Controlling Which Mesher the Program Uses

You can also use the MOPT command to control which surface meshers (triangle and quadrilateral) and which tetrahedra mesher the program uses to perform a meshing operation [AMESH,VMESH].

The MOPT command also allows you to set the order of meshing so smaller areas are meshed first

(MOPT,AORDER,ON). For large or complex volumes (including those imported from CAD packages) where SMRTSIZE proves inefficient,MOPT,AORDER,ON can be used instead.

Note

Quadrilateral surface meshes will differ based on which triangle surface mesher is selected.

This is true because all free quadrilateral meshing algorithms use a triangle mesh as a starting point.

Command(s):MOPT

GUI: Main Menu> Preprocessor> Meshing> Mesher Opts

Note

The menu path provided above takes you to the Mesher Options dialog box. References to the Mesher Options dialog box appear throughout this section.

7.3.8.3.1. Surface Meshing Options

If you need to mesh multiple volumes, you should consider using the AORDER meshing option in the Mesher Opts dialog box (or issuing the MOPT,AORDER,ON command) so your mesh is created in the smallest area first. This helps ensure that your mesh is adequately dense in smaller areas and that the mesh is of a higher quality.

The following options for triangle surface meshing are available:

• Let the program choose which triangle surface mesher to use. This is the recommended setting and the default. In most cases, the program will choose the main triangle mesher, which is the Riemann space mesher. If the chosen mesher fails for any reason, the program invokes the alternate mesher and retries the meshing operation.

To choose this option, issue the command MOPT,AMESH,DEFAULT. In the GUI, access the Mesher Options dialog box and choose Program Chooses in the Triangle Mesher option menu.

• Main triangle surface mesher (Riemann space mesher). The program uses the main mesher, and it does not invoke an alternate mesher if the main mesher fails. The Riemann space mesher is well suited for most surfaces.

To choose this option, issue the command MOPT,AMESH,MAIN. In the GUI, access the Mesher Options dialog box and choose Main from the Triangle Mesher option menu.

• First alternate triangle surface mesher (3-D tri mesher). The program uses the first alternate triangle mesher, and it does not invoke another mesher if this mesher fails. This option is not recommended due to speed considerations. However, for surfaces with degeneracies in parametric space, this mesher often provides the best results. We also recommend that you use this mesher when meshing highly anisotropic regions.

To choose this option, issue the command MOPT,AMESH,ALTERNATE. In the GUI, access the Mesher Options dialog box and choose Alternate from the Triangle Mesher option menu.

• Second alternate triangle surface mesher (2-D parametric space mesher). The program uses the second alternate triangle mesher, and it does not invoke another mesher if this mesher fails. This option is not

recommended for use on surfaces with degeneracies (spheres, cones, and so on) or on poorly paramet-erized surfaces because poor meshes may be created.

To choose this option, issue the command MOPT,AMESH,ALT2. In the GUI, access the Mesher Options dialog box and choose Alternate 2 from the Triangle Mesher option menu.

The options listed below are available for quadrilateral surface meshing. Keep in mind that quadrilateral surface meshes will differ based on which triangle surface mesher is selected. This is true because all free quadrilateral meshing algorithms use a triangle mesh as a starting point.

• Let the program choose which quadrilateral surface mesher to use. This is the recommended setting and the default. In most cases, the program will choose the main quadrilateral mesher, which is the Q-Morph (quad-morphing) mesher. For very coarse meshes, the program may choose the alternate quadrilateral mesher instead. If either mesher fails for any reason, the program invokes the other mesher and retries the meshing operation.

To choose this option, issue the command MOPT,QMESH,DEFAULT. In the GUI, access the Mesher Options dialog box and choose Program Chooses from the Quad Mesher option menu.

• Main quadrilateral surface mesher (Q-Morph mesher). The program uses the main mesher, and it does not invoke the alternate mesher if the main mesher fails.

In most cases, the Q-Morph mesher results in higher quality elements (see Figure 7.13: Quadrilateral and Q-Morph Mesher (p. 113)). The Q-Morph mesher is particularly beneficial to users whose applications require boundary sensitive, highly regular nodes and elements.

Figure 7.13: Quadrilateral and Q-Morph Mesher

Mesh (a) shows a surface that was meshed with the alternate quadrilateral mesher; mesh (b) shows the same surface, this time meshed with the Q-Morph mesher.

Notice that although both meshes shown contain one triangle element (the triangle elements are shaded in the figure), the triangle element in Figure (a) occurs on the boundary of the area. The triangle element in Figure (b) is an internal element, which is a more desirable location for it in the mesh.

For the Q-Morph mesher to be able to generate an all-quadrilateral mesh of an area, the total number of line divisions on the boundaries of the area must be even. (In most cases, turning on SmartSizing [SMRTSIZE,SIZLVL] will result in an even total number of line divisions on the boundaries.)

A triangle element (or elements) will result in the area mesh if any of these statements is true:

1. The total number of line divisions on the boundaries of the area is odd.

2. Quadrilateral element splitting is turned on for error elements [MOPT,SPLIT,ON or MOPT,SPLIT,ERR] and a quadrilateral element in violation of shape error limits would be created if the program did not split the element into triangles. (Splitting is on for error elements by default.)

3. Quadrilateral splitting is turned on for both error and warning elements [MOPT,SPLIT,WARN], and a quadrilateral element in violation of shape error and warning limits would be created if the program did not split the element into triangles.

4. Quadrilateral element splitting is turned on for either a) error elements or b) error and warning elements, and the area contains a small angle (< 30°) between adjacent boundary intervals. See Figure 7.14: Results of Quadrilateral Splitting (p. 114).

To choose this option (Q-Morph mesher), issue the command MOPT,QMESH,MAIN. In the GUI, access the Mesher Options dialog box and choose Main from the Quad Mesher option menu.

Figure 7.14: Results of Quadrilateral Splitting

(a) Quad splitting on; Results in triangle element

(b) Quad splitting off; Results in quadrilateral element Small angle

Small angle

• Alternate quadrilateral surface mesher. The program uses the alternate mesher, and it does not invoke the main mesher if the alternate mesher fails.

For this mesher to be able to generate an all-quadrilateral mesh of an area, the total number of line divisions on the boundaries of the area must be even, and quadrilateral splitting must be turned off [MOPT,SPLIT,OFF].

To choose this option, issue the command MOPT,QMESH,ALTERNATE. In the GUI, access the Mesher Options dialog box and choose Alternate in the Quad Mesher option menu. To use this mesher, you must also select either the first alternate or the second alternate triangle surface mesher.

7.3.8.3.2. Tetrahedral Element Meshing Options

The following options for tetrahedral element meshing are available:

• Let the program choose which tetrahedra mesher to use. This is the default. With this setting, the program uses the main tetrahedra mesher when it can; otherwise, it uses the alternate tetrahedra mesher.

To choose this option, issue the command MOPT,VMESH,DEFAULT. In the GUI, access the Mesher Options dialog box and choose Program Chooses in the Tet Mesher option menu.

• Main tetrahedra mesher (Delaunay technique mesher). For most models, this mesher is significantly faster than the alternate mesher.

To choose the main tetrahedra mesher, issue the command MOPT,VMESH,MAIN. In the GUI, access the Mesher Options dialog box and choose Main in the Tet Mesher option menu.

• Alternate tetrahedra mesher (advancing front mesher from Revision 5.2). This mesher does not support the generation of a tetrahedral volume mesh from facets [FVMESH]. If this mesher is selected and you issue the FVMESH command, the program uses the main tetrahedra mesher to create the mesh from facets and issues a warning message to notify you.

To choose the alternate tetrahedra mesher, issue the command MOPT,VMESH,ALTERNATE. In the GUI, access the Mesher Options dialog box and choose Alternate in the Tet Mesher option menu.