METBD 450 Lecture Notes

Chapter 5: CAD Modeling for FEA
PART 3: DEVELOPING GEOMETRY SPECIFICALLY FOR FEA

Text: Building Better Products with FEA,
by V. Adams & A. Askenazi, (Read pp. 208-218, 247)

ANSYS Modeling and Meshing Guide, Chapter 6

START SIMPLE

Consider "TOPOLOGICAL OPTIMIZATION" (also called "Layout Optimization") = BEST Use of material

Avoid "commitment" as long as possible

First model: learn the most you can as quickly as possible

Why Simplify the geometry, element types, meshing, loading ?

Don't forget to plan for assembly features, just like for loading, to make sure "surface patches" are defined in advance of being needed.

Automatic Optimization

Probabilistic Design

Mapped Meshing

Forcing a particular mesh pattern on an area or volume (usually, refers to making "all quad" or "all brick" meshes)

ANSYS Modeling and Meshing Guide, Chapter 7:   "... a mapped mesh is restricted in terms of the element shape it contains and the pattern of the mesh. A mapped area mesh contains either only quadrilateral or only triangular elements, while a mapped volume mesh contains only hexahedron elements. In addition, a mapped mesh typically has a regular pattern, with obvious rows of elements. If you want this type of mesh, you must build the geometry as a series of fairly regular volumes and/or areas that can accept a mapped mesh."

ANSYS Mapped meshing requires that an area or volume be "regular"

For an area to accept a mapped mesh, the following conditions must be satisfied:

  1. The area must be bounded by either three or four lines (with or without concatenation).    If an area is bounded by more than four lines, you can combine [LCOMB] or concatenate [LCCAT] some of the lines to reduce the total number of lines to four.
  2. The area must have equal numbers of element divisions specified on opposite sides, or have divisions matching one of the transition mesh patterns
  3. If the area is bounded by three lines, the number of element divisions must be even and equal on all sides.

To mesh a volume with all hexahedron elements, the following conditions must be satisfied:

  1. The volume must take the shape of a brick (bounded by six areas), wedge or prism (five areas), or tetrahedron (four areas). As with lines, you can add [AADD] or concatenate [ACCAT] areas if you need to reduce the number of areas bounding a volume for mapped meshing. If there are also lines bounding the concatenated areas, the lines must be concatenated as well. You must concatenate the areas first, then follow with line concatenations.  (In some cases, lines are automatically concatenated).
  2. The volume must have equal numbers of element divisions specified on opposite sides, or have divisions matching one of the transition mesh patterns for
    hexahedral meshes.
  3. The number of element divisions on triangular areas must be even if the volume is a prism or tetrahedron.

Concatenation is solely intended to be used as an aid to mapped meshing; it is not a Boolean "add" operation. Concatenation should be the last step you undertake before you execute a mapped mesh of your solid model, because the output entity obtained from a concatenation cannot be used in any subsequent solid modeling operation (other than meshing, clearing, or deleting); nor can it be used in another concatenation.

You can "undo" a concatenation by simply deleting the line or area produced by the concatenation.

Concatenation is also implied when mapped meshing from the Mesh Tool, as you pick the "corners" of an area or volume to mesh, intermediate faces, lines and keypoints are treated as "interior" features, temporarily concatenated (not saved) for the meshing operation.

VSWEEP