You can identify cells based on these metrics using the technique illustrated in Pre-Processing > Meshing > Checking the Volume Mesh > Identifying And Visualizing Bad Cells
The volume change is the ratio between a cell and its biggest neighbor. In general the volume change is bigger at the transition between prism layer and core mesh when you use a Low Reynolds meshing approach, as illustrated below:
To avoid higher volume changes than 10e-2, set the mesh size on boundaries carefully.
If you set mesh sizes that are too different on boundaries that are close one to another, you would have to change the default values for Mesh Density in order to get a smooth transition between the larger size cells and smaller ones. See also: Setting Mesh Tet/Poly Density
A prism layer must be used on all walls. For a Low Reynolds meshing approach, a fine grid should be used in the boundary layer in order to resolve it correctly. This practice in turn means that you can have high aspect ratio cells.
When using the coupled solver, you must avoid high aspect ratio cells (higher than 50-75).
For example, the following mesh is not recommended:
In order to avoid high aspect ratio cells, choose carefully the prism layer parameters.
Chevron cells are pairs of thin slender cells that share a common face whose angle is such that the line joining the cell centers does not go through the common face. An example is shown below:
When two surfaces are close, the prism layer can retract and some layers can be removed.
In order to avoid reduction of the prism layer, you can reduce, as below, the Minimum Thickness Percentage and Layer Reduction Percentage options. See also: What are the implications of changing the "Minimum Thickness Percentage" of the prism layer? and How does the Prism Layer Reduction affect the prism layer mesh?
5. "Potato chips" cellsOn curved surfaces with thin prism layer cells, depending on the mesh parameters, one can get highly concave and thin cells, also called potato chips, as illustrated below:
To improve the mesh quality of this type of cells, you can either decrease the prism layer stretching ratio in order to increase the thickness of the first cell, or increase the surface mesh density.
See also: Meshing > Volume Meshers > Prism Layer Mesher > Prism Layer Model Properties > Working with the Stretching Function6. Other recommendations
Below, some others tips & tricks about mesh which will help you to improve its quality.
In order to improve the volume mesh, you should improve the surface mesh.
In order to improve your mesh is it also recommended to change the default values for the Polyhedral Mesher Optimizer as below (for versions before 2019.1):
- 3 Optimization Cycles instead of 1
- 0.7 for the Quality Threshold instead of 0.4.
Polyhedral & Trimmer Post Optimizer
From 13.02, you can also enable the option Optimize Boundary Vertices in Polyhedral or Trimmer expert properties.
From 2019.1, this optimizer is enabled via the expert properties of the Polyhedral/Trimmer model named Run Post Mesher Optimizer. Thus, you can enabled two sub-options in the sub-node of default values named: Post Mesh Optimization. The two sub-options are Optimize Boundary Vertices and Optimize Cell Topology. See also: Pre-Processing > Meshing > Volume Meshers > Volume Mesher Controls and Values
In addition, you can use the Field Function Bad Cell Indicator. This Field Function shows the cells where the Cell Quality Remediation would be activated, so where the mesh quality is insufficient for the coupled solver. The Cell Quality Remediation model must not be used to solve the primal solution or the adjoint flow solver, but only for mesh quality post-processing. To use it, select the Cell Quality Remediation model in your Physics Continuum. Initialize the solution and create a threshold to extract the cells which have a value equal to 1 or 3 for the field function Bad Cell Indicator. If the threshold contains many cells, continue improving your mesh. Once the mesh improvements are complete, deselect Cell Quality Remediation in your Physics Continuum.See also: Simulating Physics > Remedying Cell Quality > Cell Quality Remediation Field Functions Reference
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