autougm03 acoustics

Acoustics Modeling: Sandeep SovaniAUTO UGM 2003 Confidential

Acoustics Modeling with FLUENT

Sandeep Sovani, Ph.D.Technical Support Engineer, Automotive Team

June 5th, 2003


Outline• Aeroacoustics Background

– Basics– Simulation Approaches

• Simulation Guide– Computational Aeroacoustics (CAA)– Ffowcs-Williams Hawkins Model– Fluent – Sysnoise Coupling

• Examples• Future Work• Summary

Outline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11

Simulation GuideCAA 12F-W H 13 14 1516 17 18 19 2021 22 23 24 25Sysnoise 26

Examples27 28 29 30 3132 33 34 35 3637 38 39 40 4142 43 44 45Future Work 46Summary 47


Aeroacoustics: BasicsOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




Aeroacoustics: BasicsOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11



SoundFlow

Acoustic Medium Receiver

Source


Aeroacoustics: BasicsThree types of acoustic sources:

Flow

m = m(t)

Flow

psurface = psurface(t) Turbulent Stresses

Monopole Dipole Quadrapole

Acoustic ~ Vel4Power



Flow





Aeroacoustics: BasicsOutputs Desired from Experimentation/Analysis:

• Source Strengths– Source Ranking

• Frequency Spectrum– At observer

• Directivity• Propagation





Aeroacoustics: Simulation Approaches• Source characteristics are governed by

Navier–Stokes equations– Time varying mass-flowrate (monopole)– Surface pressure fluctuations (dipole)– Turbulent stresses (quadrapole)

• Sound propagation is governed by wave equation

Source

Receiver

p’(t)





Aeroacoustics: Simulation Approaches• Wave equation is a special case of Navier-

Stokes equations

• CFD solves the Navier-Stokes equations • In theory, sound generation as well as

propagation can be simulated simply by – a transient CFD simulation

• Domain spanning from sources to receivers– monitor pressure at the receiver locations as function

of time• However, there are several practical problems





Practical problems in using CAA (Direct CFD)• 1] Frequency range (20 Hz ~ 20,000 Hz)

– Acoustic timescales are often orders of magnitude greater than turbulence timescales

– Simulation needs to be run for long real time with a small timesteps, i.e. for large no. of timesteps

• 2] Radiation to Far Field– Domain needs to extend from source to receiver– Large mesh sizes for far-field sound problems

e.g. aircraft noise heard on the ground

Aeroacoustics: Simulation ApproachesOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




• 3] Acoustic Pressure Magnitude– Magnitude of the acoustic pressure is much less than

the hydrodynamic pressure– Necessitates use of very high order discretization

schemes (5th – 6th order)

Aeroacoustics: Simulation Approaches

0

20

40

60

80

100

120

140

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02Pressure (Pascal)

SPL

(dB

)

patm ~ 1E+5 Pa





• Option 1 – Comprehensive Transient CFD Analysis– Computational Aeroacoustics (CAA)

• Option 2 – Couple CFD with Wave Equation Solver/ BEM code

– Simulate region around the source with CFD– Provide CFD pressure, velocity data as

boundary conditions to a wave equation solver/BEM code• Option 3 – Acoustic Modeling

– Simulate region around the source with CFD– Propagate sound to receiver with Analytical Models

• Option 4 – Acoustics Estimation from Local Turbulence Scales

– Use correlations that relate local source strength to local turbulence scales

Decreasing computational effort

Decreasingaccuracy

Aeroacoustics: Simulation ApproachesOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




Aeroacoustics: Simulation Approaches

LimitedGoodGoodGoodAccuracy

Steady State

TransientTransientTransientSolution Scheme

NoNoNoYesCan account for effect of sound on flow

NoNoYesNoCan propagate sound through shells

NoNoYesYesCan account for reflection

LeastModerateModerateMostExpensive computations

Option 4Turbulence Correlation

Option 3Acoustic Modeling

Option 2Coupled

CFD/BEM

Option 1CAA

Features & Limitations





Simulation Guide: CAA

D. Hendriana, S. Sovani, and M. Schiemann, “On Simulating Passenger Car Side Window Buffeting,” SAE 2003-01-1316, 2003.

• CAA (Computational Aeroacoustics)– Currently usable only for

• Near field acoustics• Low frequencies

– Useful where “hydrodynamicnoise” dominates

• Implementation:– mesh edge length =

length scale of turbulent eddieswhose timescale is 1/(max frequency)

– Time step = 1/(max frequency)/10– Run simulation for

total real time = (1/(min frequency))*10• Monitor static pressure at microphone

5





Simulation Guide: Ffowcs-Williams Hawkins

• Based on a two step approach– Simulate transient flow field accurately only

around sources– Propagate noise from source to receiver via

analytical solution of wave equation • developed by Ffowcs-Williams and Hawkins, 1969

Source Region

Navier-Stokes Equation

Acoustic Receiver

Wave EquationF-W H





Simulation Guide: Ffowcs-Williams Hawkins• Advantages

– Need CFD solution only around source– Less expense/improved accuracy

• Disadvantages– Can’t account for reflection– Can’t account for backward effect of sound on flow

• Potential Automotive Applications– Wind Noise

• Side view mirror, Wipers,Rain gutter, Cavity noise

– HVAC Duct Noise– Muffler Noise propagation





Simulation Guide: Ffowcs-Williams Hawkins• Usage

– 1] Setup mesh/turbulence-models/solver-settingsfor an accurate CFD solution around sources

• Same restriction for spatial and temporal resolution as for CAA

• mesh edge length = length scale of turbulent eddieswhose timescale is 1/(max frequency)

• Time step = 1/(max frequency)/105






– 2] Select acoustic source surfaces• Source surfaces can be impermeable (walls)

or permeable (e.g. interiors)

Receiver

Wall Source Surface

Interior Source Surface

Receiver

Duct






– 3] Select whether soundcalculation should bedone “on the fly”

• Extract AcousticsSignals Simultaneously

– If Write Source DataFiles is selectedp, u, v, w, ρ data willbe written out to filesafter every few timesteps





Simulation Guide: Ffowcs-Williams HawkinsTransient

calculations(URANS, LES, DES)

ρ, u,v,w,p onemission surfaces

Read & Compute

sound pressureSPLPSD

On-the-fly Sound Calculation

Save source data






– 4] Specify receiver points• Before running transient simulation for “on the fly” option• Before or after running transient simulation for

“save source data files” option• Receivers can be inside or outside the CFD mesh





Simulation Guide: Ffowcs-Williams HawkinsUsage

– 5] Run transient simulation• For total real time = (1/(min frequency))*10

– 6] If using “write source data files” option

• Execute “Read andCompute Sound”

• p, u, v, w, ρ data is read from stored files

• Ffowcs-Williams Hawkins integral is performed

• Sound pressure vs. time data is written out for each receiverin a separate output file





Simulation Guide: Ffowcs-Williams Hawkins7] Perform FFT of receiver sound pressure signal

to obtain PSD or SPL spectrum

FFT Utility• FFT utility is available for general analysis of unsteady data• Features

– Plot and pruning utility• Enables users to inspect and select signal

– Multiple choices of window functions • Hamming• Hanning• Barlett• Blackman





FFT Utility: Usage1. Read in plot file containing

data2. Apply “pruning” to remove

unwanted portions of the data set

3. Select Window option and x-y axes functions

4. Plot the FFT (Can optionally write FFT data to file)

Simulation Guide: Ffowcs-Williams HawkinsOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




Original Data Pruned Data

Simulation Guide: Ffowcs-Williams Hawkins


Simulation Guide: Ffowcs-Williams HawkinsOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




Simulation Guide: Ffowcs-Williams Hawkins• More information:

– Usage: Fluent 6.1 Manual– Implementation of F-W H model:

Kim S,-E., Dai Y., Koutsavdis E.K., Sovani S.D., KadamN.A., Ravuri M.R., “A Versatile Implementation of Acoustic Analogy Based Noise Prediction Method in a General Purpose CFD Code,” AIAA-2003-3202 (2003)

– Theory:Ffowcs Williams J.E. and Hawkins D.L., “Sound Generation by Turbulence and Surfaces in Arbitrary Motion,” Philosophical Transactions of the Royal Society, A264 (A1151) pp. 321-342 (1969)





Simulation Guide: Fluent-Sysnoise Coupling• Available in Fluent6.1 and Sysnoise5.6• Beneficial over F-W H when sound reflection is

important• Usage:

– 1] Setup mesh/turbulence-models/solver-settingssame as F-W H

– 2] Select source surfaces and select “write source datato files” option. Fluent will create:

• One .index file• Multiple .asd files

– 3] Create a model for the acoustic domain in Sysnoise– 4] Fluent created .index and .asd files can be directly

imported into Sysnoise





Vorticity Magnitude Contours

Examples: (1) 2D CylinderOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




• Well documented and studied• Experimental results from Revell et. al.

Lockheed Report 28074• Cylinder diameter of 0.019 m• Free stream velocity of 69.2 m/s• Reynolds number ~ 90,000• 2-D LES, ∆t = 2E-6 sec• Cylinder surface used for F-WH integration





VorticityContours





FFT of soundpressurelevel at the observer’sposition





1.321.47

0.187 0.19

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Experiment FLUENT

Comparison of LES results from FLUENT with experiment

CdStrouhal Number





100

117

102114

0

20

40

60

80

100

120

140

128D 35D

SPL

(dB)

Comparison of LES results from FLUENT with experiment for SPL

ExperimentFLUENT

Observer Location





Examples: (2) Generic Side View Mirror– Generic Side-View Mirror shape

• Half cylinder (0.2 m dia. and height)• Topped by quarter sphere• Mounted on a flat plate

Reference:Lokhande B.S., Sovani S.D., Xu J., “Computational aeroacoustic analysis of a generic side view mirror”SAE Paper 2003-01-1698, SAE NVH Conference





• Aim:– Test all three acoustics modeling strategies with CFD:

• CAA• Ffowcs-Williams Hawkins Method• Fluent-Sysnoise coupling

– Predict with each strategy: • Transient pressure fluctuation on acoustic source

surfaces (base plate and mirror body)• Sound pressure level at microphone locations away

from the mirror

Examples: (2) Generic Side View MirrorOutline 1AeroacousticsBasics2 3 4 5SimulationApproaches6 7 8 9 10 11




• Computational Domain:• Modeled hemispherical region around mirror and base-plate• Hexahedral elements. Total 1.39 million

Inlet MirrorInlet Mirror





Side View

Top View





Velocity Inlet

Pressure Far-Field

Symmetry

Walls

• Boundary Conditions:• Inlet velocity = 200 km/hr• Re = 7 × 105





• Solution Settings:– CFD code: Fluent 6.1– Solver: Segregated Implicit– Turbulence Model: LES

• Smagorinsky-Lilly sub-grid scale model– Discretization schemes:

• Time: 2nd order implicit• Momentum: 2nd order upwind• Pressure-Velocity Coupling: SIMPLE

• Transient Solution:– Timestep size: 60 microsecond– Total timesteps: 2100– Run time: 4.75 days– Hardware: 2 processors, Intel P4,

2.2 GHz, RedHat Linux





• Flow Structure: Instantaneous Vorticity Iso-Surface





Flow Structure: Transient Flow Pressure & Velocity


Microphone Locations

Side View Top View

Pt. 101

Pt. 102

B. Lokhande, S. Sovani, Fluent Inc., J. Xu, ICEM-CFD

Pt. 102

Pt. 101





SPL Spectrum

• Excessive fluctuations seen since data is presented from a single sample

Reference for Experimental Data: Hold et al. (AIAA-99-1896) and Seigert et al. (AIAA-99-1895)





10

30

50

70

90

110

0 500 1000 1500 2000Frequency (Hz)

SPL

(dB)

ExperimentalCFD - CAACFD - AA

Point 101


SPL Spectrum







SPL Spectrum






Examples: (2) Generic Side View MirrorFluent-SysnoiseCoupling

At 54 Hz





Future Work• Fan Noise

– F-W H approach is capable of handlingrotating source surfaces

– Work in progress • allow export of acoustic source

data from moving surfaces• couple Fluent and Sysnoise for

moving/rotating acoustic sources• Acoustics Estimation from Turbulence

– Work in progress to include• Lilley source term• Boundary Layer noise• Linearized Euler equation





Summary

• Acoustics modeling approaches with Fluent 6.1– Direct CAA (Computational Aeroacoustics)– F-W H Model (Ffowcs-Williams Hawkins)– Fluent-Sysnoise Coupling

• New FFT Tool for acoustics and general transient signals

• Bottom line:Accuracy of acoustic predictions is directly determined by accuracy of underlying transient CFD solution




autougm03 acoustics

Documents

simulation guidecaa

sandeep sovaniauto ugm

sysnoise 26examples27

confidentialacoustics

navierstokes equations

stokes equations cfd

sound generation

special case of navier