[samsam]

Hi,

I am performing an analysis of roughness effects on airfoils. When I introduced a roughness on the leading edge through the turbulence model, the y+ value sharply increased to about 100 around the LE.(It was less than 1 when no roughness parameter was given). So I changed the mesh size near the airfoil to make y+<1. However this causes the solution to diverge. I checked the mesh volumes and some cells have negative Vol. This didn't happen with the earlier mesh

Any ideas why?

[swiftaircraft]

The negative volume will definitely be the parameter that is giving you problems. I expect that when you refined your mesh you reduced the cell height. Did you also change the cell length. Very often you find that if you only change the height the aspect ratio of the cells increase substantially and depending on the mesher negative values can be the result.

[samsam]

Yes, I decreased cell height and also to compensate for that increased the no of cells along that edge(in ICEM-CFD). To rectify the aspect ratio change that this would cause, I increased no of cells along the airfoil as well.

This didnt solve the problem though.

[swiftaircraft]

What aspect ratio and orthogonality is Fluent showing? Are you using the double precision solver?

[samsam]

Hi,

No i wasnt using double precision solver. I tried with that and its working ok for the moment.

I have one querry though. The min orthogonality that fluent shows is 0. But the mesh metrics in ICEM-CFD seem fine:

Angle>40
Skewness> 0.65 ( I am trying to increase this)
2x2x2 Det > 0.85

Could you explain why this is happening.

[swiftaircraft]

When boundary layers are very small in height you will need to use the dp solver otherwise it cannot resolve the small numbers. What is the minimum cell height as you could also be having the same issue with the calculation of the orthogonality. The main thing is the simulation is now converging. What is the reported maximum skewness. Also do you get any error messages after running a mesh check?

[samsam]

cell height is restricted to 10^-12 m. i tried to reduce the height as much as possible to get a y+<1 even when the roughness height is specified. But since the cell height is being limited by fluent y+ doesnt reduce lesse than 10^2. mesh check shows this limitation of cell height only as problem

[swiftaircraft]

The y+ calculation is also being influenced by the surface roughness that you have inputted as part of the simulation. There is no point in reducing the cell height to less than the height of the surface roughness.