HM-3200: Tetramesh

In this step you will open and view the model file, housing.hm.

1. Start HyperMesh Desktop.
2. From the menu bar, click File > Open > Model.
3. In the Open Model dialog, open the housing.hm model file.
4. Observe the model using the different visual options available in HyperMesh (rotation, zooming, and so on). Only the geometry in the component cover is currently displayed. The file contains two parts defined by a volume of surfaces. The geometry has been cleaned such that surface connectivity is proper and surface edges that would cause sliver elements are suppressed.

In this step you will use the volume tetra mesher and equilateral triangles to create a tetra mesh for the cover.

1. Open the Tetramesh panel by clicking Mesh > Create > Tetra Mesh from the menu bar.
2. Go to the Volume tetra subpanel.
3. Set the Enclosed volume selector to surfs.
4. Select one of the surfaces in the model.
5. Verify that the 2D type is set to trias and the 3D type is set to tetras. These options control the type of elements that HyperMesh creates for the surface mesh and solid mesh of the part.
6. Verify that the toggle is set to Elems to Currrent Comp.
7. Verify that the Use curvature and Use proximity checkboxes are clear.
8. In the Element size field, type 10.
9. Click mesh.
   The tetra mesh is created.
10. If the model's mesh lines and elements are not shaded, click on the Visualization toolbar.
11. Inspect the mesh pattern that the volume tetra mesher created.

    
    
    Figure 1.

12. Reject the mesh by clicking reject.

In this step you will use the volume tetra mesher and right triangles to create a tetra mesh for the cover.

1. Select one of the surfaces in the model.
2. Set the 2D type to R-trias.
3. Click mesh.
   The tetra mesh is created.
4. Inspect the mesh pattern that the volume tetra mesher created and compare it to the first mesh that you created.
   Note: The 2D type: R-trias setting tends to create tetra elements with triangular faces that are right triangles (90-45-45 angles), while the 2D type: trias setting tends to create equilateral triangles (60-60-60 angles).

   
   
   Figure 2.

5. Reject the mesh by clicking reject.

In this step you will use the volume tetra mesher to create a tetra mesh with more elements along curved surfaces.

1. Select one of the surfaces in the model.
2. Select the Use curvature checkbox.
   The option causes more elements to be created along areas of high surface curvature. Thus, curved areas such as fillets will have more and smaller elements, which capture those features with higher resolution.
3. In the Min elem size field, type 1.0.
4. Verify that the Feature angle is set to 30.
5. Click mesh.
   The tetra mesh is created.
6. Inspect the mesh pattern that the volume tetra mesher created and compare it to the previous meshes you created. You can see that more small elements are created around the fillets.

   
   
   Figure 3.

7. Reject the mesh by clicking reject.

In this step you will use the volume tetra mesher to create a tetra mesh with more elements around small features.

1. Select one of the surfaces in the model.
2. Select the Use proximity checkbox.
   This option causes the mesh to be refined in areas where surfaces are smaller, which results in a nice transition from small elements on small surfaces to larger elements on larger, adjacent surfaces.
3. Click mesh.
   The tetra mesh is created.
4. Inspect the mesh pattern that the volume tetra mesher created and compare it to the previous meshes you created. You can see that more elements are created around the surfaces with small angles.

   
   
   Figure 4.

In this step you will prepare the display to tetra mesh the hub component using the standard tetra mesher.

1. In the Model Browser, turn off the display of every component's geometry except for hub.
2. Turn off the display of every component's elements except for hub and tetras.
   Note: There are tria shell elements in the hub component and no elements in the tetras component.
3. Return to the main menu by clicking return.

In this optional step, you will review the connectivity and quality of the tria mesh to validate its integrity for the standard tetra mesher.

1. Open the Edges panel by clicking Mesh > Check > Components > Edges from the menu bar.
2. Verify that the entity selector is set to comps.
3. Select a tria element on the hub component.
   The entire component highlights.
4. Click find edges.
   The status bar displays a message that reads, "No edges were found. Selected elements may enclose a volume."

   Note: The tetra mesher requires a closed volume of shell elements.
5. Return to the main menu by clicking return.
6. Open the Check Elements panel by clicking Mesh > Check > Elements > Check Elements from the menu bar.
7. Verify that you are in the 2-d subpanel.
8. Identify the elements that have an aspect ratio greater than 5. Aspect ratio is the ratio of the longest edge of an element to its shortest edge. This check helps you to identify sliver elements. All of the hub’s shell elements pass the check; all of the elements have an aspect ratio less than 5.
9. Identify the tria elements that have an angle less than 20. This check helps identify sliver elements. All of the hub’s shell elements pass the check; all the elements have angles greater than 20. The surface mesh is suitable for creating a tetra mesh.
10. Return to the main menu by clicking return.

In this step you will create a tetra mesh for the hub using the standard tetra mesher.

1. In the Model Browser, Component folder, right-click on tetras and select Make Current from the context menu.
2. Open the Tetra Mesh panel.
3. Go to the Tetra mesh subpanel.
4. Verify that the Float trias/quads to tetra mesh entity selector is set to comps.
   Note: By using this entity selector, HyperMesh will swap the diagonal for any pair of surface trias, which will result in a better tetra mesh quality. If you would rather keep the diagonal, see step 8.6.
5. Select a shell element on the hub component.
   The entire component is highlighted.
6. Keep the diagonal as is by activating the Fixed Trias/quads to tetra mesh entity selector and setting it to comps.
7. Click mesh.
   Tetrahedral elements are generated.

   
   
   Figure 5.

In this step you will check the quality of the hub's tetra elements.

1. In the Model Browser, only display the tetras component elements.
2. Open the Check Elements panel.
3. Go to the 3-d subpanel.
4. Identify the smallest element length among the displayed elements. If the minimum length is acceptable for a target element size of 5.0, then no further action is necessary.
5. Identify the smallest angle (tria faces: min angle) among the displayed elements. If the minimum tria face angle is no less than 10°, then the mesh quality should be acceptable.
6. Identify elements that have a tet collapse smaller than 0.3. The status bar indicates that three elements have a tetra collapse smaller than 0.3.
   Note: The tet collapse criteria is a normalized volume check for tetrahedral elements. A value of 1 indicates a perfectly formed element with maximum possible volume. A value of 0 indicates a completely collapsed element with no volume.

In this step you will isolate the element with the tetra collapse smaller than 0.2 and find the elements surrounding it.

1. With 0.3 still specified in the tet collapse< field, click tet collapse.
2. Click save failed.
   HyperMesh saves the elements that failed the tetra collapse check in the user mark.

   Note: You can retrieve the saved elements that failed the check from any panel by selecting retrieve in the extended selection menu.
3. Return to the main menu by clicking return.
4. Open the Mask panel by pressing F5.
5. Set the entity selector to elems.
6. Click elems >> retrieve.
   HyperMesh retrieves the elements that were saved in the Check Elements panel.
7. Click elems >> reverse.
8. Click mask.
   HyperMesh masks the elements and displays the three tetra element that failed the tetra collapse check.
9. Return to the main menu by clicking return.
10. On the Display toolbar, click .
    HyperMesh identifies and displays the layer of elements that are attached to the three displayed elements.
11. Click two more times.
    HyperMesh identifies and displays the layers of elements that are attached to the displayed elements.

    Note: You can duplicate the functionality of unmask adjacent using the Find panel, find attached subpanel in the Tool page.
12. In the Model Browser, turn off the display of the hub elements that were unmasked.

In this step you will remesh the hub's displayed tetra elements to improve their tetra collapse.

1. Open the Tetra Mesh panel.
2. Go to the Tetra remesh subpanel.
3. Click 3D elements: elems >> displayed.
4. Click remesh.
   HyperMesh regenerates this area of the mesh.
5. Return to the main menu by clicking return.
6. Open the Check Elements panel.
7. Find out if the tetra collapse has improved for the displayed elements by clicking tet collapse. The status bar indicates that the minimum tetra collapse is larger than the value reported before the tetra elements were remeshed.
8. Return to the main menu by clicking return.

In this optional step you will save your work.