# Does my mesh really look OK?

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.