First of all these values are relative. There are many values which are not acceptable to Fluent but CFX. For example fluent is more tolerant of angle issue than the CFX. But Fluent does not like the pyramid shape and CFX does not mind it because of its internal working.

Keeping in view they have (meshing guys) have devised some metrices with factor of safety. Following are the most important quality metrics with thier range.

Angle : Must be greater than 18 deg (But some times Fluent sometimes can work as low as 9 deg)

Quality : Must be greater than 0.3. But quality of 0.2 or greater can be tried and if solver doesn't mind it than carry on.

Expansion rate: Change of cell volume with respect to neighbouring cells. Should be less than 10 (or 20 check the CFX of Fluent manual).

Skewness : (Fluent) Must be less than 0.8. Up to 0.95 is acceptable.

Aspect Ratio : Must be less than 100 for single precision solver and 1000 for double precision solver. But due to implementation of better algorithms, I have tested and found that AR up to 8000 is OK in boundary layer with no impact on the solution. Moreover it is the characteristics of boundary layer to have the smaller cell size in the normal direction where flow gradients are steep and have bigger cell size in stream wise direction as flow gradients are not sharp. This approach is not valid for the transition prediction where you requires ten time more mesh points in the vicinity of transition.

Important thing to remember:

If you are using Fluent then make the mesh which has orthogonal quality greater than 0.01. To ensure this you need to have the

1. Min quality greater than 0.3
2. Angle greater than 18
3. Smooth cell size change


For CFX, you need to ensure this in ICEM:

1. Min quality greater than 0.3
2. Min angle 18
3. Smooth cell size change


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