Hex mesh using the Mesh, Geometry, Hex Mesh Solids command

Each of these steps is extremely important if you are going to succeed in creating a complete, correct hex mesh.

5.3.1.2 Subdividing the Solid

Once you are familiar with the types of solids that are hex-meshable you must divide your solid into these regions.

FEMAP offers a number of commands for this process.

They include the following commands all contained on the Geometry, Solid menu (refer to the commands manual for descriptions and use): Add, Remove, Common, Embed, Slice, Slice Match, Slice Along Face, Embed Face.

The Slice and Embed commands are particularly useful when attempting to create hex- meshable regions. If you need to clean up particular surfaces on solids you can use Geometry, Solid, Explode. You can then modify these sur-faces or create new sursur-faces with the surface modeling commands. The Geometry, Solid, Stitch forms the sursur-faces back into a solid.

5.3.2 Mesh Sizing

Consistent mesh sizing throughout the model is necessary for hex meshing. It is not possible to transition from a large number of elements to a small number of elements with hexahedral elements. Sizes of elements can change but the number must be consistent. This consistent sizing must propagate through the model, across the multiple solids that you have created. For this reason, local mesh sizing operations have little use in hex meshing. Global sizing and mapped surface approaches and surface linking are much more important.

The Mesh, Mesh Control, Size on Solid command (with the Hex Meshing option selected) is the primary mecha-nism to setup the necessary mesh sizing for successful hex meshing. Since many surfaces on your solids must be mapped meshed, curves on opposite sides of those surfaces must have the same number of element divisions. Once

you have properly subdivided your part, the Size on Solid command handles all sizing automatically. Simply spec-ify a nominal size.

If further mesh grading is required, or you want to modify the sizes that Size on Solid has created, you must use great care. If you manually change the mesh size along a curve, you must also manually change the mesh sizes (to the same settings) on all other curves in your solids that must match the first curve to maintain mapped meshable surfaces.

5.3.2.1 Ensuring Surface Linking

If you have subdivided your solid surface, linking is required to guarantee a continuous mesh. This is done when you specify the Hex Meshing size with Mesh, Mesh Control, Size on Solid.

If you select Hex Meshing in the Size For area, Adja-cent Surface Matching is checked and grayed.

FEMAP automatically looks at all surfaces in all selected solids and finds any coincident ones.

If Adjust Colors is checked, you can visualize which surfaces have been linked, and what solids are hex-meshable. Remove Previous Slaving will delete any surface approaches.

5.3.2.2 Specifying Sizes and Surface Approaches

FEMAP has the ability to use several different meshing approaches on surfaces via the Mesh, Mesh Control, Approach on Surface command. These approaches can be used to map mesh surfaces that would otherwise free mesh, and to match surface meshes. These methods are very useful in getting a solid to hex mesh or simply to get a better hex mesh. See Section 5, "Meshing" in FEMAP Commands for information on the different approaches that are available.

Note: It is important to remember that FEMAP will only look at the solids you select. If this command is run multiple times on different regions of the subdivided solid, the meshes will not match. To hex mesh the whole part, you must select all subdivisions at the same time.

Specifying individual mesh sizes on curves or surfaces in hex meshing is not recommended. The nature of hex meshing dictates that changes in the number of elements in one area must propagate throughout the model. FEMAP will not automatically update other sizes based on a change. You must do this manually. Be very careful, however, since because you can easily get a discontinuous hex mesh, or no hex mesh at all.

5.3.3 Hex Meshing

If you have properly subdivided your solid, and set mesh sizes and surface linking correctly the actual hex meshing is easy. Use Mesh, Geometry, Hex Mesh Solids and select the solids. The nodes on the linked surfaces can be auto-matically merged.

5.3.4 HexMesh From Elements

There are times when the hex meshing fails due to bad geometry or adjacent solids with different edge lengths. The command HexMesh From Elements provides a way to mesh geometry that has become corrupt thus causing the normal hex meshing technique to fail. This command allows you to individually map mesh the surfaces that make up the solid and then generate solid hexes from the surface mesh.

5.4 Midsurface Modeling and Meshing

Midsurfacing is a tool designed for use in certain instances. It is not a general purpose design tool. The solids used must be thin in relation to overall size, sheet metal parts or plastic injection molding parts are good examples.

Although, it is only useful for these type of “thin-walled” parts, it is an extremely valuable modeling tool for these parts. You can produce a much smaller and much more accurate model by meshing midsurfaces formed from a

“thin-walled” part, than solid meshing the thin-wall. Differences in model sizes can literally be an order of magni-tude in some cases, thereby significantly reducing run time. You even get the added benefit of a more accurate solu-tion in most cases.

The tools for creating midsurfaces are contained on the Geometry, Midsurface menu. Their operation is discussed more fully in the FEMAP Commands, but a general overview is provided below.

5.4.1 Creating Midsurfaces

The ease of midsurface creation depends greatly on the geometry of your model. Thin flat parts will be nearly com-pletely automatic. Parts with widely varying curvature, small features and/or fillets etc. will take longer and require Note: In addition to creating midsurfaces and meshing them, FEMAP provides another capability that can be

helpful in creating midsurface meshes on constant thickness parts. In this case, you can simply mesh the outer or inner surfaces of the part, and use the Modify, Move By, Offset Element command to move the elements to the midsurface. For more information, see Section 4.8.1.3, "Modify, Move By Menu" in FEMAP Commands.

Free Mesh

Four Corner Mapped Approach

manual work. Become familiar with FEMAP’s midsurfacing capabilities and attempt to prepare your model ahead of time for easy midsurface generation.

5.4.1.1 Automatic

• Fully automatic - the Geometry, Midsurface, Automatic command runs the three steps of the midsurface sequence. Be careful when running this command because the delete process may delete surfaces that you need, and they may be hard to recreate.

• Three-step automatic - Generate midsurfaces, Intersect them and then cleanup unnecessary midsurfaces. The Cleanup command, when run manually, does not actually delete the surfaces. It places them on a separate layer so you can review them to be sure you want to delete all of them. This approach is often better for very complex solids.

With either of these automatic approaches chances are good you will still need to do some manual cleanup unless the part is a simple thin-walled solid.

5.4.1.2 Manual

Manual midsurface creation and cleanup will involve all facets of geometry modeling in FEMAP. You need to be familiar with all curve, surface, and solid modification tools.

A good general approach for midsurfacing a model is provided below.