am 12 chapter 2

2-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

April 28, 2009Inventory #002645

Chapter 2

Introduction to theANSYS Meshing Application

ANSYS MeshingApplication Introduction

Introduction to the ANSYS Meshing Application



Training ManualOverview

• Introduction to the ANSYS Meshing Application• Meshing Requirements for Different Physics• ANSYS Meshing Workflow• Meshing Methods for 3D and 2D geometries• Workshop 2.1– Automatic Meshing for a Multibody Part– Program Controlled Inflation– Transferring Mesh to CFX or FLUENT




Training ManualWhat is the “ANSYS Meshing Application”?

• ANSYS has been working to integrate “best in class” technologies from several sources:– ICEM CFD – TGrid– GAMBIT– CFX– ANSYS Prep/Post– Etc.




Training ManualANSYS Meshing Application Overview

• The objective of the ANSYS Meshing Application in Workbench is to provide access to common ANSYS Inc. meshing tools in a single location, for use by any analysis type:

–FEA Simulations• Mechanical Dynamics Simulation• Explicit Dynamics Simulation

– AUTODYN– ANSYS LS DYNA

• Electromagnetic Simulation–CFD Simulation

• ANSYS CFX• ANSYS FLUENT




Training ManualMesh Specification

Purpose– For both CFD (fluid) and FEA (solid) modelling, the software performs

the computations at a range of discrete locations within the domain.

– The purpose of meshing is to decompose the solution domain into an appropriate number of locations for an accurate result.

– The basic building-blocks for a 3D mesh are:

Manifold Example: Outer casting and internal flow region are meshed for coupled thermal/stress gas flow simulation

Tetrahedrons(unstructured)

Hexahedrons(usually structured)

Prisms (formed when a tet mesh is extruded)

Pyramids (where tet. and hex. cells meet)





Considerations

• Detail: – How much geometric detail is

relevant to the simulation physics.– Including unnecessary detail can

greatly increase the effort required for the simulation.

• Refinement– Where in the domain are the most

complex stress/flow gradients? These areas will require higher densities of mesh elements.

Is it necessary to resolve this

recess?

Extra mesh applied across fluid

boundary layer

Refined mesh around bolt-hole





• Efficiency– Greater numbers of elements require more compute resource (memory /

processing time). Balance the fidelity of the simulation with available resources.





• Quality– In areas of high geometric complexity mesh elements can become distorted. Poor

quality elements can lead to poor quality results or, in some cases, no results at all!

– There are a number of methods for measuring mesh element quality (mesh metrics*). For example, one important metric is the element ‘Skewness’. Skewness is a measure of the relative distortion of an element compared to its ideal shape and is scaled from 0 (Excellent) to 1 (Unacceptable).

*Further information on mesh metrics is available in the documentation and training lecture appendices





This example illustrates an unconverged thermal field in a manifold solid casting. On closer inspection it is clear that the simulation is unable to resolve a sensible data field in the region of poor quality elements.

The example with good quality elements demonstrates no problems in the solution field.

The ANSYS Meshing Application provides many tools to help maximise mesh quality

Example showing difference between good and poor meshes:




Training ManualFEA Meshing Issues

• Structural FEA– Refine mesh to capture gradients of

concern• E.g. temperature, strain energy, stress

energy, displacement, etc.

– tet mesh dominated, but hex elements still preferred

– some explicit FEA solvers require a hex mesh

– tet meshes for FEA are usually second order (include mid-side nodes on element edges)




Training ManualCFD Meshing Issues

• CFD– Refine mesh to capture gradients of concern

• E.g. Velocity, pressure, temperature, etc.

– Mesh quality and smoothness critical for accurate results

• This leads to larger mesh sizes, often millions of elements

– tet mesh dominated, but hex elements still preferred

– tet meshes for CFD are usually first order (no mid-side nodes on element edges)




Training ManualMesh Types

• Tet Mesh and Tet/Prism hybrid





• Hex Mesh





• Tet Mesh1) Can be generated quickly, automatically, and for

complicated geometry

Mesh can be generated in 2 steps:

Step 1: Define element sizing

Step 2: Generate Mesh





• Tet Mesh2) Isotropic refinement – in order to capture gradients in one direction, mesh

is refined in all three directions – cell counts rise rapidly

Perforated plate resulting in pressure drop in x direction

x





• Tet Mesh3) Inflation layer helps with refinement normal to the wall, but still isotropic in

2-D (surface mesh)





• Hex Mesh– Fewer elements required to resolve physics for most CFD

applications

• This hexahedral mesh, which provides the same resolution of flow physics, has LESS than half the amount of nodes as the tet-mesh)

TET HEX





• Hex Mesh– Fewer elements required to resolve physics for most CFD

applications. • Anisotropic elements can be aligned with anisotropic physics

(boundary layers, areas of tight curvature like wing leading and trailing edges)




Training ManualANSYS Meshing Application Workflow

• The ANSYS Meshing Application uses a ‘divide & conquer’ approach

• A different ‘Meshing Method’ can be applied to each part in the geometry–Meshes between bodies in different parts will be non-matching or

non-conformal–Matched or conformal meshes will be generated for bodies in a

single part

• All meshes are written back to a common central database• A number of different methods are available for 3D and

2D geometry




Training ManualMeshing Methods for 3D Geometry

• There are six different meshing methods in the ANSYS Meshing Application for 3D Geometry:

–Automatic –Tetrahedrons

• Patch Conforming• Patch Independent– (ICEM CFD Tetra algorithm)

– Swept Meshing– MultiZone– Hex Dominant– CFX-Mesh




Training ManualMeshing Methods for 2D Geometry

• There are four different meshing methods in the ANSYS Meshing Platform for 2D Geometry which can be applied to Surface Bodies or Shells: – Automatic Method

(Quadrilateral Dominant)– All Triangles– Uniform Quad/Tri– Uniform Quad




Training ManualPatch Conforming Tetrahedrons

• Tetrahedrons Method with Patch Conforming Algorithm– Faces and their boundaries (edges and vertices) are respected – Includes the Expansion Factor setting, which controls the internal growth rate

of tetrahedrons with respect to boundary size– Includes inflation or boundary layer resolution for CFD– Can be mixed with Sweep methods for bodies in a single part – conformal

meshes will be generated

Tetrahedral Mesh

Swept Mesh

Prism

Tet

Pyramid

Element Shapes




Training ManualPatch Independent Tetrahedrons

• Tetrahedrons Method with Patch Independent (ICEM CFD Tetra) Algorithm– Faces and their boundaries (edges and vertices) are not necessarily respected unless

there is a load, boundary condition, or other object scoped to them– Useful for gross defeaturing or to produce a more uniformly sized mesh – Simplified version of Tetra tightly integrated into the ANSYS Meshing Application– Honors standard ANSYS Meshing Application mesh sizing controls– Tetra parts can also have inflation applied

Coarse mesh ‘walks over’ detail in surface model

Inflation layer

applied for CFD

Prism

Tet

Pyramid

Element Shapes




Training ManualSweep Method

• Produces Hexes and/or Prisms

• Body must be Sweepable

• Single Source, Single Target

• Inflation can yield pure hex or prisms

Extrusion removed to allow for swept meshing

Body split into 2 parts to allow for swept meshing

Allows for inflation layer (boundary layer resolution) for CFD

Prism

Hex

Element Shapes




Training ManualAutomatic Method

• The Automatic setting toggles between Tetrahedral (Patch Conforming) and Swept Meshing, depending upon whether the body is sweepable. Bodies in the same part will have a conformal mesh.

No inflation Programmed Controlled Inflation

Tetrahedron (Patch Conforming)Swept Tetrahedron (Patch Conforming)




Training ManualInflation

• Inflation is accomplished by extruding faces normal to a boundary to increase the boundary mesh resolution, typically for CFD

• Smooth Transition from inflated layer to interior mesh• Collision avoidance: – Stair-stepping – Layer compression

• Preview Inflation• Pre vs. Post inflation• All methods can be inflated except

for Hex-Dominant and Thin Sweep• Sweeping:– Pure hex or wedge




Training ManualMultiZone Sweep Meshing

• New feature for 12.0• Automatic geometry decomposition– With the swept method, this part would have to be

sliced into 3 bodies to get a pure hex meshWith MultiZone, it can be meshed directly!




Training Manual

• The hex-dominant meshing algorithm creates a quad-dominant surface mesh first, then hexahedral, pyramid and tetrahedral elements are filled in as needed.– Recommended when a hex mesh is desired for a body that cannot be swept– Useful for bodies with large amounts of interior volume– Not useful for thin complicated bodies where the ratio of volume to surface area is low– No boundary layer resolution for CFD

• Mainly used for FEA analysis

Prism

HexTet

Pyramid

Element Shapes

Hex-dominant mesh shown above:19,615 Hex (60%)5,108 Tet (16%)211 Prisms (1%)

7,671 pyramids (24%)

Hex-Dominant Method




Training ManualCFX-Mesh Method

• CFX-Mesh uses a ‘loose’ integration.– No Meshing Application

sizings are respected or transferred to CFX-Mesh

– Selecting Right Mouse ‘Edit…’ on the Method launches the CFX-Mesh GUI.– Define mesh

settings/controls/inflation

– Preview & generate volume mesh

– Commit the current mesh model

– Return to ANSYS Meshing

– Possible to ‘Generate Mesh’ on a CFX-Mesh method without opening the application

• Uses current or default settings

Generate Volume Mesh

Inflation layer



Pipe Tee Mesh

Workshop 2.1




Training ManualGoals

• This workshop will illustrate the use of the Automatic Meshing Method for a single body part

• The transfer of the mesh toFLUENT and CFX is also demonstrated




Training ManualSpecifying Geometry

1. Copy the pt.agdb file from the tutorial

files folder to your working directory

2. Start Workbench and double-click the

Mesh entry in the Component

Systems panel in the Toolbox

3. Right-click on Geometry in the Mesh

entry in the Project Schematic and

select Import Geometry/Browse

4. Browse to the pt.agdb file you

copied and click Open

5. Note that the Geometry entry in the

Project Schematic now has a green

check mark indicating that geometry

has been specified




Training ManualInitial Mesh

6. Double-click the Mesh entry in the

schematic or right-click and select Edit. This will

open the Meshing Application

7. In the Meshing Options panel set the Physics

Preference to CFD, the Mesh Method to

Automatic and press OK

8. Right click on Mesh and select Generate Mesh

9. Use the view manipulation tools and the axis

triad to inspect the meshBased upon choice of physics (CFD), the Meshing Application has produced a mesh accommodating curvature, a reasonable sizing strategy and automatic selection of optimal mesh methods with minimal user input. There are many ways in which the Meshing Application can control and improve the mesh. Some further mesh controls will now be demonstrated.




Training ManualNamed Selections

10.Set the Selection Filter to Faces

and select one of the pipe end

faces as shown. Right-click in the

Model View and choose Create

Named Selection. Enter velocity-

inlet-1 for the Selection Name

11.Repeat for the other two pipe end

faces using the naming as shown

12.The Named Selections just

created are listed in the Outline by

expanding Named Selections.

The names assigned here will be

transferred to the CFD solver so

the appropriate flow conditions

can be applied on these surfaces.

pressure-outlet

velocity-inlet-1velocity-inlet-2




Training ManualInflation

13.Select Mesh in the Outline and expand Inflation

in Details

14.Set Use Automatic Tet Inflation to Program

Controlled, leave other settings

15.Right click on Mesh and select Generate Mesh.

Note the inflation layers are grown from all

boundaries not assigned a Named Selection.

The thickness of the inflation layers is calculated

as a function of the surface mesh and applied

fully automatically.




Training ManualSection Planes

16.Orient the model by clicking on

the axis triad (+X Direction)

17.Click on the New Section Plane

icon in the menu bar. Left click,

hold and drag the cursor in the

direction of the arrow as

illustrated to create the Section

Plane

18.Created Section Planes are

listed (bottom left). Planes can

be individually activated using

the checkbox, deleted and

toggled between 3D element

view and 2D slice view. Try this

now (you will need to rotate the

model to see the cross-section)

After the Section Plane has been created the Section Plane cursor tool will still be active. Left clicking in the viewport and dragging will slide the Section Plane along its axis.

Clicking on either side of the Plane tool will cut the mesh on each side respectively. Clicking twice on one side will change the view to a planar slice.

When the position is finalized, select a view manipulation tool




Training ManualMesh Statistics

19. If you expand the Statistics entry

under Mesh, it will summarize the

number of nodes and elements in

the mesh

20.Under Mesh Metric select

Skewness. Note the reported

mesh quality




Training ManualTransferring Mesh to CFD

21. After the mesh has been generated, you

can transfer it to a new CFD simulation

22. In the main Workbench Window, right click

on the Mesh entry in the Meshing instance

you created on the Project Schematic and

observe that you can transfer the mesh to

a new FLUENT or CFX simulation

(Transfer Data To New >). Select either

FLUENT or CFX

23. Note that the Mesh entry now has an

Update symbol, right click the Mesh entry

and select Update. This will pass data to

the new FLUENT/CFX instance.




Training ManualFluent with Workbench Mesh

24. If FLUENT was

selected - Double

click the Setup

entry and accept

the default options

in the FLUENT

Launcher

25. FLUENT will start

with the mesh

loaded

26. Save the project

from the

Workbench File

Menu




Training ManualCFX with Workbench Mesh

27. If CFX was

selected - Double

click the Setup

entry, CFX Pre will

launch with the

mesh loaded

28. Save the project

from the

Workbench File

Menu

am 12 chapter 2

Documents

pipe end faces

fewer elements required

boundary layer resolution

poor quality elements

ansys meshing application

workbench file menu

tet mesh dominated

meshing application