In this blog, we will review some key concepts in determining the quality of the mesh used in CFD.
1) Skewness
There are generally two methods for determining the skewness:
Based on the equilateral volume:
• Applies only to triangles and tetrahedrals.
• Default method for tri’s and tet’s.
2. Based on the deviation from a normalized equilateral angle:
• Applies to all cell and face shapes.
• Always used for prisms and pyramids.
2) Smoothness and aspect ratio
• Change in size should be gradual (smooth).
• Aspect ratio is ratio of longest edge length to shortest edge length. Equal to 1 (ideal) for an equilateral triangle or a square.
3) Grid Resolution
• Pertinent flow features should be adequately resolved.
• Cell aspect ratio (width/height) should be near one where flow is multi-dimensional.
• Quad/hex cells can be stretched where flow is fully-developed and essentially one-dimensional.
• Ideally, the maximum change in grid spacing should be <20%:
4) Cell counts
• More cells can give higher accuracy. The downside is increased memory and CPU time.
• To keep cell count down:
- Use a non-uniform grid to cluster cells only where they are needed.
- If possible, use solution adaption to further refine only selected areas.
• Cell counts of the order:
- 104 are relatively small problems.
- 105 are intermediate size problems.
- 106 are large. Such problems can be efficiently run using multiple CPUs, but mesh generation and post-processing may become slow.
- 107 are huge and should be avoided if possible. However, they are common in aerospace and automotive applications.
- 108 and more are for specific research topics and are quite rare.
5) Main sources of errors
• Mesh too coarse.
• High skewness.
• Large jumps in volume between adjacent cells.
• Large aspect ratios.
• Inappropriate boundary layer mesh.
• Sharp angles between cells.
• Too many non-matching cells.
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