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Step 4: Perform the Remeshing Operation |
4.7.1.3. Remeshing Using Manual Mesh Splitting (2-D and 3-D)
You can manually split an existing mesh to obtain the solution of a nonlinear analysis which cannot otherwise converge, or to improve its accuracy.
Mesh splitting increases the number of degrees of freedom of the model by enriching the existing
mesh. It is a useful option for rezoning if the number of degrees of freedom must be increased in contact gaps, or if a new program-generated mesh or generic third-party mesh does not fully satisfy your requirements.
As shown in Figure 4.3: Rezoning Using Manual Splitting of an Existing Mesh (p. 95), the mesh-splitting remeshing method requires the REMESH,SPLIT command, used in place of commands required for other meshing methods.
Elements that have had midside nodes added or dropped (EMID) cannot be split.
The following topics related to manual mesh-splitting are available:
4.7.1.3.1.Understanding Mesh Splitting
4.7.1.3.2.Geometry Details for Mesh Splitting
4.7.1.3.3.Using the REMESH Command for Mesh Splitting
4.7.1.3.4.Mesh-Transition Options for 2-D Mesh Splitting
4.7.1.3.5.Mesh-Transition Options for 3-D Mesh Splitting
4.7.1.3.6.Improving the Local Topology of Tetrahedral Meshes via Edge and Face Swapping
4.7.1.3.7.Improving Tetrahedral Element Quality via Mesh Morphing
4.7.1.3.1. Understanding Mesh Splitting
Splitting occurs on selected solid elements in the mesh. If no solid elements are explicitly selected, the program splits all solid elements in the mesh. The mesh-splitting capability generally applies to remeshing cases in mesh-rezoning problems using the elements listed in Rezoning Requirements (p. 90).
The program splits parent elements into child elements, as follows:
•A parent quadrilateral element into four child quadrilateral elements
•A parent 2-D degenerate element into three child quadrilateral elements
•A parent triangular element into four child triangular elements
•A parent tetrahedral element into eight child tetrahedral elements
For more information, see Geometry Details for Mesh Splitting (p. 105).
Child elements inherit all shape characteristics of the parent element. Therefore, if a particular element is badly distorted and is causing convergence difficulties, simply subdividing the element by splitting it does not improve convergence.
4.7.1.3.2. Geometry Details for Mesh Splitting
The REMESH command's SPLIT option uses no geometry information; instead, it uses only information about the mesh itself (that is, nodal connectivity and nodal coordinates).
All child elements automatically inherit all attributes of the parent element from which they were generated. Quadrilateral parent elements are split into four child elements, 2-D degenerate parent elements
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are split into three quadrilateral child elements, and triangular parent elements are split into four triangular child elements, as shown in the following figures:
Figure 4.6: Splitting Quadrilateral and Degenerate Linear Elements (PLANE182)
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Figure 4.7: Splitting Quadrilateral, Degenerate and Triangular Quadratic Elements (PLANE183)
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Figure 4.8: Splitting Tetrahedral Linear Elements (SOLID285))
4.7.1.3.3. Using the REMESH Command for Mesh Splitting
To perform mesh-splitting, select the region to be rezoned (via the GUI or the ESEL command). After you have selected the target region, issue a REMESH,SPLIT command.
You can issue multiple REMESH,SPLIT commands for various parts of the mesh in the same rezoning problem (referred to as horizontal rezoning). These multiple zones can overlap or they can be isolated; however, a large number of overlaps can cause badly shaped transition elements to develop.
If the rezoned part has contact/target elements, the program generates those elements automatically for the new mesh (according to whether the underlying old mesh had the same type of contact/target elements). Isolated rigid target elements in the model remain the same throughout the analysis and cannot be remeshed; however, all contact and target elements associated with solid elements are candidates for remeshing. When you split a solid element that is associated to contact/target elements, the program deletes these associated contact/target elements. The program generates the correct
contact/target elements for the new child elements automatically at the end of the remeshing operation (REMESH,FINISH).
Because splitting refinement is mesh-based and not geometry-based, it cannot be undone after it has occurred. To create a different or new splitting scheme, or to revert to the original mesh, you must create a new rezoning environment (REZONE,MANUAL,LDSTEP,SBSTEP).
For more information about using the REMESH,SPLIT command, see Mesh-Transition Options for 2-D Mesh Splitting (p. 108) and Mesh-Transition Options for 3-D Mesh Splitting (p. 110).
4.7.1.3.4. Mesh-Transition Options for 2-D Mesh Splitting
The default REMESH,SPLIT command forces the transition elements to be mostly quadrilateral and minimizes the number of degenerated elements. The command can also be issued as:
REMESH,SPLIT,,,TRAN,QUAD
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Step 4: Perform the Remeshing Operation |
Issuing the REMESH command using the transition and quadrilateral options helps with convergence because non-degenerate elements are less prone to locking behavior.
Generating a More Localized Mesh
If you desire a more localized mesh after splitting, issue the following command:
REMESH,SPLIT,,,TRAN,DEGE
In this case, the program creates the degenerate elements in the transition zone, and the split and the unsplit regions are connected within a single element layer.
It is possible that transition elements designed in such a way can disturb the localization of the mesh. The element subdivision that occurs when transitioning from the split zone can traverse several element layers.
The following figure illustrates the options for transition element generation:
Figure 4.9: Transition Element Generation Methods for 2-D
Elements selected for splitting (ESEL) are marked as shown.
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