Minimum or Maximum Member Size

Use the maximum and minimum member size restrictions to prevent parts of the model from becoming too thick or too thin.

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DVCON_SHAPE

Applicable for Controller (SHAPE_CONTROLLER) Sensitivity (SHAPE_SENSITIVITY)
CHECK_MAX_MEM OK -
CHECK_MAX_MEM OK -

For the definition of minimum member size a radius is specified, such that normal to the surface of the model a minimum thickness of 2*radius must be achieved. If optimization displacements break this restriction, they will be readapted according to the selected main criterion. If the structure is smaller in certain regions from the very first, only growth is permitted in these areas unless the areas fit the condition. CHECK_MAX_MEM works analogously.



A typical parameter set for the member size restriction is of the following form:


DVCON_SHAPE
 ID_NAME       = ...
 ND_GROUP      = <member_size_nodes>
 CHECK_MAX_MEM = <member_size_radius>
 CHECK_MIN_MEM = <member_size_radius>
 CHECK_NDGR    = <member_size_check_group>
 TOLERANCE     = <tolerance_for_intersection_tests>
END_

Technically the implementation is based on the surface of the finite elements of the design nodes. The member size is calculated as the distance between a node and the intersection point in the negative normal direction to an element surface. If no element surface is intersected, the member size is ignored for this node.

The ND_GROUP parameter specifies the node group that is modified by the algorithm. This group should be a subset of the mesh smooth area. The distance of the nodes in this group is tested against the node group specified by CHECK_NDGR. This is an optional parameter and set by default to <MEMBER_SIZE_NODES>. This option is useful if the member size shall only be tested against certain regions like shown in the following figure.

The TOLERANCE parameter is important for intersection tests: If the intersection test for one node misses the border of the <MEMBER_SIZE_CHECK_GROUP> slightly, the element faces can be extended by the TOLERANCE value (in absolute units) so that the intersection test succeeds and the desired member size for this node is not ignored. However a too large TOLERANCE value implies the risk of getting incorrect distances. This parameter is optional and the default value is 0.1 * max element length.
Note:

  • If your initial model does not fulfill your desired member size, the member size restriction is unable to fit the model to these values.
  • If you want to ensure the minimum (maximum) member size restriction for a certain region that is not necessarily part of the mesh smooth area, choose all surface nodes of this region as CHECK_NDGR. Areas that already fulfill your restriction and are not influenced by the optimization can be left out.
  • The minimum member size restriction tends to be nonconservative. This means that the resulting member size is possibly smaller than the desired one. A workaround is to give a higher desired minimum member size value.
  • For the same reason, the maximum member size tends to be conservative.
  • The member size restrictions are not yet available for sensitivity-based shape optimization.