During mesh generation, an element's quality (which is sometimes called the element shape) is calculated. The meshing algorithm uses error limits to obtain a valid mesh. It performs extra mesh cleanup to ensure the error limits are met, such that a valid mesh is one that satisfies the necessary (minimum) conditions and can be consumed by the solver. The meshing algorithm then attempts to improve quality based on warning (target) limits. Error and warning (target) limits can be defined further as:
An error limit is the value at which an element's quality is not suitable for the solver being used, and by default the mesh would fail (see Check Mesh Quality). Priority is given to ensure there are no elements below the error limits. Error limits are determined by the physics preference as shown in the table below. You cannot change error limits. However, if you are using the Mechanical physics preference, you can choose from two sets of error limits: Standard Mechanical or Aggressive Mechanical. You can think of the error limits as the minimum quality criteria of the meshing.
A warning (target) limit has two purposes:
It is used as a warning limit. If the mesh has elements that are questionable for the solver being used, these elements can be flagged for you via the warning limit. To configure this warning behavior, set Check Mesh Quality to Yes, Errors and Warnings.
It is used as a target limit. The mesh methods will first try to improve the mesh to ensure there are no elements that do not pass the error limits. If successful, the mesh methods do further improvements to try to meet the target limits. You can think of the target limits as the quality goals for the meshing. If the mesher cannot meet the goals, a warning can be issued. To configure this warning behavior, set Check Mesh Quality to Yes, Errors and Warnings.
Note: All mesh methods use the warning (target) limits to flag warning elements if Check Mesh Quality is set to Yes, Errors and Warnings. However, not all mesh methods use the target limits to improve the mesh. Currently, only the Patch Conforming Tetra mesh method uses the target limits to improve the mesh.
You can control how the mesher responds when it reaches error and warning limits by setting the Check Mesh Quality option.
The error limits for each physics preference are:
Mechanical - Uses either of these sets of error limits, depending on the setting of the Error Limits option:
Standard Mechanical - These error limits have proven to be effective for linear, modal, stress, and thermal problems.
Aggressive Mechanical - These error limits are more restrictive than the error limits for Standard Mechanical. Aggressive Mechanical may produce more elements, fail more often, and take longer to mesh. As an alternative, you can set Physics Preference to Nonlinear Mechanical. However, doing so changes other defaults and may significantly change the mesh size and/or which features the mesh is capturing, and therefore may have a big impact on mesh quality. This is the default when Physics Preference is set to , but you can use the Error Limits option to change it.
Nonlinear Mechanical - Uses error limits as shown in the table below to produce a high quality mesh that meets the shape checking requirements of tetrahedral elements for nonlinear analysis. If the element quality cannot meet the error limits, the mesh is not desirable for nonlinear analysis. These error limits are used whenever Physics Preference is set to Nonlinear Mechanical, you cannot change them.
Note: Using the Nonlinear Mechanical option typically produces more elements and longer meshing times. If the element size is too coarse, meshing robustness may be problematic because it is sometimes difficult to get a good quality mesh that not only meets the coarse element size but also captures the features of the model. In such cases, you should reduce the element size, simplify the model, or set Check Mesh Quality to No to turn off the error checks.
Electromagnetics - Uses error limits based on element volume, face warping, and face angle. These error limits are used whenever Physics Preference is set to Electromagnetics, you cannot change them.
CFD - For non-assembly meshing algorithms, uses error limits based on element volume.
Explicit - Uses error limits based on Jacobian ratio and element volume. These error limits are used whenever Physics Preference is set to Explicit, you cannot change them.
The following table presents the error and warning (target) limits for different values of Physics Preference. The Hydrodynamics physics preference does not have error and warning limits, so it is not included in the table.
| Physics Preference | Mechanical | Nonlinear Mechanical | Electromagnetics | CFD | Explicit | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Criterion | Standard Mechanical Error Limit | Aggressive Mechanical Error Limit | Warning (Target) Limit | Error Limit | Warning (Target) Limit | Error Limit | Warning (Target) Limit | Error Limit | Warning (Target) Limit | Error Limit | Warning (Target) Limit |
| Element Quality |
< 5 x 10-4 for 3D < 0.01 for 2D < 0.75 for 1D |
< 5 x 10-4 for 3D < 0.02 for 2D < 0.85 for 1D | < 0.05 (default) |
< 5 x 10-4 for 3D < 0.02 for 2D < 0.85 for 1D | N/A | N/A | < 0.05 (default) | N/A | N/A | N/A | < 0.2 (default) |
| Jacobian Ratio (Gauss Points) |
< 0.025 | N/A | N/A | < 0.025 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |
| Jacobian Ratio (Corner Nodes) | N/A |
< 0.025 | N/A | N/A | < 0.04 (default) | N/A | N/A | N/A | N/A | < 0.001 | N/A |
| Skewness | N/A | N/A | N/A | N/A | > 0.9 (default) | N/A | N/A | N/A | > 0.9 (default) | N/A | N/A |
| Orthogonal Quality | N/A | N/A | N/A | N/A | N/A | N/A | N/A | <= 0 for meshing, not used for other methods | <= 0.05 for assembly meshing, not used for other methods | N/A | N/A |
| Element Volume | < 0 | < 0 | N/A | < 0 | N/A | < 10-30 | N/A | < 10-32, | N/A | < 0 | N/A |
| Aspect Ratio (for triangles and quadrilaterals) | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A |
| Face Angle | N/A | N/A | N/A | N/A | N/A | > 150 | N/A | N/A | N/A | N/A | N/A |
| Face Warping | N/A | N/A | N/A | N/A | N/A | > 0.4 | N/A | N/A | N/A | N/A | N/A |
| Aspect Ratio (Explicit) | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | >5 (default) |
| Characteristic Length (LS-DYNA) | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | <10% Element Size (default) |