the other response functions to the plot by highlighting them, activating the “Superimpose” toggle button and clicking the “Apply” button:


Click the “Exit” button to close the Curve Window.

4.22 Exit the SYSNOISE Session

Congratulations ! You have successfully completed a typical SYSNOISE coupled analysis sequence.

There are many possible extensions to this calculation. Some typical examples are: • Same calculation with the FEM-FEM module, • Multi-frequency analysis, • Add some acoustic absorbing linings in the van, • Acoustic contribution analysis, • Acoustic excitation instead of mechanical excitation.

You can now exit from the SYSNOISE program using the “File” ⇒ “Exit” menu entry :


5 Getting Started - Uncoupled FEM Analysis: Sound Radiation in an Engine Manifold


Sound radiation in enclosures is a common application in the industry. SYSNOISE has several ways to solve such problems, such as uncoupled BEM, coupled BEM, uncoupled FEM or even coupled BEM according to the problem nature. In this section we will focus on an uncoupled FEM application with pressure and impedance boundary conditions.

In this example, SYSNOISE will be used to compute the acoustic radiation in a engine manifold with pressure boundary condition at the inputs and anechoic impedance boundary condition at the output. For simplicity, we will not consider temperature gradients in the manifold, i.e. we will not consider inhomogeneous fluid properties.


1. Start SYSNOISE 2. Create an Acoustic Model Database 3. Define the Analysis Option: FEM FLUID 4. Import the Finite Element Mesh 5. Define Fluid Properties 6. Identify the Inlet and Outlet Surfaces 7. Import the Pressure Tables 8. Create Manual Boundary Conditions 9. Set the Analysis Parameters 10. Solve over a Frequency Range 11. Visualization of Acoustic Data Results 12. Save/Close the SYSNOISE session Each of these steps is described in detail on the following pages.


Create a new model database manifold.sdb by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar. Since no other database has been opened or created in this SYSNOISE session, this model database will be assigned the Model Number 1. Also, since no name has been given to the model, the name of the database file without extension will be assigned to the model (i.e. manifold).



In order to identify the present model as a structural model, set the FEM Frequency Structure analysis option by selecting the “Model” ⇒ “Option” menu entry in the Main Window .

Please be careful about the selection of the analysis option : if you change the option in the middle of a SYSNOISE session, the model database will be re-initialized completely and all model data and results are deleted!

If you are not sure about which option to choose, you can always activate the “Option Wizard”. This wizard sequence assists you in defining the most appropriate analysis option by requesting answers to a series of questions as shown in Chapter 2 of this manual.


5.4 Import the Finite Element Mesh

The FE mesh is a volume mesh of the manifold cavity. The mesh data (nodal coordinates and element connectivities) is contained in an external SYSNOISE Free Format file named manifold.fre. Load this mesh file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form :




Select the “Model” ⇒ “Fluid Properties” ⇒ “Material Definition” ⇒ “Fluid…” menu entry and enter the acoustic properties for standard air: speed of sound = 340 m/s ; mass density = 1.225 kg/m3. The imaginary parts of the speed of sound and of the mass density enable the user to model hysteresis and viscous dampings. Apply this fluid property to all elements.

Note: The numerical values used in the example refer to the SI International System of units (metre, kilogram, second, pascal,…).

You are allowed to use any combination of units in SYSNOISE, but you must remain consistent in the use of units when defining numerical values. If you have any doubt, do not hesitate to contact your local support representative.

5.6 Identify the Inlet and Outlet Surface

Before you can apply pressure and impedance boundary conditions, you have to identify the boundary element faces that make up the inlet and outlet surfaces.

First, change your viewing position with to have a better vew of the inlets: switch to the predefined YZ view by selecting the “Display” ⇒ “View Point” ⇒ “YZ” menu entry.

To create a set containing the element faces of the left manifold inlet, select “Geometry” ⇒ “Sets” ⇒ “Faces Selection …”, give the set the name left and use the feature selection icon in the toolbar to select the left inlet element faces in one action. Repeat the operation for the center left, center right and right inlets. Rotate the model in the Main Window display to have a clear view on the outlet : click the “Rotate” toolbar button click-hold the left mouse


button and move the mouse around. Create the set containing of the element faces of the manifold outlet and give the name outlet to the set.

These sets can be selected for display in the Main Window through the “View” ⇒ “Sets” menu entry. In the associated dialog box, Select All sets for display :

Each of the face sets is easily identified in the Main Window by means of a different face shading color (the color legend appears in the upper left part of the window).


5.7 Import the Pressure Tables

The pressure fluctuations at the inlet have been computed in a separate software package and store as tables in a SYSNOISE free format file inlet_pressure.fre.

Load this data file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form :

Select “Table” in the ‘Data-type’ drop-down list and click 'OK'.

You can then visualize the Tables by using the “Postprocess” ⇒ “Table Function …” dialog:


5.8 Create Manual Boundary Conditions

Two kind of boundary conditions are applied to the manifold, pressure boundary condition at the inlets using the four tables imported in the previous section and anechoic impedance boundary condition at the outlet.

5.8.1 Assign Pressure Boundary Conditions at the Inlets

The assignment of the pressure boundary conditions is achieved through the “Model” ⇒ “Pressure B.C.” ⇒ “Manual…” menu entry.

Select “Pressure” as boundary type and “Table” as boundary definition. Enter “1” as table number or alternatively use the “Table Selector…”. Assign the boundary condition to the first set (left) and click on the “Add” button to record the boundary condition. Repeat the operation for the other inlets using the three


remaining tables and the center left, center right and right sets. Finally, click on the “Close” button when all pressure boundary conditions are recorded.

5.8.2 Assign Impedance Boundary Conditions at the Outlet

The ρc impedance boundary condition, where ρ is the fluid mass density and c is the speed of sound, is often used to model anechoic termination such as infinite ducts.

To assign this type of boundary condition, select the “Model” ⇒ “Absorbent Panels…” menu entry.

Select “Impedance” as boundary type and “Value” as boundary definition. Enter “416.5” as impedance value, assign the boundary condition to the outlet set (outlet) and click on the “Add” button to record the boundary condition. Finally, click on the “Close” button to exit the dialog box.

5.9 Set the Analysis Parameters

Before starting the analysis, you have to set the analysis parameters of the model. These parameters control what acoustic and structural data will be saved during the analysis. For this purpose, select the “Analysis” ⇒ “Parameter…” menu entry. Select Physical as “Solution Space” and Save Potentials with a Step 1, i.e. the acoustic potentials will be saved for all analysis frequencies. For more information about the meaning of the Save/Store commands and the terms Potentials and Results in the form, consult section 3.11.


For uncoupled FEM fluid analysis with no infinite elements, it is possible to compute the acoustic response using an iterative solver. To do so, select the “Tools” ⇒ “Environment Variables…” menu entry and assign the value “1” at the SOLVITER environment variable in the SETUP section:

Make sure you have enough memory to run the problem in-core. To do so, select the “Tools” ⇒ “Environment Variables…” menu entry and assign the value “15” at the MEMORY environment variable in the SETUP section.

5.10 Solve over a Range of Analysis Frequencies

The “Inquire” ⇒ “Fmaximum…” menu entry allows you to check the maximum frequency for which the discretization of a given mesh is valid. Using the “minimum 6 elements per wavelength” criterion, this maximum frequency will


be computed on the basis of the maximum element edge length, and printed in the Echo Window. Click “OK” on the corresponding form that comes up. For the present FE mesh, the maximum frequency is 2833.7 Hz.

Then, select the “Analysis” ⇒ “Solve…” menu entry to set the frequencies for which you want to compute the acoustic response. Enter 10 To 1000 LinStep 5 as frequency range or alternatively use the “Frequency Selector”. Refer to section 3.12 for more details.

5.11 Graphical Representation of Acoustic Results

You have completed the analysis phase of your session. Now it is time to have a look at the results of your analysis, i.e. to start with the post-processing phase. As explained in section 3.14, SYSNOISE offers two general categories for acoustic data visualization:

a) Mesh post-processing, i.e. displaying the acoustic data on the Finite Element or Boundary Element mesh or on the Field Point Mesh

b) Response function plots, i.e. displaying acoustic data at a specific location as a function of frequency in the form of XY-plots

5.11.1 Mesh post-processing: color map

A typical way to visualize the pressure distribution within the manifold cavity is to represent this data for a single frequency in the form of colored fringes on the field point planes. Such a Color Map image is controlled by the “Postprocess” ⇒ “Color Map…” form. Select “Model Mesh [1] ” and “Pressure” from the pull-down menus in the “Color Map Display” form, highlight the 600.000 Hz frequency and click “Apply”.


Select “Model Mesh [1] ” and “Pressure” from the pull-down menus in the “Color Map Display” form, highlight the 600.000 Hz frequency and click “Apply”.

The pressure distribution ranges from a minimum in the middle of the manifold (blue areas) to a maximum at the extremities (red areas). Such a pressure distribution is typical for a frequency close to the resonance frequency of the first acoustic cavity mode.

5.11.2 Response functions: node potential

Response functions are pressure results or other acoustic quantities at a specific node or field point as a function of frequency.

To build a response function, go to "Analysis ⇒ Store…" and specify the type of response function you want to build (i.e. Potential). Then, give the frequency range, i.e. from 10 to 1000Hz with a linear step of 5Hz and the node list, i.e. point 14616 (center of the outlet). Click the "Apply" button to confirm these settings.


Note: You can also graphically select the points for which you want to build a response

function. Instead of typing a point number in the top field of the dialog, click on the button in the toolbar and pick the desired points with the mouse directly on the mesh.

Once created, the response function can be displayed. Select the “Postprocess” ⇒ “Response Function…” menu entry: in the form, choose field Potentials and select Potential Node 14616, select “Pressure”, set the representation format to “4: Magnitude(dB)”. Then, Click on “Option” to select the dBA scale as acoustic weighting. The “Options Dialog” will appear. Select dBA as dB Scale and click “Apply” and “Close” to close the “Options Dialog”. Finally click “Apply” on the “Response Function Dialog”. The “Curve Window” will pop-up and display the response function as a red curve.


Click the “Exit” button to close the “Curve Window”.

5.12 Compute Acoustic Response using Parallel Computing

An alternative to the actual analysis is to compute the acoustic response analysis using the parallel computation capabilities of SYSNOISE. Only the Domain Decomposition Technique is presented here. See the help on line and Installation Manual for more details on other distributed computing.

Please make sure you have a Netsolver license before running this section.

The Domain Level Distribution mechanism assigns to each processor a subset of the global acoustic domain (partitioning in sub-domains) to be solved. Since each node processes only a subpart of the domain, the memory requirement is divided by the number of processing nodes.

5.12.1 Set-up the analysis model

If you want to perform this analysis, close all open sections (see section 5.13). Next perform the steps as described from section 5.1 to 5.9 except for the setting of the environment variables in section 5.9. Use manifold_DDT.sdb as database file name.

5.12.2 Decompose the Domain.

Before to perform the analysis the domain must be decomposed in sub-domain. The Decompose command for Fluid FEM models is accessed through the “Model”⇒”Decompose…” dialog. Five decompositions are available:

• Automatic, based on the Metis Algorithms (information can be found on http://www-users.cs.umn.edu/~karypis/metis/),




• Ellipsoidal, specially designed for models containing infinite elements,

• From file, where the decomposition is imported from an external file,

• Based on material, where each domain correspond to a specific material,

• User defined, using a user implementation.

Select the type of decomposition to be performed, i.e. “From File”, and enter the filename domain.fre.

The Decompose command creates the sub-domain databases and generates the free faces at the intersections of the domains. Next, you can visualize the domain by displaying the corresponding sets (see for instance section 5.6):


5.12.3 Close the Database and End the Session

As the parallel computation are performed in batch mode, you need to end the session using the “File” ⇒ “Exit…” menu entry, which opens the following form:

Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the database and then terminate the SYSNOISE session

5.12.4 Run SYSNOISE in Parallel

See section “Running SYSNOISE in parallel” of the Installation Manual to launch SYSNOISE in parallel. Use manifold_DDT.cmd as input file.

Two domain decomposition methods are available in SYSNOISE:

• The Finite Element Tearing and Interconnecting method for the Helmholtz equation (FETI-H method) that uses a direct method on the sub-domains but an iterative one for solving the interface problem.

• The Parallel Approximate Factorization Technique (P-AFT method) that enforces the continuity at the domain interface at the end of each Krylov subspace iteration.

The propose example use the second approach to compute the acoustic response. A the end of the computation, the sub-domain databases are merged back into the original database.

NOTE: To perform domain level parallel computing, the environment variable DISTRILEVEL must be set to DOMAIN. Also, to use the FETI-H approach, the environment variable SOLVITER must be set to 0 whereas to use the P-AFT method, this environment variable must be set to 2. Domain Level Parallel Computing is only available with uncoupled FEM fluid analyses.


5.12.5 Graphical Representation of Acoustic Results

You can follow the same procedure as described in section 5.11 to check and have a deep look at the results of your analysis. To do so, you need to open the original database manifold_DDT.sdb.

5.13 Close the Databases and/or End the Session

Congratulations! You have successfully completed a typical SYSNOISE uncoupled analysis sequence.

If you want to go on and try the remaining parts of this Getting Started manual, close the SYSNOISE database using the “File” ⇒ “Close” menu entry. SYSNOISE will prompt you with the pop-up message “Do You Want to Save the Changes of This Model?” Click the “Yes” button to save the model data and the results of the calculations in the database.

Otherwise, you can end the session using the “File” ⇒ “Exit…” menu entry, which opens the following form:

Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the database(s) and then terminate the SYSNOISE session.


6 Getting Started – Numerical Engine Acoustics: Engine Acoustic Signature


A wide range of different analysis sequences has been put in place in SYSNOISE Rev 5.5 using the forward ATV-response calculation. The simplest analysis sequence is a single mono-model acoustic radiation analysis, for which the normal velocity boundary conditions can be generated from structural displacements calculated in any FEA package (e.g. NASTRAN) through the SYSNOISE Generate procedure supporting incompatible meshes.

Other solution sequences involve weakly or one-way coupled vibro-acoustic models, in which a structural model is coupled to the acoustic ATV model. The structural displacements calculated in the structural model are automatically passed onto the acoustic model, where the acoustic response is evaluated by a simple ATV-response. The structural response in this case can be calculated using the SYSNOISE structural response module or through modal superposition using an imported modal basis and corresponding modal responses (MRSPs) from another FEA package.

SYSNOISEAcoustic Model

ATV computation(IBEM, DBEM, MDBEM, FEM)

ATV p

Acoustic responseVn= [φRn] MRSP

p = <ATV> Vn

SYSNOISEStructural Model[φ]

MRSP MRSP [φR] [φRn] u

Structural responseu = [φR] MRSP

Weak

Link

Mode Transfer [φR]Normal Projection[φRn]

MRSP

[φRn][φRn]

The ATV-response calculation scheme

For this last case, an even more efficient acoustic response calculation is available, using the so-called MATV-response. MATVs are the modal counterpart of the ATVs. They express the acoustic transfer in modal coordinates from the radiating structure to a field point and thus show the acoustic contribution from each individual structural mode.

The acoustic response in the field point is obtained by simple recombination of the MATV with the corresponding structural modal responses. Working in modal coordinates results in an important data reduction. It’s clear however that they


are no longer independent of the structural model as they are linked to the structural modal basis. Whenever the structural modal basis changes, e.g. due to structural design modifications, the set of MATVs needs to be re-evaluated.

p (ω) = <MATV(ω)> MRSP(ω)

vn (ω) = [φn] MRSP

microphonep(ω) ?

MATV(ω)

<MATV>=<ATV>[φn][φ]→[φn]

The Modal Acoustic Transfer Vector concept

SYSNOISEAcoustic ModelMATV computation

(IBEM, DBEM, MDBEM, FEM)

MATV p

Acoustic responsep = <MATV> MRSP

SYSNOISEStructural Model

MRSP

Mode Transfer [φR]Normal Projection[φRn]

Weak

Link

MRSP

[φRn]

[φ]

[φRn]

Structural responseu = [φR] MRSP

MRSP [φR] [φRn] u

The MATV-response calculation scheme

See also the Release Notes, section Error! Reference source not found., for more information about ATV concepts.

The Numerical Engine Acoustics process makes use of these ATV or MATV concepts. The process makes possible the calculation of a complete acoustic signature of an engine within a day, for medium size problems (e.g. BEM model of 10000 degrees of freedom) - a calculation that up to now was practically impossible. A typical calculation of an acoustic signature of an engine for an engine speed sweep ranging from 1000 to 6000 RPM in steps of 50 RPM requires several thousands of individual response calculations, since at each RPM case several excitation frequencies need to be considered (depending on the order). The ATV approach limits the computational effort to a fraction of this. In fact, the ATVs are evaluated at fixed frequencies up to say 2000 Hz with a step of e.g. 50 Hz, independently of the different RPM cases, and are then used in a subsequent multi-RPM response calculation where the ATVs (or MATVs) are interpolated at the different excitation frequencies. All the acoustic responses can then be post-processed using a typical waterfall diagram. A further innovation is used to evaluate the acoustic power radiated by the engine. The radiated power can be accurately evaluated based on an ISO3744-1994


procedure, in which the radiated power is based on the acoustic pressure response at a limited set of field points surrounding the radiating body. This procedure is frequently used in laboratory test procedures.

Design evaluation &optimisation(Sound PressureLevel & Sound Power)

RPM

Excitation frequencies

f [Hz]

RPM

F [N] (Modal)AcousticTransferVectors

Multi-RPM acoustic responsesimulationStructural

DesignParameters

The Numerical Engine Acoustics Process

Another efficient usage of the ATV concept with the Numerical Engine Acoustics Process can be seen in a structural design evaluation and optimization loop for structural design parameters. Since the ATVs are completely independent from these design parameters (assuming the exterior radiating envelope retains the same geometry) the acoustic response can be easily evaluated at each design cycle at very low calculation cost. Such an optimization process can of course be driven by LMS Optimus, i.e. Optimus driving NASTRAN and SYSNOISE.

The different Numerical Engine Acoustics processes are schematically laid-out in the previous figure.

(We remark in passing that all processes available in the Numerical Engine Acoustics module are of course applicable to other analogous acoustic radiation problems, i.e. outside the automotive context).

In the example given here, we will run a typical calculation of an acoustic signature of an engine, for an engine speed sweep ranging from 631.72 to 5031.2 RPM with a non-constant RPM step. Forty-five RPM cases in total will be considered. The RPM range has been reduced compared to reality, so as to shorten calculation times and limit database sizes.

At each RPM case several excitation frequencies will be considered, i.e. from 300Hz to 1000 Hz with a step of 10Hz. Note that this frequency range could


vary from one RPM case to another (typically, there will be more frequencies to consider when RPM increases).

Structural modes and a forced response have been computed in an external FE code. The Modes and Modal Participation Factors (in SYSNOISE, called the ‘MRSPs’ : Modal ReSPponses) are available in external SYSNOISE Free format files.

The ATVs will be evaluated at fixed frequencies, from 300 Hz up to 1000 Hz with a step of 70 Hz, independently form the different RPM cases and will then be used in a subsequent multi-RPM response calculation whereby the ATVs will be interpolated at the different excitation frequencies.

6.2 Start SYSNOISE

Although your system manager will probably have defined an alias to start up SYSNOISE, you can always type the complete command string /path/sysnoise/5.6/bin/sysnoise * to launch the program.

* /path refers to the directory tree under which the SYSNOISE software was installed. To have the exact path, refer to the installation procedure or to your system manager

Note: SYSNOISE Rev 5.6 uses dynamic memory allocation such that it is no longer compulsory to define a command option ‘-mX’ to allocate memory at start up. The -mX Option is still valid but now defines an initial maximum memory limit for the session (it can alternatively be set by the MEMORY environment variable)









When creating a model in SYSNOISE Rev 5.5, the first thing you always have to do is to create a new database. This binary database will contain all the information related to your model and is stored in a file with the filename extension “.sdb”, which stands for “SYSNOISE database”.

To create a new database, click with the mouse on the “Create a New Model” button in the Start-Up Window.

Create a database with the name EngStr_small. The full name of the database file has to be entered as well : EngStr_small.sdb. Note that if no name is specified, a default name corresponding to the SYSNOISE database name is used.


Since no other database has been opened so far, this model database will be assigned the Model Number 1.


After the creation of the database, the next important thing is to choose the correct analysis option, i.e. what type of numerical method will be used to analyze the problem. You activate the analysis option form by clicking the “Model” ⇒ “Option” menu entry in the Main Window :

In order to identify the present model as a structural model, set the FEM Frequency Structure analysis option by selecting the “Model” ⇒ “Option” menu entry in the Main Window :


6.5 Import the ‘Surrogate’ Structural Mesh

The structural FE mesh is a surface mesh of the engine. The mesh data (nodal coordinates and element connectivities) is contained in an external SYSNOISE Free Format file named engine_str_comp.fre. Load this mesh file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.

Note: The Free Format is the SYSNOISE proprietary ASCII format. Please refer to the appendices of the SYSNOISE User’s Manual in order to get a complete description of the content of the free formatted files.



This mesh is actually NOT the structural mesh that was used for the structural modes calculation and the forced response.

Indeed, the structural mesh used in a coupled SYSNOISE analysis, must be compatible with the acoustic mesh for those parts (sets of elements) which form the fluid-structure interface. Therefore, the current structural mesh is actually only a “surrogate structure” mesh.

6.6 Transfer the Structural Modes

The structural behavior of the engine will be represented by a structural modal basis. In other words, any deformation of the engine can be written as a linear combination of the deformation shapes of the engine structural modes. This approach is known as modal superposition and is a powerful technique for structural analysis : it reduces the solution of the structural problem to finding the coefficients for the linear combination, the so-called modal responses, abbreviated MRSP.

These structural modal vectors have been computed previously by a structural FEA code and are stored in the form of a SYSNOISE Free Format data file called 44_str_modes.fre.

The Transfer procedure will project these structural modes from the original, incompatible structural mesh onto the surrogate structural mesh using a geometrical interpolation technique.


Select “File ⇒ Import …”, Set the ‘Data Type’ on “Modes Structure” and the ‘Format’ on “Free”

The transfer procedure is achieved by activating the Incompatible Mesh toggle button and clicking the Transfer Options button in the File, Import dialog to open the Transfer dialog:

Enter the file name of the original structural FE mesh, called engine_str_mesh.fre , and select the format ‘Free’. Finally, you have to determine the interpolation parameters between the structural mesh and the ‘surrogate’ structural mesh. Select “Interpolation Algorithm Number” 1 with as “Number of Nodes Used in Interpolation” 4. Observe the output in the Echo Window, which provides feedback about the import process.

You can visualize the deformation shapes of the structural modes by entering the “Postprocess” ⇒ “Deformation…” form : for “Model Mesh [1]”, choose “Structural Modes” and double-click for example on the second structural resonance frequency = 932.786 Hz. This results in a deformed display in the Main Window, as shown below. (This display can also be Animated).


6.7 Project the Structural Modes on the Element Normals

The Projected structural modes will be needed to enable the computation of the Modal Acoustic Transfer Vector (MATV) in the acoustic model.

The structural modes Φ can be projected on the element normals by using the ‘Project’ Dialog box. Go to “Analysis ⇒ Project …”, Select “Modes Structures” in the pull-down menu and click the ‘OK’ button.

The projected modes Φ can be visualized as a color plot by using the “Postprocess ⇒ Colormap …” dialog box.

n

Note: The structural modes can alternatively be projected automatically during the import procedure by activating the 'Project Modes Structure' toggle button. However, this limits the visualization capability, because full 3-d spatial motion is no longer available.


6.8 Import the MRSP’s

The Modal Responses for each RPM case and associated frequency list have to be imported in the structural model.

As already mentioned, these MRSP’s have been computed previously by a structural FEA code and have been stored in the form of a SYSNOISE Free Format data file called mrsp_45LC.fre.

Load this data file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form :

Select “MRSP” in the ‘Data-type’ drop-down list and click 'OK'.

You can then visualize the MRSP’s by using the “Postprocess ⇒ Participation Function …” dialog, if you wish also selecting your preferred Options :


6.9 Save and Close the Database

Although the current structural model will be needed later in the NEA process, it might be useful to close it while we don’t need it, so as to free some memory.

Close the SYSNOISE database using the “File” ⇒ “Close” menu entry. SYSNOISE will prompt you with the pop-up message “Do You Want to Save the Changes of This Model ?” Click the “Yes” button to save the model data and the results of the calculations in the database.

6.10 Create the “Master” Acoustic Model Database

Create a new model database by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar.

Create a database with the name EngAc_atv. The full name of the database file has to be entered as well : EngAc_atv.sdb. This model database will be assigned the Model Number 1.

This acoustic model will contain the ATVs

6.11 Define The Analysis Option

Set the present analysis option on BEM Frequency Indirect Uncoupled by selecting the “Model” ⇒ “Option” menu entry in the Main Window :


6.12 Import The Acoustic Mesh

The mesh data (nodal coordinates and element connectivities) is contained in the external Free Format file named engine_str_comp.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus.

6.13 Check the Element Normal Vectors

For a Boundary Element analysis, it is absolutely essential that the normal vectors of the elements are pointing in a consistent direction. But when the mesh was originally created, the consistency of the normal vectors may have been ignored. Therefore, SYSNOISE offers a “Check Mesh” procedure that will automatically take care of reversing element normal vectors in order to guarantee consistency.

This tool is accessed from the “Geometry” ⇒ “Check Mesh…” menu entry : activate the “Element Normal Vector Correction” toggle button in the form and click “OK” to start the procedure.


6.15 Define A Symmetry Plane We will now define a symmetry plane with respect to the XY planes. This is intended to model the reflection of the acoustic waves on the ground, such as the floor of a semi-anechoic test room. A symmetry condition corresponds to a rigid half-space condition (v=0 normal to the surface) - for instance, an infinite rigid ground surface. The definition of symmetry planes is controlled entirely by the “Geometry, Symmetry … ” dialog:

The plane of symmetry is defined by its position along the perpendicular coordinate axis ; define a symmetry plane at Z= -0.3 by clicking the Symmetry button and entering the value in the corresponding ‘Plane Z’ entry field.

6.16 Create an ‘ISO3744-1994’ Compliant Field Point Mesh

The radiated power can be accurately evaluated based on ISO3744-1994 procedure whereby the radiated power is based on the acoustic pressure response in a limited set of field point surrounding the radiating body.

The ‘ISO3744-1994’ field point mesh creation procedure automatically detects the presence of a symmetry plane, centers the field point mesh on the projected center of gravity, and fixes its radius according to the engine size. The user has the choice between a ‘Coarse’ and a ‘Fine’ field point mesh (with either 10 or 19 points on a hemisphere). Create a ‘Fine’ Field point mesh by Selecting “ Geometry ⇒ Field Point ⇒ ISO 3744-1994 ⇒ Fine Sphere “


6.17 Compute the ATVs

Select the “Analysis” ⇒ “Compute…” menu entry to compute the Acoustic Transfer Vectors from the boundary surfaces to the field points. Select ATV in the ‘Matrix to be Computed’ pull-down menu and set the frequencies for which


you want to compute the ATV. We propose to solve for the following frequency range : from 300 Hz to 1000 Hz with a step of 70 Hz, as this range covers the excitations frequencies associated with the different RPM cases.

Be aware that reducing the frequency step would greatly impact the total calculation time since the ATV computation is the most time-consuming part of the NEA process.

Also, note that the ATVs will be interpolated at the different excitation frequencies in the subsequent multi-RPM response calculation.


Save the acoustic database using the “File” ⇒ “Save” command.

6.19 Create a second Acoustic Model Database

As already mentioned in the introduction, the Modal Acoustic Transfer Vectors (MATVs) are no longer independent from the structural model as they relate to the structural modal basis.

Whenever the structural modal basis changes, e.g. due to structural design modifications, the set of MATVs needs to be re-evaluated.

Hence, it can be worth, for example in an optimization process, to keep an acoustic database containing only the acoustic mesh, field point mesh and the ATVs, therefore being completely independent from the structural modal basis. This is why we called the previous acoustic model the “Master” model.


The new model we are about to create will contain the acoustic mesh, field point mesh and MATVs. This model will therefore be dependent on the structural model.

This special data management is made possible by instructing SYSNOISE to project the ATVs in the relevant model.

First, create a new model database by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar.

Create a database with the name EngAc_matv. The full name of the database file has to be entered as well : EngAc_matv.sdb. This model database will be assigned the Model Number 2.

Then repeat the same steps as for the Master Model, but as just explained, do not compute the ATVs !! The following operations need to be carried out :

• Define The Analysis Option

• Import The Acoustic Mesh

• Check the Element Normal Vectors

• Define Fluid Properties

• Define A Symmetry Plane

• Create a ‘ISO3744-1994’ Compliant Field Point Mesh


6.20 Open the Structural Database

Since the structural databases will be needed in order to define the multi-model Link (in the next operation), it has to be opened again.


6.21 Link the Acoustic and Structural Models

In the NEA process, the acoustic and structural models need to be linked by a so-called “weak-link”. This means that the analysis will consider the one-way coupling between the engine dynamics and its acoustic behavior.

The link defines which parts of the acoustic and structural model are “interfaced”. The interface consists of the contact surface between the two geometrically compatible models, defined by a selection of FEM elements and BEM elements.

The definition of a SYSNOISE link is controlled by the two-step Link Wizard, which is started by selecting the “Model” ⇒ “Link” menu entry.

In the first wizard form, select Model Number 2 (the acoustic model that will contain the MATVs) as the “First Model” and choose all elements to be linked.


In the second wizard form, select Model Number 3 (the structural model) as the “Second Model” and choose all elements to be linked.

Finally, select the “ Fluid-Structure-Weak ” behavior because we want to analyze the one-way fluid-structure interaction between the vibrating engine and the acoustic domain. Click the “Finish” button to build the link and observe the output in the Echo Window, which confirms that the link has been built.

IMPORTANT : The link between two SYSNOISE models has a temporary character, as it is only needed for the solution of the coupled problem. As soon as one of the linked models is closed, the link will be deleted.

6.22 Project the ATVs and Compute the MATV

Using classical contribution vectors, the acoustic response in arbitrary field points can be written as :

( ) ( )[ ] ( ) ( )[ ] ( ) ffATMifvfATMfp nfp αω nΦ⋅==r

where denotes the Acoustic Transfer Matrix formed by the different Acoustic Transfer Vectors and

( )[ fATM ]( ) fα , the structural modal Participation factors

(MRSP’s). From this equation we define the modal contribution Vector Matrix as : ( )[ ]fMATV

( )[ ] ( )[ ] n ΦfATMifMATV ω=

such that the acoustic response in arbitrary field points can be seen as a superposition of modal contribution vectors using the structural mode participation factors as in

( ) ( )[ ] ( ) ffMATVfp fp α=


The “Analysis ⇒ Project …” Dialog box enables you to Project the ATVs on the structural modal basis, yielding the Modal acoustic Transfer Vectors. Note that the Projected structural modes need to be present already in the structural database and that the fluid-structure Link has to exist.

When using a ‘Master model’ approach (ATVs and MATVs are kept in two separate databases, see section 6.19), the user has also to specify the model containing the ATVs.

Activate the “ Specify Model Containing ATVs ” toggle button in the form and select the acoustic model “EngAc_atv” in the drop-down list.

As a result of this procedure, the acoustic database linked to the structural model will contain the MATVs.

6.23 Close the Master Acoustic Model Database

When using Modal Acoustic Transfer Vectors, ATVs are no longer needed to compute the acoustic response in the field points. Therefore, the so-called ‘Master’ acoustic model won’t be needed anymore.

Activate the Master Acoustic Model database by selecting the “File ⇒ Activate…” menu entry, highlight the acoustic model “EngAc_atv” in the corresponding form and click the “Apply” button.


Then Select “File ⇒ Close” to close the active database. Note that the link remains, since Model 1 was not involved in that link.

6.24 Create Compatible Load Cases

In a coupled analysis, with a BEM Indirect model and a FEM Structure model coupled by a link, the numbers of load cases in the two models must be compatible : there must be the same number of cases, with the required boundary conditions or loads in cases with the same numbers.

In Revision 5.4, the user had to create dummy load cases to match ‘real' load cases in the other model. In Revision 5.5, this procedure has completely been automated such that the user does not have to worry anymore about load cases compatibility. This procedure is initiated by selecting “Model ⇒ Load Case ⇒ Compatible ”

6.25 Set the Calculation Parameters

The calculation parameters can be set by selecting "Analysis ⇒ Parameters". First, set the solution parameter for the structural model : Select ‘EngStr_small’ in the “Solution parameters for model : ” drop-down list.

Set the solution space to “MRSP”, toggle the “Save : Displacements ” button to Off (structural displacements won’t be available, but this will save a lot of disk space) and click “Apply”.


Then, set the solution parameter for the acoustic model : Select ‘EngAc_matv’ in the “Solution parameters for model : ” drop-down list.

Set the solution space to “MATV” and specify the interpolation scheme to be used (e.g. ‘Linear’) for frequencies which were not included in the ATVs computation.

In the SYSNOISE context, the term Potentials refers to any acoustic data which is obtained on the acoustic mesh as a result of the analysis, e.g. double-layer potentials in Option BEM Indirect. The term Results refers to any acoustic data which is obtained on the Field Point. The “Save” command controls the saving of Potentials (for all nodes) or Results (for all field points) in


the database, sorted according to increasing frequency. The “Step n” command indicates that data will be saved for every nth analysis frequency.

Here, we will only select the saving of Results with a Step 1, i.e. for all analysis frequencies. Click the “Apply” and “Close” buttons to confirm these settings.

6.26 Solve over a range of RPM

As explained in section 6.22, the acoustic response in the field points can be seen as a superposition of modal contribution vectors using the structural mode participation factors as

( ) ( )[ ] ( ) ffMATVfp fp α=

The ‘Solve’ Procedure will perform this multiplication for all load cases (RPM cases) and associated frequency list.

The “Analysis ⇒ Solve” dialog features a ‘RPM’ field where each RPM case to consider has to be specified. Alternatively, Read (“File ⇒ Read”) the command file ‘rpm_solve.cmd’ that will launch the computation for all RPM cases.

6.27 Store Results

The STORE command allows you to store data (e.g. potentials or results) for a given set of locations, for all frequencies and Load cases.

Go to “Analysis ⇒ Store …”, Specify to Store the ‘Results’ at point 10 (or for ‘All’ points) and check the ‘Store for All Load Cases’ Option. Leave the ‘Frequency’ field blank in order to select all frequencies: if no frequency range is specified, SYSNOISE will automatically select all available frequencies in each loadcase.


6.28 Acoustic Results Visualization

As an example, the Active Power FP Mesh provides a useful way to assess the energy propagating from the radiating object : the following figure shows the Active Power radiated through the complete field point mesh for the first load case (i.e. 631.72 RPM), computed according to the ISO3744-1994 standard :

6.29 Waterfall Diagram

Pressure frequency responses and Radiated Acoustic Power can be displayed as a function of RPM, resulting in a so-called Waterfall diagram. These diagrams can be displayed using LMS GATEWAY (please ask LMS or your local support representative for more information) or any other Post-processing tool, but not directly in SYSNOISE. They can be exported in FREE, IDEAS and USER format. If GATEWAY is used, a session file will be automatically generated and GATEWAY started with the data loaded and the GUI environment properly set.

Go to “Postprocess ⇒ Waterfall Diagram …” and select one point (e.g. point number 10) for which you want to display the pressure waterfall diagram :

Note that new Environment Variables have been put in place so as to define how Waterfall Diagrams are treated :


• WATERFALLTOOL : name of the tool to display waterfall (e.g. GATEWAY)

• WATERFALLPATH : Path of the WATERFALLTOOL

• WATERFALLFORM : format of the waterfall file (e.g. IDEAS for GATEWAY)

• SESSION_FILE : If GATEWAY is used, the session file is automatically generated to load the waterfall data and to display it. If another tool is to be used, this variable has to be set to TRUE and a user routine generating a specific session file has to be defined.

These choices are defined in the ‘WATERFALL’ section of the Environment Variables. Using GATEWAY, the following waterfall diagram will be obtained :

The next figure is a colormap representation of the waterfall diagram, obtained with GATEWAY. Order cuts and structural natural frequencies can be identified:


7 Getting Started - Panel Contribution Analysis (PACA) : Vehicle Interior Noise


Vehicle interior noise has become a major concern in the automotive industry. Noise and vibration from primary sources such as powertrain, brakes, road/tire interaction or wind noise propagate to the body panels. In turn, the body panels vibrate and radiate noise into the car interior. The contributions from the different body panels to the overall radiated noise are not equal and may even have opposite signs. If the sound level increases as the vibration level increases, the contribution is said to be positive. If the sound level decreases as vibration level increases, the contribution is said to be negative. Finally, if the sound level is insensitive to the vibration level, the contribution is said to be neutral. This knowledge is critical for a good NVH refinement. Indeed, if the vibration of negative contribution areas is reduced, the sound level will actually increase.

Panel Acoustic Contribution Analysis (PACA) is a powerful numerical engineering tool to improve sound quality of the vehicle interior. PACA allows for a classification of the (vibrating) body panels according to their contribution to the total sound pressure and provides a good understanding of the contribution mechanisms through color maps, plots, bar charts, vector plots and Nyquist diagrams.

Starting from the geometry of a structure, and vibration shape results, SYSNOISE calculates the panel acoustic contributions

- from one or more panels, or a group of panels

- to selected field points

- at a range of frequencies

- for different load cases…

The application of Panel Acoustic Contribution Analysis is in general targeted at analyzing car interiors. In order to accommodate this type of applications the calculation of ATVs has been implemented in a Multi-Domain Boundary Element and Finite Element analysis sequences.

In this worked example, a typical case of vehicle interior noise is studied. It is shown how the different panels of the car body contribute to the overall noise at the driver's and passenger's ears.


In this case, the multi-domain direct formulation is used to represent the non-homogeneous interior acoustic domain by a set of homogeneous sub-domains that are linked together by a so-called Fluid-Fluid link. Complex frequency dependent material properties can be specified in these different domains, giving an adequate method of modeling the sound absorbing effects of the car seats as a function of the frequency. Effects of acoustic treatment at the different panels of the car body can be modeled by applying appropriate impedance boundary conditions. All quantities can be frequency dependent and defined using tables.

The procedure consists of three main steps. In the first step, the acoustic model needs to be set-up. Setting-up the acoustic model involves importing an acoustic mesh, specifying acoustic material properties, assigning impedance or admittance boundary conditions, creating panels as sets of elements, specifying field points and setting up multi-model links in case of a Multi-Domain Boundary Element model (MDDBEM)

The next step is the computation of the ATVs using the COMPUTE command. The final step consists of dedicated post-processing actions.


1. Start SYSNOISE 2. Create a Model Database 3. Define the Analysis Option 4. Import the Boundary Element Mesh 5. Check (Show) the Element Normal Vectors 6. Define Fluid Properties 7. Create Panels as Sets of Elements 8. Create Field Points 9. Assign Impedance Boundary Conditions 10. Save the Model database 11. Create The Seats Models 12. Link the Acoustic Models 13. Compute the ATV 14. ATV Visualization 15. Rename Sets 16. Create Velocity BC's from FEA Vibration Data 17. Post-processing 18. Close the Database and/or End the Session Each of these steps is described in detail in the following paragraphs.


Create a new model database by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar. This binary SYSNOISE database file will contain all the information related to the model.


Create a database with the name Car_body. The complete filename of the database file has to be entered : Car_body.sdb.


The analysis option form is activated by clicking the “Model” ⇒ “Option” menu entry in the Main Window.

To study the sound radiation in a car compartment using MDDBEM, three models will be set up, and linked together in a subsequent procedure. The analysis option that will be used for each of those models is therefore the following : BEM Frequency Direct Uncoupled Interior Element.


The Boundary Element mesh represents the bounding surfaces of the passenger compartment of a “mono volume” vehicle. Note that the acoustic domain consisting of the seats does not belong to the current acoustic domain. You can load this mesh by selecting the “File” ⇒ “Import…” menu entry in the Main Window :


The mesh forms a simple closed surface and was generated by an external mesh generator program (e.g. ANSYS, HYPERMESH, I-DEAS Master Series, MSC/PATRAN, Pro/MECHANICA or other). The mesh data is contained in an external Free Format file named carbody.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.



For a Boundary Element analysis, it is absolutely essential that the normal vectors of the elements are pointing in a consistent direction. But when the


mesh was originally created, the consistency of the normal vectors may have been ignored. Therefore, SYSNOISE offers a “Check Mesh” procedure that will automatically take care of reversing element normal vectors in order to guarantee consistency.


Note that this mesh is made up by three Sub-domains (three collections of contiguously-connected elements) and that the element normal vector correction will result in a consistent and uniform normal vector orientation for all the elements in an individual sub-domain.

In a further step, we will need to ‘Link’ this model with two other models containing the seats, by specifying the parts which are “interfaced”. Because of the seats and maybe other regions, which have acoustic properties that are different to the main air volume, a Multi-domain BE method will be used.

Therefore, we will already define two sets containing each seat’s elements. These sets can be obtained by activating the Build Sub-domain Set option of the check mesh dialog. This procedure will create a set of elements for each of the sub-domains detected in the active model.

During execution of the “Check Mesh” command, information about the mesh geometry (volume, surface area, normal direction) is printed to the Echo Window. By default, the last created set is displayed, that is one of the seats. Go to the “View” ⇒ “Sets” menu entry and select ‘Unselected All’ to come back to the former view. Press ‘Rescale’ button to center the current view.



First, change your viewing position with respect to the mesh : switch to the predefined XZ view by selecting the “Display” ⇒ “View Point” ⇒ “XZ” menu entry. Then, switch from shaded display to wireframe rendering by clicking the “Wireframe/Shade/Feature” toolbar button. Finally, display the normal vectors on the elements by clicking the “Show/Hide Vectors”

toolbar button.

The normal vectors are represented by blue arrows with the element centroid location as starting point.

You may also want to reduce the arrow size in order to have a clearer view. The arrow length can be controlled in the ‘Vector Plot’ Option of the Graphical options menu (Display ⇒ Graphical Options). For example, by changing the ‘Reference Display Size’ to 0.02 you will obtain the following figure :

You can try other predefined views from the “Display ⇒ View Point” dialog to have a better view of the normal vector arrows.



toolbar button. You can return to the default view by selecting the “Display ⇒ View Point ⇒ “User Default” menu entry.


Select the “Model ⇒ Fluid Properties…” menu entry and enter the acoustic properties for standard air : speed of sound = 340 m/s ; density = 1.225 kg/m3 . In a Boundary Element analysis, the fluid is always assumed to be homogeneous and hence these properties will apply to the whole domain. Therefore, you cannot select a particular set of elements in the Fluid Properties form : this part is inactive or “grayed out”.

Note: The numerical values used in the example refer to the SI International System of units (meter, kilogram, second, Pascal,…).

You are allowed to use any combination of units in SYSNOISE, the only golden rule is to remain consistent in the use of units when defining numerical values. If you have any doubt, do not hesitate to contact your local support representative.

7.7 Create Panels as Sets of Elements

The main interest of a PACA analysis is to determine the contribution of panels at some specific target locations. Therefore, you have to identify the boundary elements which make up the different panels. Note that the definition of the


panels could be done after the main ATV computation, as the contribution analysis is only a special post-processing of its results.

For this purpose, you can use the feature selection tool which upon selection of an element, will select all connected elements for which the angle between the normal vectors of two adjacent elements is less than a preset value, called the Feature Angle (user definable with “Tools ⇒ Environment Variable”).

If you want to remain consistent with the results presented in this manual, let SYSNOISE read the command file define_sets.cmd which automatically creates a choice of sets (go to "File ⇒ Read" and select the relevant file).

These sets can be selected for display in the Main Window through the “View” ⇒ “Sets” menu entry. In the associated form, press ‘Select all’ in order to highlight all the sets and deselect ‘Sub-Domain 000001’ as this is a set containing all the ‘exterior’ elements.


Each set is easily identified in the Main Window by means of a different element shading color for each set (the color legend appears in the upper left part of the window).

To have a better view of the sets, you can always select the predefined view “Display” ⇒ “View Point” ⇒ “Z on Top [front]”, as shown above.

7.8 Create Field points

The aim of the ATV approach is to evaluate the acoustic transfer from the radiating surface to specific field points for a range of frequencies. Therefore, these points have to be specified beforehand. These points are referred as ‘field points’ and are the numerical equivalent of microphones.

As the most interesting results are at the occupants’ ear locations, we will define points at these positions. These isolated points can be created by using the “Geometry ⇒ Field Point ⇒ Point” menu :


The point coordinates are the following :

x y z

Driver’s left ear 1 -0.47 1.02 Driver’s right ear 1 -0.2 1.02 Passenger’s left ear 1 0.2 1.02 Passenger’s left ear 1 0.47 1.02

These isolated points can be displayed by specifying in the Graphical Options (“Display ⇒ Graphical Options ⇒ Object”) to highlight individual points. Select The ‘Field Point Mesh’ Object and check the ‘Highlight Node’ Option.

You should then be able to see the individual points whose location are marked by a small diamonds.


7.9 Assign Impedance Boundary Conditions

The effects of acoustic treatment at the different panels of the car body will be modeled by applying appropriate (complex) impedance boundary conditions.

The following Impedance values are proposed :

Z= 751 - j 6640 Z= 971 + j 8798 Z = 830 + j 3030

Set Name Firewall, hatshelf, floor,engine cap , above windshield

Rearseat Roof, right side, left side, joint rear

Set Number 5, 8, 10, 13, 16 9 4, 11, 12, 17

In order to define and assign these impedance boundary conditions, Go to “Model ⇒ Absorbent Panels”, enter the correct complex impedance value and press the ‘Element Selector’ button in order to select the sets where you want to apply it.


Chose the ‘Set selection’ option for the Selection mode and pick one set and press on ‘Add entry’. Add all sets corresponding to the current impedance value and click on Ok when you’re finished. Eventually, click on Add in the ‘Boundary Definition’ dialog to accept the definition. Repeat the same procedure for each impedance that has to be applied.

The elements where impedance boundary conditions have been defined are colored (the “View ⇒ Boundary Conditions ⇒ Admittance” option is selected automatically). You can also display a colormap of the impedance by activating the “Postprocess ⇒ Colormap” dialog box , selecting the ‘Admittance B.C.’ button in the pull-down menu and pressing ‘Apply’



It is good practice to save the model database at regular intervals. This protects you from accidental corruption of the database, as all SYSNOISE operations are recorded in a temporary database _car_body.sdb, which is copied to the permanent database car_body.sdb when the “Save” command is issued. This command is found in the “File” menu.

7.11 Create the seats models

As already mentioned, we will use the sub-domain sets generated during the check mesh procedure to create the seats meshes by an ‘Export’ command.

Although we could have put the driver’s seat and the passenger’s seat in the same model, we will be put them in two separate models to show how several models can be handled in MDDBEM.

To create the seat meshes, go to “File ⇒ Export”, select ‘Set’ in the ‘Data type’ pull down menu, set the Format on ‘Free’ and pick the set corresponding to the passenger’s seat (i.e. Sub-Domain 000003). Provide also a file name to contain the mesh definition (e.g. passenger.fre)


Do the same for the driver’s seat (i.e. Sub-Domain 000002) (file name driver.fre)

Now, you have to create two new models with the same analysis option, i.e. BEM Frequency Direct Uncoupled Interior Element. The analysis option form is activated by clicking the “Model ⇒ Option” menu entry in the Main Window :

Now the passenger seat mesh (i.e. passenger.fre) has to be imported. Please refer to section 7.4 for a description of the Import procedure for a Free format Mesh File.

In the Direct BEM option, all element normal vectors point must point into the fluid region. Therefore, perform a "Check mesh" Operation as in section 7.5 but check only the option 'Element normal vector correction' (Note that even unchecked, this correction will be activated)


7.12 Define fluid Properties

The seat is given absorbent properties throughout its volume, that is by defining a fluid with appropriate complex sound and complex density. The following foam properties are proposed :

Mass density = ρ = 19 – j 11 Sound speed = c = 66 + j 24

These properties are to be defined in the “Model ⇒ Fluid properties” dialog box.


Save your model and repeat the same procedure for the driver’s seat : Create a new model, import the mesh, correct the element normal directions with check mesh and define fluid material properties.

7.13 Link the Acoustic Models

As already mentioned, a model must be defined for each domain in a MDDBEM analysis. Once created, the models must be linked together using Fluid-Fluid links.

In a MDDBEM analysis, the link operation identifies the elements of the two models that will interact . In this case, these elements are `transparent' and let the acoustic waves pass into the other model. The operation is managed by a two-step Link Wizard, which is started by selecting the “Model ⇒ Link” menu entry.

In the first wizard form, select Model Number 1 (the car body model) as the “First Model” and choose the Set Sub-Domain 000003 (set containing the passenger seat’s elements) by using the ‘Element Selector’ tool.

Instead of typing the model number, you can always click the “Model Selector” button which opens a form allowing you to select the model from a list of open databases. In the list, the databases are listed both by model number and model name, which facilitates selection of the correct model by simply highlighting it.


In the second wizard form, select Model Number 2 (the ‘passenger’ model) as the “Second Model” and type ‘all’ next to the ‘Element’ field, which corresponds to the whole passenger’s seat.

Finally, select the “Fluid-Fluid” behavior because we want to include the fluid-fluid interaction between the two different homogenous fluid. Click the “Finish” button to build the link and observe the output in the Echo Window, which confirms that the link has been built.

IMPORTANT :

The link between two SYSNOISE models has a temporary character, as it is only needed for the solution of the coupled problem. As soon as one of the linked models is closed, the link will be deleted.

Repeat the same procedure to define a Fluid-Fluid link between the ‘car body’ and ‘driver’ model. This time, the Set Sub-Domain 000002 has to be selected , as this is the set representing the driver seat :


In the second wizard form, select Model Number 3 (the ‘driver’ model) as the “Second Model” and type ‘all’ next to the ‘Element’ field, which corresponds to the whole driver’s seat :

You have now completed the pre-processing phase of your SYSNOISE acoustic analysis. You are now ready to start the computation of the Acoustic Transfer vectors.

7.14 Compute the ATVs The COMPUTE command enables the computation of ATVs for the global system (i.e. the linked models). They are stored in a global model, more precisely in a SYSNOISE binary database named ‘global.sdb’, automatically generated. At the end of the calculation, this global model becomes the active model. Note that the global.sdb file disappears as soon as a "Save as" command is issued.

Select the “Analysis” ⇒ “Compute…” menu entry to compute the Acoustic Transfer Vectors from the boundary surfaces to the field points.

Select ATV in the ‘Matrix to be Computed’ pull-down menu and set the frequencies for which you want to compute the ATV, i.e. from 10 to 190 Hz with a linear step of 20 Hz :


Clicking the “Frequency Selector…” button opens a form, in which the analysis frequencies are specified.

Click the “OK” button on the “Compute” form to start the computations.

Please note that you can always interrupt a SYSNOISE analysis run by

pressing the right-most “Interrupt” toolbar button

In SYSNOISE Rev 5.5, a new Environment Variable named ‘MEMORY’ defines the memory threshold that SYSNOISE cannot exceed. If the memory needed to run the analysis is larger than this value, SYSNOISE will prompt for a new memory limit not to exceed :


The default value for the ‘MEMORY’ environment variable is 10 Megawords. This default value can later be modified at any time, by using the “Tools ⇒ Environment Variables” dialog box, or by using the -mX option with the startup command, where X is the maximum memory in Mwords. In batch mode, if more memory is needed SYSNOISE will stop. Therefore, make sure the maximum memory is correctly defined.

The following diagram summarize the process we went through to perform the MDDBEM analysis using ATVs :

Domain1(model 1)

Domain2(model 2) DomainN

(model N)

Link

Compute ATVs

Merge

GlobalModel

7.15 Rename Sets

For consistency reasons (there can be sets having the same name and/or number in the linked databases), the sets names and numbers do not remain the same in the ‘global’ database. The previous set definition can be retrieved by making SYSNOISE read the command file named ‘Rename_sets.cmd’. This will rename and renumber the current sets so as to recover the previous definition. The command file can be read by going to “File ⇒ Read” and selecting the desired command file.


7.16 ATV Visualization

A wide range of post-processing capabilities is available in SYSNOISE to classify the contributions and get a better understanding of the acoustic radiation mechanism.

The dedicated PACA SYSNOISE post-processor enables you to generate a series of different contour plots.

The Acoustic Transfer Vectors can be displayed in various ways : The following ATV plot shows the relative influence of each element of the boundary, on a selected field point (i.e. point number 11) for equal nominal velocities at 90 HZ. By inspecting this figure one can easily grasp the physical meaning of ATVs.

This can be obtained by selecting “Postprocess” ⇒ “Colormap …” and by choosing ‘ATV’ in the pull-down menu. Select a frequency and field point from the list and press the ‘Apply’ button. Instead of selecting a frequency from the list you can also specify a frequency at which the ATV should be interpolated through the frequency Interpolation option. Both Spline and Linear interpolation are available. The spline interpolation can be tuned by the SPLINE_TOLERANCE environment variable (default = 0.0005). The tolerance specifies the threshold for coefficients to be kept in the spline-interpolation formulae. By default ATVs are shown as density-quantities, i.e. scaled to the local element area. For further color map options please refer to the general SYSNOISE User’s manual.


In order to have a better view inside the car compartment , you can instruct SYSNOISE to only display a limited number of sets using the “Display” ⇒ “Graphical Options” menu entry.

Select the “View Port” radio button, toggle the “Display Mesh When Displaying Sets” button to off and click “Apply” and “Close”. This will only show the sets selected for display in the “View” ⇒ “Sets” menu; the other elements remain invisible. Therefore, go to the “View” ⇒ “Sets” menu and select all sets but the ‘car_body’, ‘right side’ and ‘right window’ sets.

You should obtain the following view :


7.17 Create Velocity BCs from FEA Vibration Data

One particular element or panel may be significant in the final response, depending on how it vibrates. So, surface vibration shapes are Imported into SYSNOISE.

This is a standard procedure, with many different options for the interface formats.

The vibration shapes can come from a structural Finite Element model, from a structural response solution inside SYSNOISE, or even from measurements (using special data expansion methods). Please refer to the SYSNOISE User's Manual for more information.

Multiplying the vibration shape vectors by the Acoustic Transfer Vectors gives the actual response due to the vibrations.

Because this is a vector multiplication, not a solution of the BE equations, there is a tremendous benefit in speeding-up the calculation: the result can be ‘almost’ instantaneous! This is one of the main benefits of the ATV approach.


In this example, we will use structural vibration shapes of the car body due to a mechanical point load (e.g. unit force on a mount point) computed by a Nastran Forced Response Analysis over a frequency range from 10 Hz to 190 Hz, with a step of 20 Hz. These response vectors have been stored in an external Free format file named disp_car_body.fre (hence, you won’t need the specific Nastran Interface to achieve this example).

It is important to note that in acoustics, only the normal component of vibration is used because the “in-plane” vibration component (relative to the panel surface) does not generate any sound waves. Hence, we will now automatically convert the vibration vectors into SYSNOISE normal velocity boundary conditions on the acoustic boundary element mesh for the complete frequency range. This conversion process is achieved by the GENERATE wizard, a sequence of dialogs which is started by selecting the “Model” ⇒ “Vibrating Panels” ⇒ “FEA…” menu entry.

7.17.1 Specify location for velocity boundary conditions


The velocity boundary conditions have to be applied on the car body since no excitation is applied on the seats. Therefore, Select the corresponding Element Set by using the Element Selector Tool or enter directly 'Set 1' in the element definition field of the “Step 1” wizard dialog. Click the “Next” button to proceed to the next dialog.

7.17.2 Specify FEA vibration data source

In the “Step 2” wizard dialog, you have to specify from which data file SYSNOISE is to retrieve the vibration data.

The structural response vectors consist of harmonic displacements (six components, i.e. three X,Y,Z translations and three X,Y,Z rotations) stored in a free format file, but these vectors could also be stored in the form of structural velocities or accelerations. Therefore, you have to specify the data type as “Displacements”.

Activate the “Frequency Selector” button and specify the frequency range for which velocity boundary conditions will be generated : from 10 to 190 Hz with a linear step of 20 Hz. Finally, enter the name of the free format file (i.e. disp_car_body.fre) and select the format FREE from the pull-down menu.


7.17.3 Specify structural mesh for FEA vibration data

The structural response vectors have been obtained on a structural Finite Element mesh of the car body. In general, the original FE mesh on which the vibration data has been obtained will be incompatible with the SYSNOISE acoustic Boundary Element mesh because of different node locations, different mesh densities, etc. Therefore, SYSNOISE can interpolate the data from the structural FE mesh to the acoustic BE mesh by means of a geometrical interpolation algorithm between each acoustic node and the surrounding


structural nodes. The current mesh differs from the structural car body mesh only by the seats that are not present in the former. In the “Step 3” wizard dialog, activate the “Use Other File” and chose the carbody.fre mesh. Press then ‘Finish’. This will use the default algorithm and settings but note that they can be modified by clicking again on the 'Next' button. Please refer to the SYSNOISE manual for more information.

7.17.4 Visual check of velocity boundary conditions

SYSNOISE contains tools which allow to check the normal velocity boundary conditions created by the GENERATE sequence. Open the “Postprocess” ⇒ “Color Map…” form. For “Model Mesh [4] “ select the “Velocity BC” entry from the pull-down menu, highlight 50.000 Hz as frequency and set the “Amplitude” filter. Click the “Apply” button to get the following image in the Main Window display :


Click on the “Hide All” toolbar button to “clean” the display and to return to the original display of the complete boundary element mesh.

7.18 Post-processing

7.18.1 Element Contribution

The contributions to a field point from individual elements can be plotted by selecting ‘Element Contribution’ in the ‘Color Map Display’ dialog box.

Note that the computation is carried out 'on the fly', and that no additional data are stored in the database. As further explained below, the real part of the third and fourth normalizations enable you to evaluate the positive or negative contribution of each element to the total sound. A positive contribution means that the sound level increases as the vibration level increases whereas a negative contribution means that the sound level decreases as the vibration level increases.

As an example, the element contribution plot at 30 Hz corresponding to the field point number 11 shows two zones of opposite contribution effect that somehow 'compensate' each other (for unit velocities).


This shows how the different regions of a specific structural vibration shape cause unwanted noise at the receiver’s location, which gives important information about the most effective place for acoustic surface treatment or structural stiffening or damping. More theoretically, the element contribution is obtained by a coefficient by coefficient multiplication of the structural velocity boundary condition vector and the ATV. Indeed, as already pointed out in the introduction, the ATV formulation leads to the following equation :

[ ] bvATVp T=

where the Acoustic Transfer Matrix [ ]ATV is formed by the different Acoustic Transfer Vectors and where the subscript b stands for boundary. This equations can be rewritten as :

[ ] ∑∑==

==ne

ee

Te

ne

ee vATVpp

11

where ne is the total number of elements.

7.18.2 Panel Contribution

a) COLORMAPS

The contribution of a panel is computed by the summation of element contributions over the panel elements:


[ ] ∑∑ ==pe

eT

epe

ec vATVpp

where stands for panel elements. ep

The contributions can be normalized using 4 different schemes (The asterisk (*) stands for the complex conjugate) that can be selected from the ‘Normalization factor’ pull-down menu.

It should be noted that the sound produced by a given panel can be in phase or out of phase with respect to the total sound. When the sound produced by the panel is in phase with respect to the total sound, the real part of normalizations

pppc

∗ and 2p

ppc∗

is positive. Similarly, when the sound produced by the panel

is out of phase with respect to the total sound, the real part of normalizations

pppc

∗ and 2p

ppc∗

is negative.

A positive contribution means that the sound level increases as the vibration level increases whereas a negative contribution means that the sound level decreases as the vibration level increases

A colormap of the panel contribution can be obtained, but as long as we only have isolated field points, we will not see anything (ATVs to a field point mesh need to be created) !

b) CONTRIBUTION DIAGRAMS

SYSNOISE enables the generation of a series of contribution diagrams in the form of bar charts, vector plots or Nyquist diagrams. A new Contribution Function dialog has been designed giving the user full flexibility to generating a wide range of different contribution plots. The contributions are in general dependent on the frequency, the panel and the field point. Therefore, you need to select what data should be used along the x-axis. Once this selection is made the selector for the corresponding list becomes a multi-selector while the selector for the other list is reduced to a single item selector. The old contribution function dialog is still available as an extra entry in the Post-processing dialog. The multi selector supports single picking holding down the Ctrl-key or range picking holding down the Shift-key. By default XY-plots are generated. Vector plots or Nyquist diagrams can be generated selecting the Polar-option.

At a selected frequency, the contributions of all panels can be shown as vectors: this shows relative magnitude and phase (some panels will be out-of-phase and have a canceling effect).


The contribution of one panel to one field point can be shown through a frequency range

The contributions of several panels at one frequency can be shown as a bar chart and positive and negative contributions can easily be identified. It should be noted that the bar-chart representation can be set through the Function Options dialog.


The contributions of several panels can be also shown in the same way, but at several frequencies (‘Nyquist’-type plot). As already mentioned, the contribution of one panel can also be shown as a contour plot across a grid of field points.


Congratulations ! You have successfully completed a typical SYSNOISE PACA MDDBEM analysis sequence.

If you want to go on and try further examples, for example from elsewhere in this Getting Started manual, close the SYSNOISE database using the “File” ⇒ “Close” menu entry. SYSNOISE will prompt you with the pop-up message “Do You Want to Save the Changes of This Model ?” Click the “Yes” button to save the model data and the results of the calculations in the database.


Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the global.sdb database and then terminate the SYSNOISE session.


8 Panel contributions in exterior analysis

This section is intended to point out that panel contribution analysis can be applied not only to interior acoustics problems, but also to any generic exterior radiation problem. In the following example, the acoustic radiation of an engine due to a structural excitation is considered (through the generation of velocity BCs from FEA vibration-data). The ATV technique is used to determine efficiently how the different "panels" of the engine contribute to the overall noise.

As this example is not meant to give a full step by step explanation of the analysis sequence, please refer to the User's manual for more information.

First, open the SYSNOISE database named atvrsp.sdb by using the "File" ⇒ "Open" dialog. This database contains the boundary element mesh, the field point mesh as well as the ATVs from the boundary surfaces to the field points for a set of frequencies (i.e. from 100Hz to 300Hz with a step of 50Hz).

Normal velocity boundary conditions have also already been generated from structural displacements calculated in a FEA package through the SYSNOISE 'Generate' procedure. These excitations have been defined from 100Hz to 300Hz with a step of 50Hz.

All contribution 'Post-processing' capabilities presented in section 7.18 can be used in the current example. As an illustration, the next figure shows the contribution of a panel (corresponding to the set named "front") as a contour plot across the field point mesh at 250 Hz. The following figure shows the contribution of a panel to a specific field point as a Nyquist diagram.

Note that the real part of the contribution along with the fourth normalization

scheme ( 2pc pp

∗) is shown so as to identify the positive or negative contributions,

as explained in section 7.17.3.


9 Getting Started – Inverse Numerical Acoustics : Engine Noise Identification


In the Inverse Numerical Acoustics process the concept of ATVs is used in an ‘inverse’ way (see figure below). The input-output relation is used to calculate back to the radiating surface to derive the normal structural velocity from measured acoustic pressure levels in a grid of field points. The technique can thus be used for acoustic source localization and quantification. In other words, the new Inverse Numerical Acoustics capabilities in SYSNOISE Revision 5.5 give you a tool to identify the likely sources of known noise. The technique is of particular interest when the source is rotating or moving, too light or too hot to be instrumented by accelerometers. In contrast with Near-field Acoustic Holography, limited to simple sources and pressure measurement surfaces, this Inverse method can be used for any arbitrary geometry. (Note: the method is purposely not referred to as the Inverse Boundary Element method since it is based on the more general concept of Acoustic Transfer Vectors which can be calculated by other methods such as the Finite Element method, or even measured). The method is based on inverting the input-output relation between the surface degrees of freedom and the pressure measurement grid by means of Singular Value Decomposition.

p (ω) = <ATV(ω)> vn (ω)

vn (ω) ?

ATV(ω)

[ATV(ω)]-1

microphonep(ω)

The SYSNOISE INVERSE NUMERICAL ACOUSTICS module is based on the use of ATV vectors. The main advantage of this approach lies in the fact that the technique is general and can be used either with an indirect variational boundary element formulation or with a direct collocation element formulation.

Using contribution vectors the pressure at given field points is given by


[ ] bvATMp T= (1)

where the Acoustic Transfer Matrix [ ]ATM is formed by the different Acoustic Transfer Vectors, bv the boundary normal velocity and p is the pressure at the given field points. Generally, p is the unknown, bv is the given and

is calculated using SYSNOISE BEM variational or BEM collocation modules. But for a wide range of problems, known as inverse problems, becomes the unknown and the given, - generally measured). For this class of problems, equation (1) needs to be inverted :

[ ]ATMv

pb

[ ]( ) pATMvb1T

−=

From a pure mathematical point of view this can be exactly done only if [ ] is square and definite. Unfortunately this condition occurs rarely and [ ] is generally rectangular. Furthermore, the conditioning of

ATMATM

[ ]ATM

]

is such that equation (1) leads to inaccurate results. To solve these problems, a singular value decomposition of [ is performed in order to obtain its pseudo-inverse. The singular values below a user-definable threshold value are removed in order to increase the matrix conditioning and therefore, the solution accuracy.

ATM

More precisely, the such that: 1 −iσ 1εσσ <i are set to zero, where the iσ are

the positive singular values of [ ]ATM . The scalar 1 is the threshold value and ε is a user-defined tolerance.

εσ

The accuracy can also be improved by constraining the normal velocity at some boundary points e.g. where the normal velocity is measured or where the boundary is assumed to be rigid.

The Inverse Numerical Acoustic module will be illustrated on an engine noise identification problem. The Inverse module will be used to calculate the surface vibrations which would produce known acoustic results. These known acoustic results are available in a data file and have actually been obtained by a ‘forward’ calculation that is documented in section 9.15. These results must therefore be seen as ‘simulated’ pressure measurements.


1. Create a Model Database

2. Define The Analysis Option

3. Import The Boundary Element Mesh

4. Check the Element Normal Vectors


5. Define Fluid Properties

6. Import Field point mesh

7. Compute ATV

8. Import The 'Inverse' Pressure Data

9. Apply Zero Inverse Velocity on the back elements

10. Find Optimal Regularization Tolerance

11. Inverse Analysis

12. Display The Velocity Distribution

Each of these steps is described in detail in the following sections.


Create again a new model database by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar. This binary SYSNOISE database file will

contain all the information related to your acoustic model : acoustic mesh, acoustic properties, boundary conditions, acoustic results (after the analysis),

etc…

Name the new database Engine_inverse.sdb. Since no other database has been opened so far, this model database will be assigned the Model Number 1.

9.3 Define The Analysis Option

Set the present analysis option on BEM Frequency Indirect Uncoupled by selecting the “Model” ⇒ “Option” menu entry in the Main Window.


9.4 Import The Boundary Element Mesh

Usually, the acoustic mesh on the object is simplified : often, a sort of ‘envelope’ as can be seen below. The SYSNOISE mesh-coarsening tools can be useful when making such a mesh. Even though the ‘enveloping’ mesh, on which the surface data are predicted, is very approximate (compared to the ‘real’ surface geometry) we will see that the pressures re-calculated in the field are very accurate. The mesh data is contained in an external SYSNOISE Free Format file named EngInv.fre. Load this mesh file by selecting the “File” ⇒ “Import” menu entry in the Main Window, which gives access to the “Import” form.

You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.


9.5 Check the Element Normal Vectors

For a Boundary Element analysis, it is absolutely essential that the normal vectors of the elements are pointing in a consistent direction. But when the mesh was originally created, the consistency of the normal vectors may have been ignored.

Therefore, SYSNOISE offers a “Check Mesh” procedure that will automatically take care of reversing element normal vectors in order to guarantee consistency.


This tool is accessed from the “Geometry ⇒ Check Mesh…” menu entry : activate the “Element Normal Vector Correction” toggle button in the form and click “OK” to start the procedure.




9.7 Import Field Point Mesh

We will now create a field point mesh that will simulate measurement grids. Field points are additional points in which you want to evaluate acoustic quantities and are the numerical equivalent of microphone positions.


Although it is possible to create a field point mesh directly in SYSNOISE, using ‘standard’ geometrical shapes such as point, line, plane, sphere, box, cylinder etc, we will import it from an existing file.

Go to “File ⇒ Import” to import the field point mesh. The field point mesh data are contained in an external Free Format file named fpm.fre. Set the data type to ‘Point’ and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog

Note that for SYSNOISE Inverse Numerical Acoustics, the shapes and positions of the measurement grids are arbitrary : they don’t have to ‘conform’ to the surface of the radiating object in any special way, neither are they limited to planes or other special shapes !


9.8 Compute ATVs

As already mentioned, the Inverse Numerical Acoustics module uses the ATVs in an ‘inverse’ way. Therefore, the next step is to compute the model’s Acoustic Transfer Vectors. Select the “Analysis” ⇒ “Compute…” menu entry to compute the Acoustic Transfer Vectors from the boundary surfaces to the field points. Select ATV in the ‘Matrix to be Computed’ pull-down menu and set the frequencies for which you want to compute the ATV, e.g. 900 Hz.

ATVs define the relationships between nominal surface velocities and pressures at the field points

- they are independent of actual vibration shapes

- the calculation involves full BEM solutions, but a smart methodology implemented in SYSNOISE Revision 5.5 gives a big speed-up compared to earlier ‘conventional’ BEM solutions.

Acoustic Transfer Vectors can be displayed (use the “Postprocess ⇒ Colormap“ menu) and can be stored and re-used (to be combined with several sets of pressure measurements in different inverse analyses, for example)

For an Inverse problem, the Acoustic Transfer Vector display can indicate how strongly one part of the surface can contribute to the measured pressure at a microphone location. See for example the following picture where the ATV corresponding to point number 20 (position marked by a square) has been plotted :


9.9 Import the ‘Inverse’ Pressure Data

The pressure results (‘measurements’) for the microphone positions now have to be imposed. The pressure measurements will be simulated by the pressure results obtained in a SYSNOISE radiation analysis that is documented in section 9.15. You can load these pressure results by selecting the “File” ⇒ “Import…” menu entry in the Main Window : Set the ‘data type’ to “Inverse Pressure” and the ‘interface format’ to “Free” by selecting these entries from the respective pull-down menus. You can type the filename, i.e. inverse_pressure.fre directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.

The imported pressure field can then be displayed on the field-point mesh by using the “Postprocess ” ⇒ “ Colormap …” menu (select ‘Inverse Pressure’ in the pull-down menu)


9.10 Apply Zero Velocity on Selected Elements

We will assume that surface velocities are negligible on the lower face of the engine. This can indeed be observed after completing the GENERATE sequence of the radiation analysis documented in section 9.15.

In the inverse calculation, it can be useful to constrain the normal velocity to zero where surfaces velocities are negligible; and/or to use velocity measurements (i.e. specify imposed velocity BC's by using a response function through a table), as it decreases the number of unknowns; it may also improve accuracy. By doing so, we assume this boundary to be rigid.

First, create a Set containing the elements of the lower engine face. Select “Geometry” ⇒ “Sets” ⇒ “Elements Selection …”, give the set a name (i.e. back_elements) and use the feature selection tool to select the desired elements in one action (the view has to be rotated, first).


It is now possible to apply a zero inverse velocity on the nodes of the element set. Select “Model” ⇒ “Inverse Velocity …”. Leave the “value” field on ‘0’ and select ‘Set 1’ in the ‘Where’ field by using the ‘Node selector’ dialog.


9.11 Regularization Analysis

As described in section 9.1, the pseudo-inversion of the Acoustic Transfer Matrix is achieved through the use of Singular Value Decomposition. Nevertheless, singular or ill-conditioned matrices contain null or small singular values. This results in infinite or huge terms in the inverse matrix. Whereas the infinite terms clearly lead to an infinite solution, the huge terms lead to a solution that will be very sensitive to the right hand side variations. Therefore, small errors on the RHS will result in huge errors on the solution. To avoid this problem, the small or null singular values need to be dropped. This process is called truncated singular value decomposition. The singular values are dropped as soon as they are smaller than a relative value called tolerance parameter. The choice of this tolerance is a trade off between regularization and loss of information. Therefore, the critical issue of the method is to correctly select the tolerance parameter such that the errors are sufficiently reduced without causing unacceptable loss of information. This is usually achieved through the use of the so-called L-curve where the norm of the solution is plotted versus the rest. The vertical part of the L-curve corresponds to the solutions obtained using low regularization whereas the horizontal part corresponds to the solutions obtained using strong regularization. Obviously, the optimal regularization can be found at the intersection of the two parts.

To build such regularization curve, select “Analysis”⇒”Regularization…”. Select All points and set the frequency for which you want to compute the L-Curve, i.e. 900Hz.

Once computed, you can visualize the L-Curve, select the “Postprocess” ⇒ “L-Curve” Set the frequency for which you want to compute the L-Curve, i.e. 900Hz, select Amplituded (log) as Y-Axis and place a marker at 0.001. Click “Option”, and enter [-0.005,0.02] as X Range and [1.4,2.2] as Y Range. Click “Apply” to save your settings then “Close” to go back to the “L-Curve Response Function” dialog.


Click “Apply” and the “Curve Window” will pop-up and display the response function as a red curve. When placing a market, the corresponding tolerance parameter is displayed in the “Echo Window”. For this particular example, the position of the marker gives an SVD tolerance of approximately 0.023.

This tolerance value can be directly used in the Inverse Analysis (next section) or can be stored in a table together with optimal values for other frequencies. This table can then be used in the Inverse Analysis to account for the frequency dependency of the optimal tolerance parameter.

9.12 Inverse Analysis

The Acoustic Transfer Vectors are combined with the vectors of field-point pressures to solve the Inverse problem.


Starting from the set of known acoustic results (measured pressures) and an assumed vibrating structure with unknown vibrations, SYSNOISE will calculate the surface vibrations which would produce the known acoustic results. The inverse method solves actually the following question: what vibration or pressure on the object could be causing the results measured on the microphone grids?

Select “Analysis” ⇒ “Inverse …” to define the inverse calculation parameters:

Type ‘All’ in the ‘Points’ Field and set the ‘Frequency’ parameter on 900 Hz. The ‘Interpolate’ pull-down menu enables you to use frequency-interpolation to obtain results at frequencies which were not included in the ATVs computation - which can give another improvement in efficiency.

The ‘Tolerance’ parameter relates to the scalar ε used to withdraw ‘too small’ singular values (accuracy improvement method). Note that this parameter has no influence on the calculation time. Please refer to the introduction for more details on the theoretical background of the method.

Click the ‘Apply’ button to start the inverse calculation. Allow some time for SYSNOISE to complete these calculations.

9.13 Display the Velocity Distribution

Finally, the surface velocities which would cause the measured pressures in the field, can be printed and displayed. Go to ‘Postprocess’ ⇒ ‘Colormap …’ and select ‘Velocity B.C.” in the ‘Color Map Display’ dialog box. You can obtain the following figure :


As already mentioned, the Inverse results were calculated from field-point pressures which actually came from the radiation analysis described in the section 9.15. After completing this ‘optional’ analysis, the surface velocity distribution from the Inverse solution can be compared with the original ‘real’ values :


It is important to note that the meshes are not strictly identical and the left model is therefore an equivalent source. It is also important to point out that the velocity BCs can be exported in Free and Universal formats and re-used in another models.

9.14 Optional Extension : Verification Based on Pressures

9.14.1 Re-calculate Pressure Field (Optional)

The pressure at the field points can be re-calculated for verification purposes, from the surface results predicted by the Inverse solution, and compared to the original field-point values (e.g., measured). The pressures are obtained by a standard Solve and Post-process field points analysis sequence.

Select the “Analysis” ⇒ “Solve…” menu entry to set the frequencies for which you want to compute the acoustic response. In the ‘Frequency’ field enter 900 (HZ).

As a result of the preceding acoustic response analysis, we have obtained the acoustic pressure and acoustic normal velocity distribution on the boundary element surface. The Field Point Post-processing enables you to compute

the acoustic pressure and other acoustic quantities at each field points, from the acoustic data on the BEM surface. Select “Analysis” ⇒ “Process Field

Points…”. In the form, the “Frequency Selection” is automatically set to the


frequency selection used for the last analysis. Select all field points for the “Points Selection” :

Click the “OK” button to start the field point post-processing

The following figure shows, in the right window, the pressures re-calculated from predicted surface data, and in the left window, the original (“inverse”) pressure. Note that the scale is the same for both models.

9.15 Annex : Forward Calculation of Pressures

The optional analysis proposed above, allowed an assessment of the accuracy of the inverse method, by comparing re-computed field-point pressures. This section describes how the pressure data simulating pressure measurements at the field points was obtained. It is not necessary to carry out these steps to perform the Inverse example, unless the file of field-point pressures which is Imported into the Inverse model has been lost or corrupted. The forward calculation is presented here for completeness.


9.15.1 Create a Model Database


Create a database with the name Engine_simulation. The full name of the database file has to be entered as well : Engine_simulation.sdb. This model database will be assigned the Model Number 2.

9.15.2 Define The Analysis Option

The aim of this analysis is to compute the radiation of an engine due to a known vibration vector, so as to obtain pressure results at selected field points. These acoustic results were actually considered as the simulated pressure measurements in the inverse analysis.

We will therefore setup a BEM acoustic model and convert the vibration vector into normal velocity boundary conditions on the acoustic boundary element mesh by using the GENERATE wizard.

Set the present analysis option on BEM Frequency Indirect Uncoupled by selecting the “Model” ⇒ “Option” menu entry in the Main Window :


9.15.3 Import the Boundary Element Mesh

The mesh data (nodal coordinates and element connectivities) is contained in an external Free Format file named Engine.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. Again, the mesh in this example is used as a verification test, but in reality it will often be a ‘real vibrating’ structure.

9.15.4 Check Mesh

Repeat the operations carried out in section 9.5.

9.15.5 Define Fluid Properties



9.15.6 Generate Velocity Boundary Conditions

In this example, we will use a single structural vibration shape of the engine due to a mechanical excitation, computed by a FE code at 900Hz. Note that the same procedure can be carried out for any frequency range. The response vector has been stored in an external Free format file named EngStr_disp_900Hz.fre

It is important to note that in acoustics, only the normal component of vibration is used because the “in-plane” vibration component (relative to the panel surface) does not generate any sound waves. Hence, we will now automatically convert the vibration vectors into SYSNOISE normal velocity boundary conditions on the acoustic boundary element mesh. This conversion process is achieved by the GENERATE wizard, a sequence of dialogs which is started by selecting the “Model” ⇒ “Vibrating Panels” ⇒ “FEA…” menu entry.

The velocity boundary conditions have to be applied on all elements. Therefore, type ‘All’ for the element definition of the “Step 1” wizard dialog. click the “Next” button to proceed to the next dialog.


In the “Step 2” wizard dialog, you have to specify from which data file SYSNOISE has to retrieve the vibration data

The structural response vector consist of harmonic displacements (six components, i.e. three X,Y,Z translations and three X,Y,Z rotations) stored in a free format file, but this vector could also be stored in the form of structural velocities or accelerations. Therefore, you have to specify the data type as “Displacements”. Activate the “Frequency Selector” button and specify the frequency range for which velocity boundary conditions will be generated : 900 Hz. Finally, enter the name of the free format file (i.e. EngStr_disp_900Hz.fre) and select the format ‘Free’ from the pull-down menu.


The structural response vector has been obtained on a structural Finite Element mesh of the engine.

In general, the original FE mesh on which the vibration data has been obtained will be incompatible with the SYSNOISE acoustic Boundary Element mesh because of different node locations, different mesh densities, etc…

Therefore, SYSNOISE enables interpolation of the data from the structural FE mesh to the acoustic BE mesh by means of a geometrical interpolation


algorithm between each acoustic node and the surrounding structural nodes. In this example however, the structural mesh and the acoustic mesh are the same for the sake of simplicity. In the “Step 3” wizard dialog, activate “Use Current Mesh” and Press then ‘Finish’.

You can easily check that the velocity boundaries have been generated by using the “Postprocess ⇒ Colormap” menu.

9.15.7 Import Field Point Mesh


9.15.8 Solve and Field Point Post-processing

Please refer to section 9.14.1.

9.15.9 Export Pressure Results

We will now export the pressure results to a file. Results can be written to a file using an external interface format by selecting the ‘Data Type Results’ in the File, Export dialog. The external file will contain (among others) the pressure values at each field point. Select the ‘Free’ format and give the file a name, i.e. ‘inverse_pressure.fre’


10 Getting Started – Volume Distributed Quadrupoles: Jet Noise problem


The problem of jet noise can be described using Lighthill Analogy [11].

The pressure radiated by a turbulent flow can be expressed as:

yji

jiyji

ij dVyyyxGvvdV

yyyxGTxp

∂∂∂

≈∂∂

∂= ∫∫

ΩΩ

)|()|()(2

0

2 rrrrr ρ

where T is Lighthill tensor that is well defined by the term ij jivv0ρ for subsonic

isentropic and high Reynolds flows. The Green function )|( yxG rr is the free

field Green function given as:

||4)|(

||

yxeyxG

yxik

rrrr

rr

−=

−

π

Using this analogy, the problem is equivalent to the problem of noise propagation in a medium at rest where a distribution of quadrupole sources is present.

nozzle jet Quadrupole distribution

This example is dedicated to the application of the aero-acoustics analogy and therefore the aero-acoustic sources to the simulation of a subsonic jet noise. The flow data that are required for such an application is the flow velocity in the turbulence region. The nozzle of the jet is represented by a cylinder and the source region by a volume where equivalent distributed quadrupoles are generated.

Please be aware that the data used in this example have no physical meaning and are used for the tutorial purposes only. These “dummy” velocity data are


provided in a free format file but real flow velocity data can be imported from the CFD codes: STAR-CD or CFX5.6.

The complete analysis sequence is summarized below:

1. Start SYSNOISE 2. Create a Model Database 3. Define the Analysis Option: Indirect BEM 4. Import the Model Mesh 5. Define Fluid Properties 6. Import the volume for the quadrupoles distribution 7. Generate Distributed quadrupoles 8. Define the Field Point Mesh 9. Incident Field Analysis 10. Post-processing 11. Close the Database and/or End the Session

Each of these steps is described in detail in the following paragraphs.

10.2 Start SYSNOISE

Although your system manager will probably have defined an alias to start up SYSNOISE, you can always type the complete command string /path/sysnoise/5.6/bin/sysnoise -m5* to launch the program.

* /path refers to the directory tree under which the SYSNOISE software was installed. To have the exact path, refer to the installation procedure or to your system manager










Create a database with the name jet. The complete filename of the database file has to be entered : jet.sdb.




The analysis option used is the following : BEM Indirect Variational Uncoupled Unbaffled Frequency.

10.5 Import the Model Mesh

In this example the jet nozzle is represented by a rigid cylinder. You can load this mesh by selecting the “File” ⇒ “Import…” menu entry in the Main Window.

The file containing the mesh data is an external Free Format file called cyl.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.




Select the “Model ⇒ Fluid Properties…” menu entry and enter the acoustic properties for standard air: speed of sound = 340 m/s; density = 1.225 kg/m3. In a Boundary Element analysis, the fluid is always assumed to be homogeneous and hence these properties will apply to the whole domain. Therefore, you cannot select a particular set of elements in the Fluid Properties form : this part is inactive or “grayed out”.




10.7 Import the volume for the quadrupoles distribution

The quadrupoles distribution in the source region simulating the jet noise is represented by a volume containing hexahedral 8 nodes elements. In our example, this volume is a 3D box. You can load this mesh by selecting the “File” ⇒ “Import…” menu entry in the Main Window. The field point mesh is written in an external Free Format file called box.fre. Set the data type to “Point” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.

Remark1: Please note that ONLY hexahedral 8 nodes elements are allowed to support the distributed quadrupole sources.

Remark2: The field point mesh appelation is used only to refer to the sources location.

After clicking the “OK” button, the 3D fieldpoint mesh is read in and displayed in the Main Window :


10.8 Generate the quadrupoles distribution

By selecting the “Model” ⇒ “Sources” ⇒ “Volume Distributed Quadrupoles…” menu entry in the Main Window, you can generate the quadrupoles distribution on the 3D field point mesh.

In “Step 1” of the wizard dialog, define the field elements on which the quadrupole sources will be generated. Set the “Field Element selector” to all and click the “Next>” button.

In the “step 2” wizard dialog, you have to import the velocity data i.e. the strength of the quadrupole sources. These data are provided in frequency domain. Please note that the data can be provided in time domain. In this case an FFT is performed within SYSNOISE to transform it from time to frequency domain.

Set the data type to Frequency and the frequency to 100. The velocity data are stored in an external Free format file called velocity.fre. Set the interface format to “Free”. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog


“Step 3” consists in defining the CFD mesh where the data have been stored. This mesh can be the same as the one supporting the quadrupoles sources. In our case, this mesh is different from the original one. Select “Use Other File” and select the Free format file finebox.fre.

In this 4th step you can define the interpolation parameters used for the interpolation between the CFD mesh and the acoustic mesh:

After clicking the “Finish” button, the quadrupoles distribution is generated. You can then visualize the 6 different components of the Lighthill tensor by using the “Postprocess” ⇒ “Colormap” menu entry.


10.9 Define the Field Point Mesh

You can now create a field point mesh which define the microphones position at which the radiated field will be evaluated by selecting the “Geometry ⇒ Field Point…” menu entry. You can define different geometries. Select for instance the “Cylinder” geometry and set the “Center” to 1 0.1 0.1, the “Normal Vector” to –1 0 0, the “Radius” to 1 1 1, set the “Divide (Section)” and the “Divide (Length)” to 10, and set the “Length” to 3:


10.10 Incident Field Analysis

You can then select the “Analysis ⇒ Incident Field…” menu entry to compute the pressure field radiated from the quadrupoles distribution on the field point mesh.


You can visualize the radiated pressure field from the quadrupoles distribution by selecting the “Postprocess ⇒ Color Map…” menu entry.

By using the “Wireframe/Shade/Feature” toolbar button and defining the graphical options such that:

the results will be displayed in such way:




Otherwise, you can end the session using the “File” ⇒ “Exit…” menu entry, which opens the following form:

Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the jet.sdb database and then terminate the SYSNOISE session.


11 Getting Started – Surface Dipoles Distributed Sources problem – Multi Domain Analysis


Flow-induced noise from an open car sunroof is one of the major noise sources in modern cars and represents one of the problems the engineer is facing at the design stage. The example considered in this section is an illustration of how to use the new aero-acoustics features of SYSNOISE Rev5.6 namely dipoles in this case, to simulate the noise from an idealized sunroof.

Please be aware that the data used in this example are built for the tutorial purposes only and have no physical meaning. These flow pressure data are provided in a free format file, but real flow pressure CFD data can be imported from CFD codes such as STAR-CD, CFX5.6 or FLUENT 6.1.18.

In this example, a distribution of surface dipole sources [12] is created on the car body (exterior and interior) and a Multi-DBEM analysis is performed. Two models are required for this analysis, a DBEM interior representing the car cabin and a DBEM exterior representing the exterior of the car. These two models are linked together and coupled via the opening i.e. the roof. The procedure is summarized below:

Set up an interior model:

1. Start SYSNOISE 2. Create an Interior Model Database 3. Define the Analysis Option: Direct BEM Interior 4. Import the interior Model Mesh 5. Define symmetry planes 6. Check (Show) the Element Normal Vectors 7. Define Fluid Properties 8. Create a Set of Elements (for Dipoles BC and duplicated nodes) 9. Generate the surface distributed dipoles 10. Duplicate Nodes 11. Save the Interior Model database

Set up an exterior model:

12. Create an Exterior Model Database 13. Define the Analysis Option: Direct BEM exterior 14. Import the exterior Model Mesh 15. Check (Show) the Element Normal Vectors


16. Define symmetry planes 17. Create a Set of Elements (for Dipoles BC and duplicated nodes) 18. Generate the surface distributed dipoles 19. Duplicate Nodes 20. Save the Exterior Model database

Link the two models and solve

21. Link the Interior & Exterior Models 22. Solve Analysis 23. Activate the Interior Model 24. Define a field point mesh 25. Process Field Point Analysis 26. Activate the Exterior Model 27. Define a Field Point Mesh 28. Process Field Point Analysis 29. Close the Databases and/or End the Session

These steps are described in details in the following paragraphs.

11.2 Create an Interior Model Database

To solve the problem in the car cabin, an interior model is created. This can be done by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar. This binary SYSNOISE database file will contain all the information related to the model.

Create a database with the name Car_int. The complete filename of the database file has to be entered : Car_int.sdb.




To set the interior model, set the analysis model option to: BEM Frequency Direct Uncoupled Interior Element.


For a Multi-DBEM analysis, a closed boundary element mesh is required. A closed car cabin mesh is provided in a Free format file. To import this mesh go to “File” ⇒ “Import…” menu entry in the Main Window :

and select car_int.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.



11.5 Define Symmetry Planes

The imported mesh represents only half a car cabin. To represent the second half, a symmetry plane can be defined using the “Geometry” ⇒ “Symmetry…” menu entry, set the symmetry plane at Y=0.75. Another symmetry plane can also be defined to take into account the effect of an infinite rigid ground. Set the symmetry at Z=0.

After clicking the “OK” button, the planes are displayed in the Main Window :



toolbar button.


11.7 Define the sets for BC

Three main sets need to be defined. A set of elements where the dipole sources are generated (open_car_int), a set of elements representing the coupling surface that links the interior problem with the exterior (roof) and a last set of nodes (coupling nodes) that need to be duplicated.

To define the different set, you can do it either manually or by reading the command file (set_int.cmd) “File” ⇒ “Read…” menu entry. To do it manually, go to “Geometry” ⇒ “Sets” ⇒ “Elements selection…” menu entry.

Create a first set with the name whole defining the whole mesh and set the number to 1. Set the item type to “Elements” and then type all in the Element Selection Box.


Create a second set that represents the roof. Set the name to roof, the number to 2, the item type to “Elements” and then select the elements 694 To 709 Step 1.

To create the car cabin with the open roof, use the “Geometry” ⇒ “Sets” ⇒ “Operation Between Sets…” menu entry. Set the name to open_car_int, the number to 3, the item type to “Elements”, and define the operation between Set 1 and Set 2: select the set called “whole” using the “Set 1 selection…” menu entry (or type directly 1), select the set called “roof” using the “Set 2 selection…” menu entry (or type directly 2), and set the “Operation” to “Difference”.

To define the “coupling nodes” use “Geometry” ⇒ “Sets” ⇒ “Nodes selection…” menu entry. These nodes are located at the border of the roof. Set the name to


coupling_node, the number to 4, the item type to “Nodes”, and select the nodes 569 To 573 625 627 629 578 To 582.

All the sets defined can be displayed on the Main Windows by selecting the “View” ⇒ “Sets…” menu entry:






11.9 Define the surface distributed dipole source

It is time now to define the sources on the surface of the car cabin. These sources are generated on the set number 3 that is the open_car_int.

Select the “Model ⇒ Sources ⇒ Surface Distributed Dipoles…” menu entry. This opens the Generate Wizard dialog box.

The first step is to define the location of the sources. Select the location using the “Element Selector…” menu entry. Set the Selection Mode to “Set Selection” and select the set called “open_car_int” in the pull-down menu entry, click on “Add Entry” and “OK” button. Then the “Next >” button opens a new window.


In this “Step 2” of the Generate Wizard dialog box, define the pressure results used to generate the surface dipoles distribution. Set the Data Type to “Frequency”, set the Frequency to 100, and select the file called pressure.fre and the Format “Free”. Then click on the “Next >” button to open the next window. Please note that the pressure data can be provided either versus time or frequency. In the case of a time dependent data, an FFT is performed automatically within SYSNOISE.

In “Step 3” of the Generate Wizard dialog, you have to define the CFD mesh on which CFD pressure data are provided. Select “Use Other File” and select the file called car.fre, and the Format “Free”. This file contains the complete mesh of the car (with the interior and the exterior surfaces).


In “Step 4” you can define the interpolation parameters. In this example, the mesh car_int.fre is a subset of the mesh car.fre which contains the interior and the exterior of the car. Therefore, there is one-to-one correspondence between the nodes of the two meshes, except for the exterior part of the car. We can thus choose the “Interpolation Algorithm Number” 1 with a “Number of Nodes Used in Interpolation” of 1 and select a sufficiently small value for the “Maximum Distance to Interpolating Node”, e.g. 0.05 m.

11.10 Define the coupling nodes and elements

In many analysis options, it is necessary to duplicate some of the nodes due to discontinuities in the geometry or in the boundary conditions. For instance, when nodes are shared between elements having surface dipole boundary conditions (known pressure and velocity) and coupling elements (unknown pressure and velocity), it is necessary to duplicates the nodes at the interfaces to insure data consistency.

Select the “Geometry” ⇒ “Duplicate Nodes…” menu entry. Then set “Nodes to be Duplicated” to Set 4 (or select the set called “coupling_nodes” in the “Element Selector…” dialog).

Set “Elements to be Disconnected” to Set 2 (or select the set called “roof” in the “Element Selector…” dialog).


11.11 Save the Interior Model Database


11.12 Create an Exterior Model Database

To solve the problem in the exterior of the car cabin, an exterior model is created. This is done by selecting the “File” ⇒ “New…” menu entry in the Main Window menu bar.

Create a database with the name Car_ext. The complete filename of the database file has to be entered: Car_ext.sdb. This model is assigned automatically the Number 2. This Number is used to identify the model when multiple databases are opened in the same session.


The analysis option dialog is activated by clicking the “Model” ⇒ “Option” menu entry in the Main Window.


11.14 Import the Exterior Model Mesh The mesh data is contained in an external Free Format file called car_ext.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog. After clicking the “OK” button, the Boundary Element mesh is read in and displayed in the Main Window :

11.15 Define Symmetry Planes

As explained in section 11.5 two symmetry planes can be defined at Y=0.75 and Z=0:



This is done the same way as in section 11.6. You can also visualize the element normal vectors:


11.17 Define the Different Sets for the Analysis

Three main sets need to be defined: a set of elements representing the whole mesh where the dipoles are generated, a coupling set of elements that is the sunroof and the coupling nodes as explained in section 11.7.

You can do it either manually as an exercise, following the same procedure as in section .. or read the command file: set_ext.cmd.


See section 11.8

11.19 Define the Surface Distributed Dipole Source

This is done the same way as in section 11.9

11.20 Define the coupling nodes and elements

Follow procedure 11.10

11.21 Save the Exterior Model Database

This is done using the “File” ⇒ “Save” command.


This step consists in linking the interior and the exterior models. The coupling surface in this case is the sunroof. To link the two models, use the “Model ⇒ Link” menu entry and select the models that will be linked and the coupling elements.

In the first wizard dialog, select Model Number 1 (i.e. Car_int) as the “First Model” and set the coupling elements to Set 2 (i.e. the roof) by using the ‘Element Selector’ tool.


In the second wizard dialog, select Model 2 (i.e. Car_ext) as the “Second Model” and set the coupling elements to Set 2 (i.e. the roof).

Select the “Fluid-Fluid” behavior options and click the “Finish” button to build the link and observe the output in the Echo Window, which confirms that the link has been built.

IMPORTANT:


You have now completed the pre-processing phase of your SYSNOISE acoustic analysis. You are now ready to start the computation of the solving analysis.

11.23 Solve Analysis

To solve the problem, select the “Analysis ⇒ Solve…” menu entry and set the frequency to the frequency of analysis i.e. 100.



The dipole boundary conditions can be visualized. To do this, select first the “View ⇒ Objects…” menu entry, and then use the “Postprocess ⇒ Color Map…” to visualize the values on the exterior model mesh:

11.24.1 Field Point Mesh

You can now create a field point mesh, which defines the positions at which the radiated field will be evaluated.

To compute the radiated field inside the car for instance, activate the first model database (the interior model) by using the “File” ⇒ “Activate” menu entry:


Next, create a field point mesh by selecting the “Geometry ⇒ Field Point…” menu entry. This shows a list of the commonly used geometrical shapes for field point meshes: point, line, plane, sphere, box, cylinder… Select for instance the “Plane” geometry and set the coordinates of three points on the plane:

To compute the sound on the field point mesh, select the “Analysis ⇒ Process Field Points…” menu entry. Set the frequency to the frequency of analysis i.e. 100 and select all the points of the field point mesh.

As a result of the analysis, you can visualize the radiated pressure on the field point mesh by using the “Postprocess ⇒ Color Map…” menu entry.


You can also compute the radiated field outside the car, by activating the second model database (the exterior model). Then, import the field point mesh by selecting the “File ⇒ Import…” menu entry and select the file plane.fre with the format “Free” and set the data type to “Point”:

Next process the field points by selecting the “Analysis ⇒ Process Field Points…” menu entry. Set the frequency to 100 and select all the points of the field point mesh.


As a result of the analysis, you can visualize the radiated pressure on the field point mesh by using the “Postprocess ⇒ Color Map…” menu entry.

You can change the graphical options to get the same minimal and maximal values for the SPL inside and outside the car cabin.


11.25 Close the Databases and/or End the Session

Congratulations ! You have successfully completed a typical SYSNOISE Multi-DBEM analysis sequence.


Otherwise, you can end the session using the “File” ⇒ “Exit…” menu entry, which opens the following dialog :

Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the car.sdb database and then terminate the SYSNOISE session.


12 Getting Started – Fan Source problem – Multi Domain Analysis


12.1.1 Noise from a rotor

The noise produced by a rotating machine has two main characteristics [13,14]:

- The broadband noise

- The discrete frequency noise

The broadband noise from a rotor is due to random loading forces on the blades, which are induced by the absorption of atmospheric turbulence. Its study requires the use of statistic tools and is not considered in SYSNOISE Rev5.6.

The discrete frequency noise is due to the periodic interaction of the incoming air with the blades of the rotor. The noise is at the blade passing frequencies and harmonics (BPFH) and can be decomposed into three main parts:

- the thickness noise or the noise due to the air displacement induced by the rotation of each blade.

- the loading noise or the noise due to the interaction of flow with the blades. This represents the noise due to the loading forces (lift and drag) on the blades.

- the noise due to vortex shedding in the wake of the rotor.

A mathematical expression of the radiated pressure from a rotor can be written by using the Ffowcs-Williams & Hawkings formulation [Ffowcs]. This is simply an application of the Lighthill analogy to the case of flow induced noise in the presence of rotating rigid bodies.

In this formulation, all the sound sources are put into the right-hand side of the wave equation. Its first term is a quadrupole term and corresponds to the noise due to vortex shedding in the wake of the rotor. The second term is a dipole and corresponds to the loading noise. The third term is a monopole and represents the thickness noise. As we consider only rotating machines with small blades, which represent the major applications in the automotive and home appliance sector, the thickness noise can be neglected when compared to the loading


noise. In a same way, the quadrupole term is negligible compared to the dipole term.

By using the mathematical derivation from Roger [14], the pressure radiated by a rotor is expressed as an infinite sum of characteristic free field radiation modes (obtained by each harmonic of the Fourier series of the total force on a compact blade segment, weighted by Bessel functions).

12.1.2 Rotor/Stator interaction

Flow

RROOTTOORR

Flow

SSTTAATTOORR SSTTAATTOORRRROOTTOORR

When a stator is present, two configurations are possible: either the rotor is placed downstrean of the stator (this is called the stator-rotor interaction or the inlet guide vane case), either it is placed in between the flow and the stator (this is called the rotor-stator interaction or the outlet guide vane case).

Two noise mechanisms occur:

- the downstream element creates flow disturbances and the perturbations can move upstream towards the upstream element of the fan. This is called the potential-interaction noise.

- The vortices shed in the wake of the upstream element are travelling downstream and interact with the blades of the downstream element. This is called the wake-interaction noise.

The potential-interaction noise is generally negligible compared with the wake-interaction noise and it is therefore not considered here.

NB: Each of these configurations (rotor only, inlet or outlet guide vane case) are represented in SYSNOISE either as a single fan source or a set of fans sources located at the center of the fan

12.1.3 Description of the example

In this worked example, the axial fan consists of a rotor, a stator and a duct. The rotor and the stator have ten blades. The duct has an internal radius of 100mm and a total length of 1000mm. The fan is placed 300mm from the inlet


of the duct. The blade radius is 90mm at the shroud. The rotating speed of the rotor blades is 314.159 rad/s (i.e. 3000 rpm).

Axial fan

inlet outlet

flow

L = 1000mm

L1 = 300mm

In this example, the multi-DBEM analysis is used to model the noise emanating from the ducted Rotor/Stator source. Two models are required for such analysis: an interior model, representing the interior of the duct and the Rotor/Stator source and an exterior domain. The two models are linked and coupled via the coupling surfaces that are the inlet and outlet of the duct. As a first step, the Rotor/Stator is represented as a single fan source. A distribution of sources is presented

The complete analysis sequence consists of the following steps:

Set up an interior model:

1. Start SYSNOISE 2. Create an Interior Model Database 3. Define the Analysis Option: Direct BEM Interior 4. Import the Model Mesh 5. Check (Show) the Element Normal Vectors 6. Define Fluid Properties 7. Create a Set of Elements 8. Import Table file containing the forces on the blade 9. Define the Fan Source 10. Save the Interior Model database

Set up an exterior model:

11. Create an Exterior Model Database 12. Define the Analysis Option: Direct BEM exterior 13. Import the Model Mesh 14. Check (Show) the Element Normal Vectors 15. Create a Set of Elements 16. Save the Exterior Model database

Link the two models and solve:

17. Link the Interior & Exterior Models 18. Activate the Interior Model


19. Solve harmonic 1 2 3 20. Activate the Exterior Model 21. Define the Field Point Mesh 22. Process Field Point Analysis 23. Post-processing 24. Close the Databases and/or End the Session

Each of these steps is described in detail in the following paragraphs.

12.2 Create an Interior Model Database


Create a database with the name Fan_int. The complete filename of the database file has to be entered : Fan_int.sdb.



To study the sound radiation from a fan source using Multi-DBEM, two models are set up, and linked together in a subsequent procedure. The analysis option used for the first of those models is the following : BEM Frequency Direct Uncoupled Interior Element.



In this example, a ducted fan radiating from the inlet and outlet of a rigid cylinder is considered. The duct mesh is closed as required for a DBEM analysis. You can load this mesh by selecting the “File” ⇒ “Import…” menu entry in the Main Window :

The mesh data is stored in an external Free Format file called cylinder.fre. Set the data type to “Mesh” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog.








toolbar button.



12.6 Define sets

A set needs to be defined for the analysis. This set is the coupling surface of the interior and exterior models and represent the top and the bottom of the duct.

The set definition tool is accessed from the “Geometry” ⇒ “Sets” ⇒ “Elements selection…” menu entry. Create the first set with the name radiating_BC, the number 1. Set the item type to “Elements” and then select the top and bottom

faces of the cylinder by using the buttons and






12.8 Define the fan source

A fan source in SYSNOISE Rev5.6 can be either a discrete source or a distribution of discrete source.

For both types of sources, the forces on a blade of reference is required from the CFD computations.


In the case of the discrete fan source, the resulting force is applied at one point of the blade. The representation of the fan by a single fan source is a good approximation the size of the blades are small compared to the wavelength (compact source approximation).

Integration of the pressure fluctuations

over the surface

Resulting force on the

blade

When the size of the blade is very large and the fan cannot be considered as a compact source, the blade can be subdivided in a set of sub-segments where each sub-segment can be replaced by an equivalent source. In this case, the forces exerted by each segment are needed to define the set of sources [Roger].

Integration of the pressure fluctuations

over the surface

Resulting forces on

the different segments of

the blade

X Y

Z

In the two following sections, the definition of both types of sources, i.e. discrete or distributed, is considered. You may choose the type of source you want to define in your analysis: follow either the steps described in section 12.8.1 (if you want to define a discrete fan source) or the steps described in section 12.8.2 (if you want to define a generate fan source).


12.8.1 Discrete Fan Source

Before defining a discrete fan source, the table containing the force on the blade (inlet case) or the vane (outlet case) has to be imported. Select the “File ⇒ Import…” menu entry. The force data is contained in an external Free Format file called “data_freq.fre”. Set the data type to “Table” and the interface format to “Free” by selecting these entries from the respective pull-down menus. You can type the filename directly in the entry field, or alternatively click the “FileBox” button to select the filename from the File Selector dialog. Please note that the forces data can also be provided as a function of time, in this case an FFT is required.

Next select the “Model ⇒ Sources ⇒ Discrete Sources…” menu entry and set the source type to “Fan”. Give a name to the fan source: “fan” e.g. SYSNOISE Rev 5.6 gives you the possibility to compute the radiated sound from either a rotor-stator interaction (outlet configuration), either a stator-rotor interaction (inlet configuration), or a rotor without the presence of a stator.

In our example, the fan consists of a rotor and a stator placed upstream of the rotor. Select therefore the “Stator+Rotor (Stator at Inlet)” configuration. Then, set the “Number of Rotor Blades” to 10, the “Number of Stator Vanes” to 10, and the “Rotation Speed” to 3000 rpm. The Blade Loading Force table file has been imported in a previous step. The file contains three tables, one for each component of the force (the radial component, the tangential component and the axial component). In the discrete fan source, only one force is given at one position on the blade. Set this position “At radius” to 0.06 in the “Source Definition” window. This position is given relatively to the center of the rotor. Then, set the “Table number” to “1 2 3”. These numbers correspond to the radial, tangential and axial components of the force, respectively. Finally, set the “Rotor location” to the vector 0 0 0 and the “Rotor Direction” to the vector 0 0 1.


You can visualize the fan source in the duct (cylinder) by clicking the “Wireframe/Shade/Feature” toolbar button.

As explained above, when the fan has large size blades and cannot be approximated by a single source, the blade reference is subdivided in sub-segments and the forces are computed on each sub-segment. Next, based on these data, a set of sources corresponding to each sub-segment is defined.


12.8.2 Generate Fan Source

Select the “Model ⇒ Sources ⇒ Generate Fan Source” menu entry. As mentioned in the previous section, Sysnoise gives the possibility to compute the radiated sound from either a rotor-stator interaction (outlet configuration), either a stator-rotor interaction (inlet configuration), or a rotor without stator.

In our example, the fan consists of a rotor and a stator placed upstream of the rotor. Select therefore the “Stator+Rotor (Stator at Inlet)” configuration. Then, set the “Number of Rotor Blades” to 10, the “Number of Stator Vanes” to 10, and the “Rotation Speed” to 3000 rpm. Then, set the “Source location” to the vector 0 0 0 and the “Source Direction” to the vector 0 0 1.

The surface of the rotor blade is divided here into three portions (or segments). The Blade Loading Force acting on each portion of the blade is stored in a free-format file called ‘gen_data_freq.fre’. The file contains three tables per segment. In total, the file contains therefore nine tables: one for each component of the force acting on each segment. The position of the force on the blade is given in another file. In the example, the file is a free format file called ‘line.fre’ and contains the positions (in the reference frame XYZ) of the three forces.

You can visualize the fan source in the duct (cylinder) by clicking the “Wireframe/Shade/Feature” toolbar button.




12.10 Create an exterior model database


Create a database with the name Fan_ext. The complete filename of the database file has to be entered : Fan_ext.sdb (see section 3.2). Since this is the second database to be opened in this SYSNOISE session, it will be assigned the Model Number 2 automatically. This Model Number is used to identify different models when multiple databases are opened in the same session.





Please see section 12.4


This can be done the same way as described in section 3.5.

You can also visualize the element normal vectors:

12.14 Define a set

Define the coupling set an explained in section 12.6



Please see section 12.7




This step consists in linking the interior and the exterior models. The coupling surface in this case is the inlet/outlet of the duct. To link the two models, use the “Model ⇒ Link” menu entry and select the models that will be linked and the coupling elements.

In the first wizard dialog, select Model Number 1 (the interior model i.e. Fan_int) as the “First Model” and set the coupling elements to Set 1 (inlet/outlet of the duct) by using the ‘Element Selector’ tool.

In the second wizard dialog, select Model Number 2 (the exterior model i.e. Fan_ext) as the “Second Model” and set the coupling elements to Set 1

Finally, select the “Fluid-Fluid” behavior option.

IMPORTANT :



You have now completed the pre-processing phase of your SYSNOISE acoustic analysis. You are now ready to start the computation of the solving analysis.

12.18 Solve at harmonics of the BPF

First activate the first model database (the interior model) by using the “File ⇒ Activate…” menu entry and selecting the interior model.

You can then select the “Analysis ⇒ Solve…” menu entry to set the harmonics of the Blade Passing Frequency at which you want to perform the analysis. You can for example compute the radiated acoustical field at the first harmonic (“Harmonic” set to 1) or at the two first harmonics (“Harmonic” set to 1 2). The analysis may also be done by setting the frequency rather than the harmonic number.


12.19.1 Model mesh

As a result of this acoustical analysis, the acoustical pressure on the surface of the boundary element mesh (the cylinder) and the acoustic normal velocity. You can visualize the results: first select one of the two model meshes by using the “View ⇒ Objects…” menu entry, and then use the “Postprocess ⇒ Color Map…” to visualize the surface pressure on the model mesh:


You can also visualize the acoustic modes propagating in the duct by clicking the “Animation…” button in the “Color Map Display” window.

12.19.2 Field Point Mesh

You can now create a field point mesh, which define the point positions at which the radiated field will be evaluate.

First, activate the second model database (the exterior model). Then, create a field point mesh by selecting the “Geometry ⇒ Field Point…” menu entry. This shows a list of the commonly used geometrical shapes for field point meshes: point, line, plane, sphere, box, cylinder… Select for instance the “Sphere” geometry and set the “Sphere Radius” to 1, the “Division Factor” to 10, and the position at the vector 0 0 0.

Then, do the process field points analysis by selecting the “Analysis ⇒ Process Field Points…” menu entry. You have to set the frequency corresponding to the


harmonics you chose in the solve process (section 3.18). Compute for instance the radiated sound pressure level at the frequency corresponding to the first harmonic: set the frequency to 500 and select all the points of the field point mesh.

As a result of the analysis, you can visualize the radiated pressure on the field point mesh by using the the “Postprocess ⇒ Color Map…” menu entry.

For the discrete fan source case you obtain:

For the generated fan sources you obtain:





Activate the “Save the Model Changes” toggle button and click the “OK” button. This will save the model data and the results of the calculations into the fan.sdb database and then terminate the SYSNOISE session.


References

[1] F. Gerard, M. Tournour, N. El Masri, L. Cremers, M. Felice, A. Selmane, “Numerical Modeling of Engine Noise Radiation Through the Use of Acoustic Transfer Vectors - A Case Study”, Proc and exposition report, SAE Noise and Vibration Conference. April 30 – May 3, 2001, Traverse City, Michigan, USA M.

[2] Tournour, L. Cremers, P. Guisset, F. Augusztinovicz, F. Marki, “Inverse numerical acoustics based on acoustic transfer vectors”, Proc. ICSV-7, Garmisch-Partenkirchen, Germany, Jul. 4-7 2000.

[3] F. Augusztinovicz and M. Tournour, “Reconstruction of source strength distribution by inverting the boundary element method”, In "Boundary Elements in Acoustics. Advances and Applications", ed. O. von Estorff, WIT press, 2000.

[4] M. Tournour, P. Brux, P. Mas, X. Wang, Colin McCulloch, Philippe Vignassa, “Inverse Numerical Acoustics of a Truck Engine”, Proc and exposition report, SAE Noise and Vibration Conference. May , 2003, Traverse City, Michigan, USA

[5] F. Gerard, M. Tournour, N. El Masri, L. Cremers, M. Felice, A. Selmane, “Acoustic Transfer Vectors for Numerical Modeling of Engine”, Sound and Vibration, pp20-25, July 2002.

[6] L. Cremers, M. Tournour and C.F. McCulloch, “Panel Acoustic Contribution Analysis based on Acoustic Transfer Vectors”, Proc. of The 2001 International Congress and Exhibition on Noise Control Engineering. The Hague, The Netherlands, 2001 August 27-30.

[7] Brezinski C. “Padé type approximation and general orthogonal polynomials”. ISNM Vol. 50, Birkhauser Verlag, Basel, 1980

[8] M. Magolu monga Made. “Incomplete factorization-based preconditioning for solving the Helmholtz equation”. Int. J. Num. Methods Eng., 50:1077-1101, 2001.

[9] M. Magolu monga Made and H.A. van der Vorst. “Parallel incomplete factorizations with pseudo-overlapped subdomains”. Parallel Computing, 27:989-1008, 2001.

[10] A. de La Bourdonnaye, C. Farhat, A. Macedo, F. Magoulès and F.-X. Roux. “A non-overlapping domain decomposition method for the exterior Helmholtz problem”, Contemporary Mathematics, 218:42-66, 1998.

[11] Lighthill, M.J. (1952), “On Sound Generated Aerodynamically: Part I: General Theory”, Proc. Roy. Soc., A211, 564-587.

[12] Curle, N. (1955). “The influence of solid boundaries upon aerodynamic sound”, Proc. Roy. Soc. Lond., A23, 505-514.

[13] Ffowcs Williams, J.E. and Hawkings, D.L. (1969), “Sound Generation by Turbulence and Surfaces in Arbitrary Motion”, Phil. Trans. Roy. Soc., A, Vol. 264, No. 1151, pp.321-344.

[14] Roger, M. (2000).”The acoustic Analogy some Theoretical Background”, VKI-LS-2000-02, Noise in Turbomachines, Von Karman Institute for Fluid Dynamics.

[15] Z. El Hachemi, R. Hallez and al. “Application of Aero-acoustic Analogy to Automotive Industry”. JSAE 2002



We have been very happy to guide you across this getting started manual and help you understand and explore SYSNOISE standard and new features. We hope this has been useful to you and has given you already a good overview on the functionalities of our software.

Please do not hesitate to contact LMS International or your local support offices should you have any request or need any further information. We will be glad to answer your questions and provide you with all the help you might need.

Looking forward serving you and making you happy with our solutions …

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