AM 12 Chapter 4

4-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

April 28, 2009Inventory #002645

Chapter 4

Common MeshControls

ANSYS MeshingApplication Introduction

Common Mesh Controls



Training ManualOverview• Meshing Procedure• Physics Based Settings• Global Mesh Controls

– Relevance and Relevance Center– Element Size– Initial Size Seed– Smoothing and Transition– Span Angle Center– Advanced Sizing Function

• Local Mesh Controls– Edge, Face, and Body Sizings– Sphere and Body of Influence– Contact Sizing– Mapped Face Meshing– Match Control– Refinement– Pinch Controls

• Inflation• Workshop 4.1 Global and Local Mesh Controls• Workshop 4.2 Mapped Face Meshing for a Swept Mesh




Training ManualMeshing Procedure

1. Set target physics (structural, CFD, etc.). This will be automatically setif you generate the mesh as part of a physics based system (i.e. FLUENT, CFX, or Mechanical)

2. Set meshing methods

3. Specify mesh settings (sizings, controls, inflation, etc.)

4. Create named selections for convenience

5. Preview mesh and adjust settings if necessary

6. Generate mesh

7. Check mesh quality

8. Prepare mesh for analysis




Training Manual

• Different types of analyses have different meshing requirements– Structural – coarser mesh with higher order elements– CFD – finer, smoothly varying mesh, boundary layer resolution

• Different CFD solvers have different requirements as well

– Explicit Dynamics – uniformly size mesh desired

• Possible to set common defaults by setting the Physics Preference

Physics Preference Option

Sets the following automatically ...

Solid Element Midside Nodes Default

Relevance Center Default

smoothing transition

Mechanical Kept Coarse Medium Fast

CFD Dropped Coarse Medium Slow

Electromagnetic Kept Medium Medium Fast

Explicit Dropped Coarse High Slow

Physics Based Settings




Training ManualPhysics Based Settings

• Mechanical • CFD




Training ManualMesh for Default Mechanical Settings




Training ManualMesh for Default CFD Settings

Acts on Edges and Faces

Acts on VolumeActs on All




Training ManualNo Advanced Size Function

Acts on Edges




Training ManualSpan Center Coarse




Training ManualFast Transition




Training ManualEffect of Keeping Midside Nodes




Training ManualEffect of Shape Checking

• Mesh node and element count identical to default Mechanical Mesh




Training ManualMesh Quality• Mechanical Settings • CFD Settings




Training ManualMesh Sizing Strategy; Mechanical

• Seek efficient approach to resolve critical features withminimal input from user

• Define or accept defaults for a few global mesh sizing settings

• Make global adjustments with Relevance and Relevance Center

• As needed, define sizing for bodies, faces, edges, sphere ofinfluence to exert more control over how sizes are set as themesh is generated




Training ManualMesh Sizing Strategy: CFD

• Rely on Advanced Size Functions to refine the mesh whererequired

– Curvature (default on)– Proximity

• Identify the smallest feature in the model– Set the min-size such that this feature is sufficiently resolved– If that will lead to an over-refined mesh

• Scope a Hard size on this feature below min size

– Use pinch control to remove small edges and faces• Ensure that the pinch tolerance is always smaller than the local min size

• As needed, define soft sizing's using bodies, faces, edges or body of influence to exert more control over how sizes are set as themesh is generated




Training ManualRelevance and Relevance Center

Coarse Medium Fine

0

100

-100

0

100

-100

0

100

-100

• A slider bar to refine or coarsen the mesh

Relevance




Training ManualGlobal Element Size

• Element Size sets the element size used for the entire model. This size will be used for all edge, face, and body meshing. This option does not appear when the Advanced Size function is used

– Default value based on Relevanceand Initial Size Seed

– Desired value can be entered




Training Manual

• Initial Size Seed– Controls the initial mesh seeding for each

part. – This is ignored if an element size is specified– Active Assembly: bases the initial seeding on

the diagonal of the bounding box that encloses only parts that are unsuppressed.The mesh could change as parts aresuppressed and unsuppressed

– Full Assembly: bases the initial seeding on the diagonal of the bounding box that encloses all assembly parts regardless of the number of suppressed parts. As a result, the mesh never changes due to part suppression

– Part: bases the initial seeding on the diagonal of the bounding box that encloses each particular individual part as it is meshed. The mesh never changes due to part suppression.

Initial Size Seed




Training Manual

• Smoothing (Advanced Size Function is off)– Mesh Smoothing attempts to improve

element quality by moving locations of nodes with respect to surrounding nodes and elements. The following options control the number of smoothing iterations along with the threshold metric where the mesher will start smoothing.

• Medium (Mechanical)• Medium (CFD, Emag)• High (Explicit )

• Transition (Advanced Size Function is off)– Transition controls the rate at which adjacent

elements will grow • Slow (CFD, Explicit) produces a smooth

transition• Fast (Mechanical, Emag) produces a more

abrupt transition

Smoothing and Transition




Training ManualSpan Center Angle

• Span Angle Center sets the goal for curvature based refinement for edges. The mesh will subdivide in curved regions until the individual elements span this angle. The following choices are available:

– Coarse – 91° to 60° – Medium – 75° to 24° – Fine – 36° to 12°

– Only used if Advanced Size Function is off




Training ManualSpan Angle Center

45°

12°




Training ManualAdvanced Size Function

• Standard Size Function • Advanced Size Function

• Without the advanced size function, edges are meshed according to the specified element size, refined for curvature andproximity, adjusted for defeaturing or pinch controls, and then passed to the face andvolume meshers





• Options and defaults:– Proximity & Curvature– Curvature (default)

• Default 18 degrees

– Proximity• Default 3 cells per Gap (2D and 3D)• Default accuracy; 0.5

– Increase to 1 if proximity is not honored





• With curvature

• With curvature and proximity (5 cells in gap)




Training ManualAdvanced Size Function: Curvature




Training ManualAdvanced Size Function: Fixed

• Local mesh sizes must be setvia mesh controls as no localized refinement due tocurvature or proximity willoccur

• Gradation between the sizeswill occur according to thespecified growth rate




Training Manual

• Below is the list of local controls available (availability depends on the mesh method used)

– Sizing– Contact Sizing– Refinement– Mapped Face Meshing– Match Control– Pinch– Inflation

Local Mesh Controls




Training ManualLocal Sizing

• Local Sizing:– “Element Size” specifies average element edge

length on bodies, faces, edges, or vertices– “Number of Divisions” specifies number of

elements on edge(s)– “Sphere of Influence” elements within the

sphere have a given average element size– Available options above depend on which

entities are scoped:– Options will be different if Advanced Size

Function is used

Entity Element Size # of Elem. Division Sphere of InfluenceBodies x xFaces x xEdges x x xVertices x




Training ManualFace Sizing

• Local Sizing:– “Element Size” specifies average element

edge length on face(s)– “ “Sphere of Influence” elements within the

sphere have a given average element size– For anything other than a vertex, a sphere

of influence requires that a CoordinateSyetem be defined




Training ManualFace Sizing – Element Size




Training ManualSphere Of Influence – Coordinate System




Training ManualFace Sizing – Sphere of Influence




Training ManualFace Sizing: Effect of Scope

“Sphere of Influence” face sizing (shown in red) has been defined. Elements lying in that sphere for that scoped entity will have a given average element size.

A general Sphere of Influence (Point Sizing) would control the mesh on all faces that it touched

Scoped to single surface

Scoped to 3 surfaces

• Multiple entities could be selected

• All scoped entities within sphere are affected by size settings

• Face Sizing (sphere of influence)




Training ManualFace Sizing with Advanced Size Function




Training ManualScoped Sizes with Size Function

• On meshed entity– Edge, Face, Body

• Hard– Size is Constant– Biasing (edge)– This can be used to

assign a size smaller than the min size

• Soft– Size is Max– Local Curvature

angle– Local Growth

rate (value has to be smaller than Global)




Training Manual

• Edge Sizing (bias)– An edge may be discretized with a

bias towards one end, both ends, or the center

– Consider:

• A swept mesh is used with the source face as shown

• Edge sizings are specified for the two pairs of edges on the source face.

• The edge sizings are biased so as to give a finer mesh near the edges

Edge Sizing




Training ManualVertex Sizing

– Sizing can also be defined for Vertices

– A Vertex Sizing is a Sphere of Influence where the center of the sphere is defined by a model vertex. The sizing will be defined for all entities over the entire sphere

• Vertex Sizing




Training ManualBody of Influence with Size Function

• Body of Influence– Body of influence (BOI) is only

active when the Advanced Size Function is on

– The BOI can be any CAD Line, Face or Solid Body

– The BOI is not meshed, it just acts as a source of constant size

– To implement:• Pick Geometry• Pick Bodies of Influence• Assign parameters

– Element size – Local growth rate




Training ManualContact Sizing

• Contact Sizing– provides a way of generating

similar-sized elements on contact faces between parts (meshes will be of similar size but will not be conformal)

– An “Element Size” or “Relevance” can be specified for a given contact region

In this example, the contact region between the two parts has a Contact Sizing specified (by Element Size). Note that the mesh is now consistent at the contact region.




Training ManualRefinement

• Refinement– Element refinement divides existing mesh– Valid for faces, edges, and vertices– Not available for Patch Independent Tetrahedrons or CFX-Mesh– An ‘initial’ mesh is created with global and local size controls first, then

element refinement is performed at the specified location(s).– Refinement level can vary from

1 (minimal) to 3 (maximum).– A refinement level of “1” breaks up the edges

of the elements in the ‘initial’ mesh in half.– Not recommended for CFD as cannot be used

with inflation

For example shown, the left side has refinement level of 1 whereas the right side is left untouched with default mesh settings.




Training ManualMapped Face Meshing

• Mapped Face Meshing – allows for the generation of structured meshes on surfaces:– In example below, mapped face meshing on the internal

cylindrical face provides a more uniform mesh pattern.

– If surface cannot be mapped mesh for any reason, meshing will continue and this will be shown in Outline Tree with the icon





• Mapped Face Meshing– If the face selected for a Mapped Face

Meshing is defined by two loops, then the Radial Number of Divisions field is activated. This specifies the number of divisions across the annular region when sweeping.

– This is useful for putting in a number of layers across a thin annulus




Training ManualMatch Control

• Match Control– Matches mesh pattern on periodic faces to facilitate cyclic

symmetry analyses typical of rotating machinery

Cut Boundaries

Full Model Cyclic Symmetry Model

Matched Faces




Training ManualCreating a Match Control

• Match Control (Procedure):– Insert “Match Face Meshing” control under Mesh branch– Identify faces of symmetry boundary– Identify the coordinate system (Z axis is rotation axis)

Rotation CS




Training ManualMatch Control and Advanced Size Function

– Scoped Advanced Size function information is also matched• Scoped Size can be added on either side with identical results

– Periodic Matching only needs one coordinate system

– Periodic matching can handle common edges, multiple faces and inflation layers




Training ManualPinch Controls

• When pinch controls are defined, defeaturing occurs when the mesh is generated.

• The Pinch feature works on vertices and edges only; faces and bodies cannot be pinched.

• The Pinch feature is supported for the following mesh methods:

– Patch Conforming Tetrahedrons– Thin Solid Sweeps– Hex Dominant meshing– Quad Dominant Surface Meshing– All Triangles Surface meshing




Training ManualAutomatic Pinch Control




Training ManualAutomatic Pinch Generation

• With automatic pinch generation user can pinch features under a defined size to remove features leading to bad quality elements

• Note that the pinch tolerance should be smaller than the local min size

• Can be used in CFD for removals of – Slivers– Short edges– Sharp angles




Training ManualInflation Options

• Use Automatic Inflation– Program Controlled

• All faces not part of Named selections• No internal faces between shared bodies

• Inflation options – Smooth Transition (default for 2D and Tet

meshing– First Layer Thickness– Total Thickness (Default for others)

• Inflation Algorithm– Pre (TGrid)

• for Tri/Patch conforming Tet/Sweep

– Post (ICEM CFD)• for Patch non-conforming Tetra

• Collision Avoidance– Compression (default for Fluent)– Stair-Step (default for CFX)

• Additional detail on settings in Chapter 5




Training ManualInflation for Tetrahedrons and MultiZone

• Inflation– When the Mesh Method is set to Tetrahedrons or MultiZone,

inflation layers are scoped to a body or bodies by selecting the faces from which inflation is desired




Training ManualInflation for Swept Mesh

• For a swept mesh, inflation is applied by selecting the edges from which inflation is desired on the source face

• The Src/Trg Selection on the should therefore be set to either Manual Source or Manual Source and Target




Training ManualGenerating a Mesh

• Generate Mesh– Generates the entire volume mesh

• Preview Surface Mesh– For most methods (but not the Tetrahedral Patch Independent method),

this option is significantly faster. As such, it is often useful to preview the surface mesh first.

– If Generate Mesh fails because element quality parameters could not be met, Preview Surface Mesh is useful

• It allows you to see the surface mesh, so you can see where it need improving




Training ManualSection Planes

• In the Meshing Application, section planes can display internal mesh• Find the “New section plane” button on toolbar• Can display

– Elements either side of plane– Cut or whole elements– Elements on the plane

• Multiple planes can be used





• Exterior mesh





• Create section plane, display one side





• Switch side of section plane displayed





• Display whole elements




Training ManualNamed Selections

• Named Selections allow users to group together vertices, edges, surfaces, or bodies together

– Named Selections can be used for defining mesh controls, applying loads and supports in a structural analysis,etc.

– Named selections will appear as named regions when themesh is imported into CFX-Pre or Fluent

– Provides an easy method to reselect groups that will be referenced often when defining contact regions, boundaryconditions, etc.

– Can be used to facilitate program controlled inflation

• Note:– Only one type of entity can be in a

particular Named Selection. For example, vertices and edges cannot exist in the same Named Selection.

– Named Selection groups can be imported from DesignModeler and from some CAD systems




Training Manual

• The visibility of Named Selections can be toggled on and off to either hide or display geometry or mesh

Named Selections



Effect of Global and LocalControls on Mesh Size andQuality

Workshop 4.1




Training ManualGoals

• This workshop will illustrate the use and effect of global and local mesh sizing controls including use of the Curvature and Proximity features in the Advanced Size function

• The geometry has both thin-walled regions and regions of high local curvature




Training ManualSpecifying Geometry

1. Copy the tee.agdb file from the

files folder to your working directory

2. Start Workbench and double-click

the Mesh entry in the Component

Systems panel at the right

3. Right-click on Geometry in the Mesh

entry in the Project Schematic and

select Import Geometry/Browse

4. Browse to the tee.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 ManualMeshing Options

7. Right-click on Mesh and click Edit to

open the Ansys Meshing

8. On the Meshing Options Panel at the right,

set the Physics Preference to Mechanical

and the Mesh Method to Patch Conforming

Tetrahedrons and click OK.

9. Look in the Mesh Outline and verify that the

Patch Conforming Method has been

assigned to the single body in the geometry




Training ManualDefault Mechanical Mesh

10. Left click on Mesh expand the Sizing and

Statistics Entries. Select Skewness as the

Mesh Metric. Right click on Mesh and

Generate the Mesh. Note the coarseness

of the mesh and the statistics.




Training ManualCFD Mesh

11. Change the Physics Preference to CFD

and the Solver Preference to Fluent.

Verify that the Advanced Size Option is

set to Curvature

12. Right-click on Mesh and generate the

mesh. Notice the much finer mesh and

the improvement in the mesh metric.




Training ManualSection Plane

13. Orient the Model view so that it is edge on as

shown. Click on the new section plane icon

14. Draw a section plane that splits the model

down the middle. Orient the view so that it is

parallel to the axis of the tee. Click on the

Show Whole Elements icon. Note that there

is only one element through the thickness of

the thin regions




Training ManualAdding Proximity Sensing

15. Click in the Use Advanced Size Function and change

the setting to On: Proximity and Curvature. This will

add proximity sensing to the meshing algorithm.

16. Leave the view as is with the Section Plane active.

Generate the mesh again (this will take some time).

Note that there are now multiple elements across the

thickness and the large increase in mesh count.




Training ManualIncreasing the Minimum Size

17. Click in the Minimum Size box in the Mesh Settings

and enter 0.005 for the Min Size.

18. Regenerate the mesh Note that there are still

multiple elements across the thickness but the mesh

count is considerable smaller.




Training ManualUsing a Face Sizing

19. Set the Advanced Size Function back to

Curvature and toggle off the section plane

20. Right-click on the mesh entry in the

Outline and insert a Sizing. Pick the

outer cylindrical face as shown and click

Apply.





21. Set the element size to 0.005 [m]. Regenerate the

mesh. Notice that the mesh on the selected face is

finer than the mesh on adjacent faces.





22. Toggle the Section Plane back on and orient the view so that it is parallel to the

axis of the tee. Note that there are only multiple elements through the thin sections

where the face sizing is active




Training ManualCoodinate System for Sphere of Influence

23. Right click on Coordinate Systems and insert a

Coordinate System. Set the Define By option to Global

Coordinates and enter [0 [m], 0.1 [m], and 0.08 [m])

as the Origin X, Y, and Z. With the Section Plane toggled

off, the coordinate system should appear as shown

below.




Training ManualBody Sizing (Sphere of Influence)

24. Suppress the Face Sizing via a right-click.

25. Right click on Mesh and insert a Sizing. Pick the Body

and set the Type to Sphere of Influence. Click in the

Sphere Center entry box and select the Coordinate

system you created.




Training ManualBody Sizing

26. Set the Sphere Radius to 0.01 [m] and the Element Size

to 0.005 [m]. The model display will update to preview

the extent of the Sphere of Influence.





27. Regenerate the mesh with the Section Plane toggled off.

Note the limited extent of the Sphere of Influence.





28. Toggle the Section Plane back on and rotate the view so that it is parallel to the

axis. Note there are only multiple elements through the thin regions near the

Sphere of Influence.



Mapped Face Meshingand Edge Sizings for a Swept Mesh for a Thin Annulus

Workshop 4.2




Training ManualGoals

• This workshop will illustrate the use and effect of a Mapped Face Meshing control for a Swept Mesh to enforce a radial number of divisions across the width of a thin annulus

• An Edge Sizing is also set for edges on the Source and Target faces to help generate a high quality mesh




Training ManualSpecifying Geometry

1. Copy the elbow.agdb file from the

files folder to your working directory

2. Start Workbench and double-click the

Mesh entry in the Component

Systems panel on the left

3. Right-click on Geometry in the Mesh

entry in the Project Schematic and

select Import Geometry/Browse

4. Browse to the elbow.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 ManualInsert Sweep Mesh

7. Close the Meshing

Options Panel at the

right without setting

anything.

8. Right-click on Mesh and

insert a Method. Select

the body and set the

method to Sweep.

9. Set the Src/Trg selection

to Manual Source and

select the face shown as

the Source.




Training ManualInitial Mesh

10. In the Mesh settings, set

the Physics to CFD and

the Solver Preference to

Fluent. Expand the

Sizing and Statistics

entries

11. Set the Mesh Metric to

Skewness and generate

the mesh

12. Note that there is only

one element across

most of the annular

thickness and the

skewness is relatively

high





13. Since the source face has an

inner and outer loop, adding a

mapped Face Meshing will

allow you to specify the radial

number of divisions

14. Right-click on Mesh and insert

a Mapped Face Meshing.

Pick the source face for the

sweep and set the Radial

Number of Divisions to 3.




Training ManualEffect of Mapped Face Meshing

15. Generate the mesh. Note that

there are now 3 elements

across the thickness of the

annulus but there is some

twist to the mesh as it sweeps

from the source to the target.

The skewness is also still

rather high for such a simple

geometry.

16. Right click on Mesh and Insert

a Sizing. You will define a

consistent edge sizing for the

4 edges on the source and

target faces to help improve

the quality




Training ManualEdge Sizing

17. Pick the four edges which

bound the source and target

faces and apply the election to

the Geometry entry box

18. Set the Type to Number of

Divisions and set the number

to 20. The Model Display

should update to reflect the

settings




Training ManualFinal Mesh

17. Regenerate the mesh. Note the improvement in the mesh quality metrics.

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