Even when an analysis terminates due to severe mesh distortion, rezoning enables you to continue the analysis and complete the simulation. You can also use rezoning to improve analysis accuracy and convergence when the mesh is distorted but does not terminate the analysis.
To illustrate how rezoning works in a case where the analysis terminates, assume that the following initial mesh and boundary conditions exist:
The simulation terminates at TIME = 0.44. Rezoning begins on the deformed mesh at substep 7 (TIME = 0.40):
After remeshing the selected region, an acceptable new mesh is ready:
Based on the new mesh, the simulation concludes successfully at TIME = 1.0:
For a more detailed example, see Rezoning Examples.
The purpose of rezoning is to repair a distorted mesh to overcome convergence problems caused by the distortion.
Rezoning is effective only when the mesh distortion is caused by a large, nonuniform deformation. Rezoning cannot help if divergence occurs for any other reason such as unstable material, unstable structures, or numerical instabilities.
Unstable Material
Most nonlinear material models, especially those employing hyperelastic materials, have their own applicable ranges. When a deformation is too large or a stress state exceeds the applicable range, the material may become unstable. The instability can manifest itself as a mesh distortion, but rezoning cannot help in such cases. While it is sometimes difficult to determine when material is unstable, you can check the strain values, stress states, and convergence patterns. A sudden convergence difficulty could mean that material is no longer stable. The program also issues a warning at the beginning of the solution indicating when hyperelastic material could be unstable, although such a warning is very preliminary and applies only to cases involving simple stress states.
Unstable Structures
For some geometries and loads, a deformation may cause a snap-through, or local buckling. Such behavior can also manifest itself as a mesh distortion, but one that rezoning cannot repair. The effect is usually easy to detect by closely checking the deformed region or the load-versus- time (displacement) curve.
Numerical Instabilities
A condition of numerical instability can occur when a problem is nearly overconstrained. The constraints can include kinematic constraints such as applied displacements, CP, and CE, and volumetric constraints introduced by fully incompressible material in mixed u-P elements. In many cases, numerical instability is apparent even in the early stages of an analysis.
For a successful rezoning, the new mesh must be of a higher quality than the old mesh. If the new mesh is not better than the original mesh, rezoning cannot improve convergence, and can even worsen convergence problems.