am 12 chapter 5

5-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

April 28, 2009Inventory #002645

Chapter 5

Tetrahedral Meshwith Inflation

ANSYS MeshingApplication Introduction

Tetrahedral Mesh with Inflation



Training ManualOverview

• Tetrahedral Meshing Algorithms• Inflation Options for Patch Conforming – Algorithms– Pre and Post– Advanced Options– Collision Avoidance

• Patch Independent Meshing– Defeaturing– Proximity refinement– Curvature refinement

• Workshop 5.1 Tetrahedral Mesh with Inflation for a Tee Mixer (Patch Conforming)

• Workshop 5.2 Fluid and Structural Meshes for an Auto Manifold (Patch Independent)




Training ManualTetrahedral Meshing Algorithms

• Patch Conforming– All faces and edges are respected by default (although this can be

changed with pinch controls and virtual topology and there is default defeaturing based on the minimum size limit)

– For moderately clean CAD (i.e. native CAD, Parasolid, ACIS, Etc.)– Possible to combine with sweeps in a multibody-part to generate conformal

hybrid tet/prism and hex meshes– Works with advanced size functions– Surface mesh Volume mesh

• Patch Independent– Useful for CAD with sliver faces, many surface patches, short edges, etc.– Built in defeaturing/simplification based on meshing technology– Based on ICEM CFD Tetra/Prism Octree method– Volume mesh Surface mesh




Training ManualInflation for Patch Conforming Tetrahedrons

• Basic settings include Inflation Option as well as Pre and Post Inflation Algorithms




Training ManualInlation Option – Smooth Transition

• Smooth Transition (Default)– Uses the local tetrahedral element size to compute each local initial height and

total height so that the rate of volume change is smooth. Each triangle that is being inflated will have an initial height that is computed with respect to its area, averaged at the nodes. This means that for a uniform mesh, the initial heights will be roughly the same, while for a varying mesh, the initial heights will vary. – Transition Ratio

• Volume-based size change between the last layer of elements in the inflation layer and the first elements in the tetrahedron region.

• When the Solver is set to CFX, the default for Transition Ratio is 0.77. For all other physics preferences, including CFD when Solver Preference is set to Fluent, the default is 0.272.

• The difference in treatment arises because the Fluent solver is cell-centered so that the mesh element is equal to the solver element, while the CFX solver is vertex-centered with the solver element constructed from the mesh duals about a node.




Training ManualInflation Option – Thickness Options

• Total Thickness– Creates constant inflation layers using the values of the Number of Layers and

Growth Rate controls to obtain a total thickness as defined by the value of the Maximum Thickness control. Unlike inflation with the Smooth Transition option, with the Total Thickness option the thickness of the first inflation layer and each following layer is constant

• First Layer Thickness– creates constant inflation layers using the values of the First Layer Height,

Maximum Layers, and Growth Rate controls to generate the inflation mesh. Unlike inflation with the Smooth Transition option, with the First Layer Thickness option the thickness of the first inflation layer and each following layer is constant.




Training ManualInflation Algorithms

• Pre– TGrid algorithm– Default setting for all physics type. The surface mesh will be inflated

first, and then the rest of the volume mesh will be generated.– Does not support different number of layers on adjacent faces.– Also applicable to Sweep and 2D meshing.

• Post– ICEM CFD algorithm – A post processing technique that works after the tetrahedral mesh is

generated is used. Valid only for patching conforming and patch independent tetrahedrons.




Training ManualAdvanced Inflation Options

• Visible if View Advanced Options under the Global Inflation Options under Mesh is set to Yes– Collison Avoidance

• Layer Compression (Default for Fluent)• Stair Stepping (Default for CFX)

– Growth Rate Type– Maximum Angle– Fillet Ratio– Use Post Smoothing– Smoothing Iterations




Training ManualCollision Avoidance

• Layer Compression– If advancing inflated fronts from different

faces are about to collide, the inflation layers are compressed in order to leave enough room for a layer of tets

– If layer compression cannot resolve collision, layers may be removed as in stair stepping described below. A warning message may be generated and the quality of the mesh may be impacted,which is of special concern for FLUENTusers

• Stair Stepping– Inflation layers are “peeled off” to

prevent collision of advancing fronts in order to leave enough room for a layerof tets




Training ManualInflation: Compression vs. Stair-stepping

Layer Compression: Stair-stepping:




Training ManualPatch Independent Tetrahedrons

• The Patch Independent Tetrahedral methods has controls similar to the Advanced Size Function for Curvature and Proximity as well as an explicit tolerance for defeaturing geometry.

• Remember that the volume mesh is created first and then projected to vertices, edges, and faces to create the surface mesh. You can alwaysforce faces, edges, or points to be respected by creating named selections or by setting Define Defeaturing Tolerance to No




Training Manual

• “Filters” in/out edges based on size and angle. If set to Yes, a Define Defeaturing Tolerance field appears where you enter a numerical value. There are several basic cases, including the following:– Two approximately parallel spaced edges (fillet or

chamfer) closer than the tolerance, as shown in Fig. 1• If the face (fillet or chamfer) between spans by more than 15

degrees, one edge is kept and the other is dropped. Nodes will only line up along one edge. If the face between spans by less than 15 degrees, both edges may be dropped and patch independent mesh will walk over this feature without capturing it explicitly.

• If the tolerance is less than the fillet/chamfer size, and the deflection is greater than 15 degrees, both edges are kept.

– A small hole with a diameter smaller than the tolerance as shown in Fig. 2.

• No edges are dropped. You should defeature manually in this case.

Defeaturing Tolerance

Fig. 1

Fig. 2




Training ManualExamples of Defeaturing

Geometry No Defeaturing Defeaturing Tolerance of 1




Training ManualProximity Refinement – Cells in Gap

• Num Cells Across Gap– (displayed only when Curvature and Proximity Refinement is set to Yes).

This is the number of cells desired in narrow gaps. This sets the goal for the proximity based refinement. The mesh will subdivide in tight regions toward this goal, but the refinement is limited by the Min Size Limit is reached. It will not override this limit. The default is 1.




Training ManualCurvature Refinement – Span Angle

• Span Angle– (displayed only when Curvature and

Proximity Refinement is set to Yes). Sets the goal for the curvature based refinement. Similar to setting for the Advanced Size Function This refinement is also limited by the Min Size Limit. The following choices are available:

• Coarse – 91 degrees to 60 degrees• Medium – 75 degrees to 24 degrees• Fine – 36 degrees to 18 degrees




Training ManualInflation for Patch Independent Tetrahedrons

• Settings similar to those for Patch Conforming, but only Post algorithm since the surface mesh does not exist before the volume mesh is created.



Tetrahedral Mesh with Inflationfor a Tee Mixer

Workshop 5.1




Training ManualGoals

This workshop demonstrates the creation of a mesh for the fluid portion ofa mixing tee using the patch conforming tetrahedral mesher with an inflation layer to resolve the wall boundary layer. This mesh will be used later in the course to set up a CFD simulation so the workshop also demonstrateshow to export a mesh database for future analysis.




Training ManualImporting Geometry

1. Start Workbench and

select the Import option in

the menu bar and change

the filter to Geometry File.

2. Specify the mixer-tee.agdb

file from the Tutorials

folder and note that an

entry appears in the

Project Schematic with a

green check mark.

3. Expand the Component

Systems entry at the left

and drag Meshing onto the

DM instance in the project

schematic. Note the

linkage that appears.




Training ManualSupressing Solid Parts and Setting the Method

4. Double click the Mesh entry in System B

on the Project Schematic to open up

ANSYS Meshing.

5. Note that there are 5 parts and 5 solid

bodies. The four Solid entries comprise

the solid portion of the mixing tee while the

body named Fluid is the fluid region

6. Since we will focus first on the fluid region,

right-click and suppress the four solid

bodies under geometry in the Outline

7. Right-click on Mesh and select Insert

Method. Select the fluid body and set the

Method to Tetrahedrons and the Algorithm

to Patch Conforming




Training ManualPhysics Preference

8. Set the Physics Preference to CFD and the

Solver Preference to Fluent

9. Expand the Sizing entry and set the Max

Face Size to 0.015 [m] and the Max Tet

Size to 0.03 [m].

10. Expand the Inflation entry and set Use

Automatic Tet Inflation to None. This

means that you will need to define inflation

layers manually




Training ManualPreviewing the Surface Mesh

11. Right click on Mesh and select Preview Surface Mesh. The mesh resolution of the

surfaces appears to be reasonable




Training ManualInflating the Method

12. Right click on the Patch Conforming Method

and choose Inflate this Method

13. Pick the five side surfaces of the model

as shown below and set the Inflation Option to

First Layer Thickness and enter a value of

0.001 m. Set the Maximum Layers to 5.




Training ManualGenerating the Inflated Mesh

14. Right click on the mesh and choose

Generate Mesh to create the inflated volume

mesh.




Training ManualSkewness Mesh Metric

15. Expand the Statistics Entry and set the Mesh Metrics Option to Skewness. The

maximum value of 0.742 is suitable for the Fluent Solver




Training ManualUnsuppressing the Solids

16. Select the four solid parts under geometry in

the project outline and right-click to unsuppress

them. You might want to mesh the solids for a

conjugate heat transfer calculation in either

CFX or FLUENT or perhaps for a one-way

FSI calculation.




Training ManualMesh Method for the Solid Parts

17. Insert a Patch Conforming Tetrahedral Method and assign it to the four solid parts




Training ManualBody Sizing for the Solid Parts

18. Right-click on Mesh in the Outline and

insert a Sizing. Select the 4 solid

bodies in the Model View (set the

selection filter to bodies if needed) and

apply the selection to the geometry.

Enter a sizing of 0.04 [m].




Training ManualCombined Mesh for Fluid and Solid Parts

19. Regenerate the mesh. Note the new mesh count and quality metric




Training ManualNamed Selections for CFD

20. You will use the fluid portion of the mesh

to set up a CFD tutorial later. Since the

solid bodies are not needed for this,

suppress the four solid bodies again.

21. To help prepare the mesh for CFD, you

will create named selections for the 3 end

faces. To create a named selection, set

the selection filter to Faces, pick a face in

the Model View, right-click and choose

Create Named Selection. Do this for the

low-y face in the model using inlet-y as

the Name for the Selection.




Training ManualPreparing the Mesh for CFD

22. Repeat this for the high-Z face (inlet-z) and the high-Y face (outlet)




Training ManualExporting the Mesh File

22. While still in ANSYS Meshing, click on

File/Export and save the file as a meshdat

file. Note the location as you will import this

file later in the course when setting up a fluid

flow simulation using the fluid mesh.

23. Save the Project and exit Workbench.



Fluid and Structural Meshes for an Auto Manifold

Workshop 5.2




Training ManualGoals

This workshop demonstrates the creation of a mesh suitable for a conjugate heat transfer (CHT) flow simulation, or a fluid-structure interaction (FSI) simulation. However, the geometry presents potential difficulties.

• The geometry contains two bodies. One represents a solid manifold and the other the fluid region.

• The mesh in the fluid body will be of CFD quality, with inflation, whilst the solid mesh will be of structural quality.

• The Patch Independent mesh method is used to avoid problems arising from the nature of the CAD (see later).

• This same geometry will later be meshed using with the Patch Conforming Method with Virtual Topology in Workshop A.1.




Training ManualOpen Workbench

1. Launch ANSYS 12.0 Workbench (WB) from the START menu

2. Click on Component Systems in the Toolbox on the LHS of the WB main panel

3. Double click the Mesh option




Training ManualGeometry Import

4. Right click on the Geometry button in the RHS of the WB panel and select Import geometry.

5. Import the Auto-manifold.agdb file6. Double click on the Mesh button (in cell A3) to launch the Meshing

Application




Training ManualGeometry Review7. The geometry was created by importing an IGES file for the manifold into

DesignModeler and extracting the fluid region. • On the right-hand-side of the screen, Select ‘Mechanical’ for the Physics• Select ‘Automatic’ for the Meshing Method, then ‘OK’ that window.• The units of the geometry should be set to mm.

8. Named Selections were created in DesignModeler. Expand the Named Selections object and click on each to see where they have been applied.

• If you cannot see the Named Selections in the Meshing Application, see next slide.




Training ManualNamed Selections

• If the Named Selections don’t appear in the Meshing Application, it is probably because the option to bring them in is switched off.

• Activate the option by right clicking on the Geometry button on the Project Page, and then select Properties.

• Ensure Named Selections is checked, and the Selection Key is blank.

• Right-Click on Geometry and select “Update”

• Right-Click on Mesh and select “Refresh”




Training ManualProblem Geometry

cusps

Short edge

Narrow sliver faces

• The geometry includes many faces of various sizes and shapes and this would lead to problems for Patch Conforming methods. Virtual Topology could be an option to address this, but instead a Patch Independent approach will be used.




Training ManualSurface Meshes

refinement

small angles

• Patch Conforming Method• Short edges – unwanted

refinement and too many elements.

• Cusps and narrow faces – poor quality elements with small angles.

• Patch Independent Method• Mesh not constrained by

geometry.• Coarser mesh possible.• Elements are more regular

in size and shape.




Training ManualInserting the Mesh Method

9. Left click on Mesh in the tree and check its details• Make sure the Physics preference is set to Mechanical

– This ensures that the solid body will have a mesh suitable for structural mechanics

10.Assign the Meshing method:• Set the Cursor Mode to Body

• RMB (Window) select Select All

• RMB select Insert -> Method– Depending on your Workbench settings,

sometimes a default Mesh Method is already created for you. You can either delete it and create the new one, or modify its details as follows.

11.Set Details of Method• Set the Method to Tetrahedrons

• Set Algorithm to Patch Independent

• Set Max Element Size to 6 mm

• Set Min Size Limit to 2 mm

Cursor Modes




Training ManualSetting Element Size for Fluid Region

12. Body Sizing• Maintain the Cursor Mode of Body Selection

• Select the Fluid body

• RMB (Window) select Insert -> Sizing

13. Set Details of Body Sizing• Set Element Size to 5 mm




Training ManualAdding a Boundary Layer Mesh to the Fluid

19. Insert Inflation• Maintain the Cursor Mode of Body Selection

• Select the Solid Body

• RMB (Window) select Suppress Body

• RMB (Window) select Select All

• RMB (Window) select Insert Inflation

20. Set Details of Inflation• Set the Cursor Mode to Face selection

• To select Inflation on all wall surfaces:– Pick one face– Drop down the Extend

menu and Extend to Limits– Apply selection

• 126 Faces

• Change Inflation Option– Total Thickness

• Set Maximum Thickness to 5 mm




Training ManualVolume Mesh Generation

21. Mesh the Bodies• Set the Cursor Mode to Body selection

• RMB (in Window) select Unsuppress all Bodies

• RMB (in Tree) select Generate Mesh

• In Statistics set Mesh Metrics to Skewness– Around 150 000 elements were created with a worst

skewness of about 0.9




Training ManualReview Volume Mesh

22. Review the Mesh• Examine the surface mesh of the fluid region to see that it does not conform

to the underlying surface topology– You can Hide the solid mesh region to make this easier by right clicking on it in

the geometry part of the Tree

• Try using a Section Plane ( ) to look at the interior of the mesh




Training ManualConclusion

23. The mesh is now complete

• RMB (Tree) select Update– This ensures everything in the project so far is up to date

• Select File > Close Mesh to close the Mesh application

• In the WB panel select File > Save Project As… and give the project a name

• Exit from ANSYS Workbench by selecting File > Exit

• Note: Because the Mechanical mesh preference was used, the mesh elements will contain midside nodes. This is a benefit for mechanical analysiswhich can use higher order tetrahedral elements. If the mesh is then used for CFD, these midside nodes will be automatically ignored.

am 12 chapter 5

Documents

advanced size

min size limit

patch independent

growth rate

smooth transition

patch conforming

patch independent

patch conforming