WORKSHOP 10 HEX VS TET SOLID ELEMENT MESH

WORKSHOP 10 HEX VS TET SOLID ELEMENT MESH VS WS10-1

WS10-2

Problem Description This workshop is for creating a tetrahedral and hexahedral element mesh for a geometric solid. The tetrahedral mesh can be created quickly and easily using the standard tet mesher TetMesh. Creating the hexahedral mesh involves doing much more work. The task involves dividing the geometry into two parts. Groups are used to segregate the parts. The hex mesh for the first part is created by creating a combination of IsoMesh and Paver meshes, then sweeping them to create the hex solid elements. The hex elements for the second part are created by using the mesher IsoMesh. First, the blue colored parametric solids must be created. The techniques used are simple. There is one difficulty associated with creating two solids; they are very skewed, and this affects the meshing. The hex element meshes are combined, and the continuity of them is investigated. WS10-3

Suggested Exercise Steps 1. Create a database. 2. Import a parasolid file with single solid. 3. Create a tet mesh using TetMesh; the quality of the elements is looked at. 4. Close the database, and create a new one. 5. Import the parasolid file again. 6. Disassemble the solid and remove the unwanted surface vertices. As finding all of the vertices can become an arduous task delete the surfaces and solid, and import a different model file with already prepared surfaces. 7. Create a plane to break the surfaces into two groups, then break the surfaces with the plane. 8. After breaking the surfaces place each set into a different group. 9. Begin with the base model, and create a parametric surface to fill the hole. 10. Create mesh seeds for first two surfaces 11. Create Quad4 elements, at the base, to be swept. WS10-4

Suggested Exercise Steps (continued) 12. Create Hex8 Elements by Sweeping Quad4s at the base. 13. Create Seeds, IsoMesh, Then Sweep to Hex8s. 14. Create Mesh Seeds in Preparation for Paver Mesh. 15. Create Paver Mesh Using Seeds. 16. Complete Sweeping Quads to Create Hexs. 17. Delete Quad Elements and Connect Hex Elements. 18. Post Group Top With Surfaces for creating parametric Solids. 19. Create Parametric Solids From parametric Surfaces. 20. Create Curves to be Used for Creating Surfaces. 21. Create Parametric Surfaces From Curves. 22. Create Parametric Solids From Surfaces. 23. Display Both Parts of the Model. 24. Create Mesh Seeds to force Congruent Hex Meshes. WS10-5

Suggested Exercise Steps (continued) 25. Create Hex Meshes for the Top Part of Model. 26. Connect Hexs in Top Part of Model. 27. Determine Quality of Hex Mesh. WS10-6

Step 1. Create a Database a d Create a new database. a. File/New. b. Enter tet as the file name. c. Click OK. d. Choose Based on Model for Tolerance. e. Select MSC.Nastran as the Analysis Code. f. Select Structural as the Analysis Type. g. Click OK. b c g f e WS10-7

Step 2. Import Parasolid File a c d Import the parasolid.xmt file. a. File / Import. b. Change the Source to Parasolid.xmt. c. Select part.xmt. d. Click on Apply. e. Click on OK when the import summary appears. WS10-8 e

Step 3. Create a TetMesh Mesh a TetMesh the solid. a. Elements: Create/Mesh/Solid. b. Set Elem Shape to Tet. c. Set Mesher to TetMesh. d. Under Input List select Solid 1. e. Under Global Edge Length enter 5.0. f. Click Apply. g. Change to Iso 3 view and Smooth shaded. d c WS10-9

Step 3. Create a TetMesh Mesh (Cont.) Check the quality of the elements a. Elements: Verify/ Tet/All. b. Click on Apply. a Test Aspect Edge Angle Face Skew Number Failed 0 3020 3381 Worst Case Max = 4.0146294 Max = 77.154922 Max = 80.138222 At Element 6420 4599 6420 Collapse 0 Min = 0.063722752 6420 This method was straightforward and quite simple. It can be observed that 6420 Tet10 Elements were created. Normal Offset Tangent Offset Jacobian Ratio Jacobian Zero 131 5 0 0 Max = 0.26516628 Max = 0.33153021 Max = 9.2409439 Min = 0.20035997 3033 5181 3623 2050 b WS10-10

Step 4. Close the DB and Create a new DB e Create a database and specify the New Model Preferences. a. File/Close. b. File/New c. Enter hex8 as the file name. d. Click OK. e. Choose Based on Model for Tolerance. f. Select MSC.Nastran as the Analysis Code. c d h g f g. Select Structural as the Analysis Type. h. Click OK. WS10-11

Step 5. Import the Parasolid File Again a b Import the parasolid file part.xmt. a. File/Import. b. Select part.xmt. c. Click on Apply. d. Click on OK when the import summary appears. c WS10-12 d

Step 6. Disassemble the Parasolid Solid Prepare the solid so that a hex mesh can be created. Start by disassembling the parasolid. a. Geometry: Edit/Solid/ Disassemble. b. For Solid List select Solid 1. c. Click on Apply. d. Select Yes when asked to delete the original solids. e. Click on the Label Control icon and turn on the surface labels. b a c WS10-13

Step 6. Disassemble the Parasolid Solid (Cont.) Note that the surfaces are trimmed (indicated by the magenta lines). Where possible create parametric surfaces (green) from trimmed surfaces. To do this do: a. Geometry: Create/Surface/Curve b. Geometry: Edit/Surface/ Remove Vertex It may be difficult to remove all the vertices that should be, because of the difficulty in identifying them. Therefore, delete the model and import an IGES file with all the surfaces already edited. c. Delete the model. d. File/Import e. Change the Source to IGES. f. Select the file surfaces.igs. g. Click on Apply. b a Surfaces from surfaces.igs In order to simplify the exercise the newly imported surfaces are already edited. WS10-14

Step 7. Create a Plane to Break the Surfaces Break the surfaces to create two regions for creating hex8 elements. A plane * is used to break the surfaces. a. Geometry: Create/Plane/ 3 Points. b. For Point 1 List select Point 17. c. For Point 2 List select Point 18. d. For Point 3 List select Point 5. b c d * * WS10-15

Step 7. Create a Plane to Break the Surfaces (Cont.) Break the surfaces with the newly created plane. a. Geometry: Edit/Surface/ Break. b. Option: Plane. c. Surface List: select all surfaces. d. Break Plane List: select Plane 1. e. Select Yes for All when asked to delete original surfaces. c b a d WS10-16

Step 8. Place each set of Surfaces Into Different Groups a Create two groups for different regions of the model. Start with the base region of the model. a. Group/Create. b. For New Group Name enter base. c. Check Unpost All Other Groups. d. Go to Preferences/Picking and change the Rectangle/Polygon Picking to Enclose entire entity e. Change to the Right side view f. For Entity Selection include the surfaces shown in the figure. g. Click on Apply. b c f f WS10-17

Step 8. Place each set of Surfaces Into Different Groups (Cont.) Shown here is the base region of the model. The entities in the two regions of the model are to be placed in different groups to facilitate meshing. WS10-18

Step 8. Place each set of Surfaces Into Different Groups (Cont.) b Post the group default_group and create a group for the remaining region of the model. a. Post the group default_group. b. Group/Create. c. For the New Group Name enter top. d. Check Unpost All Other Groups. e. Go to Preferences/Picking and change the Rectangle/Polygon Picking to Enclose any portion of entity f. Switch to the Right side view and under Entity Selection include the surfaces shown in the figure. c d f g. Click on Apply. f WS10-19

Step 8. Place each set of Surfaces Into Different Groups (Cont.) WS10-20

Step 9. Create a Parametric Surface to Fill Hole Enclose the base region with surfaces by creating a parametric surface where the plane is. a. Post the group base. b. Go to the Iso 1 view. c. Geometry: Create/ Surface/Curve. d. Set the Option to 2 Curve. e. Under Starting Curve List select Surface 22.3. f. Under Ending Curve List select Surface 13.2. c d e f WS10-21

Step 10. Create Mesh Seeds for First two Surfaces Create a mesh seed for the base so that a quad mesh can be applied (the quad mesh will later be swept to create a hex8 mesh). a. Elements: Create/Mesh Seed/ Uniform. b. Enter 9 for the Number. c. For Curve List select Surface 13.1. d. Set the Number to 3 and select Surface 26.1 for the Curve List. e. Set the Number to 4 and select Surface 26.2 and Surface 8.4 for the Curve List. b a c WS10-22

Step 11. Create Quad4 Elements to be Swept a Mesh a surface using the Paver. a. Elements: Create/Mesh/ Surface. b. Set Elem Shape to Quad. c. Set Mesher to Paver. d. Set Topology to Quad4. e. For Surface List select Surface 13. f. Remove the check for Automatic Calculation, and enter 3.0 for Global Edge Length. g. Click on Apply. e c f WS10-23

Step 11. Create Quad4 Elements to be Swept (Cont.) Mesh another surface using IsoMesh. a. Elements: Create/Mesh/ Surface. b. Set Elem Shape to Quad. c. Set Mesher to IsoMesh. d. Set Topology to Quad4. e. For Surface List select Surface 26. f. Remove the check for Automatic Calculation, and enter 3.0 for the Global Edge Length. g. Click on Apply. WS10-24

Step 11. Create Quad4 Elements to be Swept (Cont.) Here are the surface meshes for part of the base. The meshes will be swept to create a hex mesh for the base part of the model. WS10-25

Step 12. Create Hex8 Elements by Sweeping Quad4s Sweep the surface meshes to the first section. a. Elements : Sweep/ Element/Extrude. a b. Under Mesh Control, enter 3 for Number. c. Click on OK. d. Select on Direction Vector and select the Tip and base points icon. Then, select Tip and Base points as shown in the figure. b e. For selecting the Base Entity List entities use the Meshed entity icon, then the Meshed Surface icon. f. Select Surfaces 13 and 26 for the Base Entity List. g. Click on Apply. d e f d g WS10-26

Step 12. Create Hex8 Elements by Sweeping Quad4s (Cont.) WS10-27

Step 12. Create Hex8 Elements by Sweeping Quad4s (Cont.) Sweep the free faces of the hex elements to the next section. a. Elements : Sweep/Element/ Extrude. b. Enter 1 for Number under Mesh Control, and click OK. c. Click on Direction Vector and select on Tip and base points icon. Then, select Tip and Base points as shown in the figure. d. Go to Preferences/Picking and select Enclose entire entity. e. Change to Right side view and click on the Element s face icon. f. Under Base Entity List select the element faces as shown in the figure. g. Click Apply. f c c f WS10-28

Step 12. Create Hex8 Elements by Sweeping Quad4s (Cont.) The elements have been swept twice to create the model illustrated below. Note that the nodes line up as they should. WS10-29

Step 13. Create Seeds, IsoMesh, Then Sweep to Hex8s a IsoMesh the surface in a corner (Surface 18) to create elements that will be swept. First, because of the curvature (90 degree bend) it is necessary to create mesh seeds. a. Switch to the Iso 1 view and zoom into Surface 18. b. Elements: Create/Mesh Seed/ Uniform. c. Enter 4 for the Number. d. For Curve List select Surface 7.2. d WS10-30

Step 13. Create Seeds, IsoMesh, Then Sweep to Hex8s (Cont.) Mesh another surface using IsoMesh, then sweep the elements created. a. Elements: Create/Mesh/ Surface. b. Set Elem Shape to Quad. c. Set Mesher to IsoMesh. d. Set Topology to Quad4. e. For Surface List select Surface 18. f. Remove the check for Automatic Calculation, and enter 3.0 for the Global Edge Length. g. Click on Apply. e e h. Now, sweep the mesh just created Elements: Sweep/Element/ Extrude. i. Enter 1 for Number under Mesh Control, and click OK. j. Click on Direction Vector and select on Tip and base points icon. Then, select Tip and Base points as shown in the figure. j j WS10-31

Step 13. Create Seeds, IsoMesh, Then Sweep to Hex8s (Cont.) Finish sweeping the elements. a. Elements : Sweep/ Element/Extrude. b. For selecting the Base Entity List entities use the Meshed entity icon, then the Meshed Surface icon. c. Select Surface 18 for the Base Entity List. d. Click on Apply. WS10-32

Step 14. Create Mesh Seeds in Preparation for Paver Mesh Paver mesh Surface 12 so that the mesh can be swept to create hex elements for part of the base. The mesh to be created must match that already created by sweeping. Mesh seed are to be created using existing nodes. a. Elements: Create/Mesh Seed/ Tabular. b. For the Coordinate Type select Node and Point. c. Under the Nodes or Points List select the Nodes as shown in the figure. d. Select Surface 12.2 under Curve List. e. Click on Apply. d b a c d c e WS10-33

Step 14. Create Mesh Seeds in Preparation for Paver Mesh (Cont.) WS10-34

Step 15. Create Paver Mesh Using Seeds Mesh the surface where one of its edges was just seeded. a. Elements : Create/Mesh/ Surface. b. For Elem Shape select Quad. c. For Mesher select Paver. d. For Topology select Quad4. e. Select Surface 12 for Surface List. f. Use 3.0 for Global Edge Length g. Click on Apply. WS10-35

Step 15. Create Paver Mesh Using Seeds (Cont.) WS10-36

Step 16. Complete Sweeping Quads to Create Hexs Complete sweeping element faces and 2D quad elements to the side of the model. a. Elements: Sweep/Element/ Extrude. b. Go to Mesh Control and enter 8 for Number, and click OK. c. Click on Direction Vector and select on Tip and Base points icon. Then, select the Tip and Base points as shown in the figure. d. Under Base Entity List click on the Element s Face icon. e. Switch the view to Top view, and select the elements as indicated. f. Click on Apply. c c d e WS10-37

Step 16. Complete Sweeping Quads to Create Hexs (Cont.) WS10-38

Step 16. Complete Sweeping Quads to Create Hexs (Cont.) Complete sweeping 2D quad elements to the side of the model. a. Elements: Sweep/Element/ Extrude. b. Go to Mesh Control and enter 8 for Number, and click OK. c. Click on Direction Vector and select on Tip and Base points icon. Then, select the Tip and Base points as shown in the figure. d. Under Base Entity List click on the 2D element icon. e. Switch the view to Top view, and select the elements as indicated. f. Click on Apply. d c c WS10-39 e

Step 16. Complete Sweeping Quads to Create Hexs (Cont.) WS10-40

Step 17. Delete Quad Elements and Connect Hex Elements Delete 2D elements, then equivalence. a. Elements: Delete/Element. b. Elements: Equivalence/All/Tolerance Cube. Delete 2D Elements Equivalence WS10-41

Step 17. Delete Quad Elements and Connect Hex Elements (Cont.) Completed hex mesh for the base part of the model. Now, verify that all the hex elements are connected. a. Elements: Verify/Element/Boundaries Free Edges Copyright 2004 MSC.Software Corporation WS10-42

Step 18. Post Group Top With Surfaces for Blue Solids a Create parametric solids(blue) for the remainder of the model, and IsoMesh them with hex elements. Post the group top. a. Group/Post. b. Under Select Groups to Post select top. c. Click on Apply, then Cancel. d. Go to the Iso 1 view. Copyright 2004 MSC.Software Corporation b WS10-43

Step 19. Create Parametric Solids From Surfaces a Create the first parametric solid using two surfaces a. Geometry: Create/Solid/ Surface. b. Set the Option to 2 Surface. c. Select Surface 23 for the Starting Surface List. d. Select Surface 3 for the Ending Surface List. b c d Copyright 2004 MSC.Software Corporation WS10-44

Step 19. Create Parametric Solids From Surfaces (Cont.) a Create the second solid. a. Geometry: Create/Solid/ Surface b. Select Surface 17 for the Starting Surface List. c. Select Surface 2 for the Ending Surface List. Solids 1 and 2 have been created. b c Copyright 2004 MSC.Software Corporation WS10-45

Step 20. Create Curves to be Used for Creating Surfaces a To create the remaining solids it will be necessary to create some curves, surfaces from the curves, then solids from the surfaces. a. Geometry: Edit/Curve/ Break. b. Rotate the view to get the following view of the model. c. Set the Option to Parametric. d. Enter 0.7 for the Break Point. e. Select Surface 4.2 and 4.4 for the Curve List. f. Increase the point size. Copyright 2004 MSC.Software Corporation e c d e e WS10-46

Step 20. Create Curves to be Used for Creating Surfaces (Cont.) Break curves again to obtain all needed curves. a. Geometry: Edit/Curve/Break. b. Set the Option to Parametric. c. Set the Break Point to 0.5. d. Under Curve List select Curve 1 and 3. e. Click on Apply. Copyright 2004 MSC.Software Corporation WS10-47

Step 21. Create Parametric Surfaces From Curves Create surfaces using the curves just created and previously existing surface or solid edges. a. Geometry: Create/Surface/ Curve. b. Option: 2 Curve. c. Select curves or edges for Starting Curve List and Ending Curve List. Starting Curve List Ending Curve List Surface 1.4 Curve 2 Surface 1.2 Curve 4 Solid 2.2.3 Curve 6 Solid 2.2.1 Curve 8 Surface 5.4 Curve 5 Surface 5.2 Curve 7 Copyright 2004 MSC.Software Corporation WS10-48

Step 22. Create Parametric Solids From Surfaces Create the parametric solids. a. Geometry: Create/Solid/ Surface. b. Set Option to 2 Surface. c. Select the surfaces, as specified in the table, for Starting Surface List and Ending Surface List. Starting Surface List Ending Surface List Surface 27 Surface 28 Surface 29 Surface 30 Surface 31 Surface 32 Copyright 2004 MSC.Software Corporation WS10-49

Step 24. Create Mesh Seeds for Congruent Hex Meshes IsoMesh the five new parametric solids. It is desired to have the hex mesh in Solid 1 and the base part of the model congruent. The fact that there is a 2D IsoMesh associated to Surface 26 should control the IsoMesh of Solid 1. However, Solid 1 is very distorted. This will prevent the mesh created in it from being closely congruent to the mesh on Surface 26. So, mesh seeds are to be created on the other two edges of Surface 26. a. Elements: Create/Mesh Seed/ Tabular. b. Select the Node and Point toggle. c. Select the nodes shown in the figure. It may be easier to select the nodes by erasing the five solids that were just created. See the next page. Copyright 2004 MSC.Software Corporation WS10-52

Step 24. Create Mesh Seeds for Congruent Hex Meshes (Cont.) Continuation of creating mesh seed using tabular approach. a. Select the Node and Point toggle. b. Select the nodes shown in the figure. c. Select the two edges for Surface 26 indicated by the red lines. Surface 13 adjacent to Surface 26 so its edges will be picked. d. Before clicking Apply the tolerance needs to be changed because of the distorted shape of Solid 1. It is recommended to use 0.05 instead of the default 0.005. e. Apply. Copyright 2004 MSC.Software Corporation WS10-53

Step 25. Create Hex Meshes for Top Part of Model IsoMesh the five new solids a. Elements: Create/Mesh/Solid b. Select Hex for Elem Shape c. Select IsoMesh for Mesher d. Under Solid List select Solid 1:5 e. Specify 3.0 for Global Edge Length f. Apply Copyright 2004 MSC.Software Corporation WS10-54

Step 25. Create Hex Meshes for Top Part of Model (Cont.) IsoMesh issues an error message. Meshing Solid 1 with the constraint from the mesh on Surface 26, and the solid s poor shape has caused the failure. This is seen on the next pages. Solid 1.1.3 Solid 1.2.3 Copyright 2004 MSC.Software Corporation WS10-55

Step 25. Create Hex Meshes for Top Part of Model (Cont.) Copy the five solids, Solid 1:5, away from the model. Copy the mesh on Surface 26 to the end of Solid 6, and associate it to the end of the solid, Solid 6.3. IsoMesh all five solids, Solid 6:10. The same error message is issued as before. Delete the 2D mesh associated to the end of Solid 6, and IsoMesh all five solids. As can be seen all the solids were meshed. So, the mesh on Surface 26 is causing the problem. However, it is worth while to see that the shape of Solid 1 is related to the problem. This is shown on the next page. Copyright 2004 MSC.Software Corporation WS10-56

Step 25. Create Hex Meshes for Top Part of Model (Cont.) Reintroduce the 2D mesh on the end of Solid 6. Then, extrude the same end of Solid 6 to create a parametric solid, Solid 11. IsoMesh Solid 11. The meshing succeeded. So, given Solid 1 as it is, the 2D mesh on Surface 26 is preventing the meshing from occurring. However, the shape of Solid 1, given the 2D mesh on Surface 26, is preventing the meshing from occurring. Is this the complete story? Proceed to find out. Copyright 2004 MSC.Software Corporation WS10-57

Step 25. Create Hex Meshes for Top Part of Model (Cont.) IsoMesh just Solid 1. a. Elements: Create/Mesh/Solid b. Select Hex for Elem Shape c. Select IsoMesh for Mesher d. Under Solid List select Solid 1 e. Specify 3.0 for Global Edge Length f. Apply So, Solid 1 meshed! Try meshing other solids. g. h. i. j. k. l. Under Solid List select Solid 2 Specify 3.0 for Global Edge Length Apply Under Solid List select Solid 3:5 Specify 3.0 for Global Edge Length Apply So, the problem was related to Solid 1 and 2, and perhaps others. Copyright 2004 MSC.Software Corporation WS10-58

Step 26. Connect Hexs in Top Part of Model Connect the hex elements. a. Elements: Equivalence/All/ Tolerance Cube b. This model has some possible problems, so set the equivalencing tolerance to 1.0. c. Apply It is detected that the tolerance is > the smallest element edge. It is reset so it is slightly < the smallest element edge. Copyright 2004 MSC.Software Corporation WS10-59

Step 26. Connect Hexs in Top Part of Model (Cont.) Determine if all the hex elements are connected. First look at element free edges. Then, look at element free faces. a. Elements: Verify/Element/Boundaries Free Edges b. Apply c. Display just free faces using Display/Finite Elements Show Only Free Faces (In Wireframe) Copyright 2004 MSC.Software Corporation WS10-60

Step 26. Connect Hexs in Top Part of Model (Cont.) Look for any free faces at just certain geometric interfaces. To do this create 2D quad elements on element free faces. a. Elements: Create/Element/Edit b. Shape: Quad c. Topology: Quad4 d. Pattern: Elem Face e. Select Free face of element picking icon f. Under Face select any desired hex element free faces g. Erase everything except the 2D quad elements just created. h. Rotate the model so that can see if there are any hex free faces where there shouldn t be. Copyright 2004 MSC.Software Corporation WS10-61