2.2. Nonlinear Mesh Adaptivity Usage Considerations

Nonlinear mesh adaptivity through refinement can generally improve solution accuracy. It can help to capture local deformations in more detail, useful in applications such as rubber sealing for small cavities, local necking, or local buckling. Without a sufficiently fine mesh, these phenomena are difficult to simulate, yet it is often infeasible to start with a very fine mesh.

Two methods are available for refining the mesh:

Refinement via general remeshing refines the mesh when used with a contact-based (NLADAPTIVE,,,CONTACT,NUMELEM and NLADAPTIVE,,,CONTACT,CZM)), energy-based (NLADAPTIVE,,,ENERGY), or position-based (NLADAPTIVE,,,BOX) criterion. During refinement, the program reduces the element edge to approximately 75 percent of the original edge length, and may also create transition layers to connect the refined regions to the unrefined regions. Compared to a split mesh, the quality of a refined mesh is generally better and should be the first choice; however, it may still not lead to an improvement over the old mesh.
Refinement through splitting splits each element edge into two edges. Typically, it cannot repair a distorted mesh. In some cases, it can exacerbate mesh distortion by creating smaller elements. To reduce the effect, the program uses built-in morphing and topology repair after splitting; even so, mesh-quality improvement is slight. Avoid splitting or refinement in highly distorted regions. Doing so may lead to convergence difficulties.

Nonlinear mesh adaptivity via general remeshing can remove mesh-distortion problems when used with the mesh-quality-based criterion. Try this option if a problem cannot be solved because of mesh distortion. Although general remeshing to remove distortion cannot occur at the same time as mesh splitting, it can occur at the same time as general remeshing for refinement, enabling the program to better process difficult large-deformation analyses and achieve improved solution accuracy. The general remeshing method to remove distortion supports self-contact in 2D analyses with PLANE182 and PLANE222, and in 3D analyses with SOLID187 and SOLID227.

Nonlinear mesh adaptivity through coarsening via general remeshing can reduce the numerical effort by reducing the overall degrees of freedom in the model.

Coarsening via general remeshing can coarsen the mesh when using contact-based cohesive zone material (NLADAPTIVE,,,CONTACT,CZM), energy-based (NLADAPTIVE,,,ENERGY), or position-based (NLADAPTIVE,,,BOX) criteria. During coarsening, the program attempts to increase the original equivalent element size of the selected elements by a default value of approximately 50 percent for energy-based and position-based criteria, and by a required input value VAL3 > 1.0 via NLMESH,SRAT,,VAL3 for contact-based criterion with cohesive-zone material. It may also create transition layers to connect the coarsened regions to the remaining regions.
Refinement and distortion always take precedence over coarsening. Coarsening in regions of highly distorted elements may not lead to an increase in the element size as the remesher always attempts to improve the mesh-quality.
For contact elements with cohesive zone material (NLADAPTIVE,,,CONTACT,CZM), coarsening with the nonlinear adaptivity criterion is indirectly activated via NLMESH,SRAT,,VAL3, where VAL3 > 1.0. Regions where the active cohesive zone meets the criterion are refined, and the debonded (damaged) regions are coarsened.