LES crash and velocity bug

[Cbarnaud]

Hello,
I am struggling with LES on CS 6.0.0.
I have two inlets and one outlet and even when I choose 1m/s for each inlet (normally 30 m/s) my velocity increase strongly to 100 m/s and way upper after few iterations. I precise the geometry is a tube for one of the inlet, so it should not increase that much in this part. All the other parameters are default.
This mesh/geometry works well with URANS/RANS (with few oscillations) but it seems I can't use LES.
I think the problem comes from my mesh, which is very huge and I get from someone else, I wish I have not to re-do it because of its size.
Is there a way to solve this, without modifying the mesh? Else, is there a test case that proves that the mesh is the heart of the problem?

Thank you,
All the best
CB

[Yvan Fournier]

Hello,

LES might be a bit more sensitive to the mesh. What type of elements du you use/what is the mesh quality ? If you use tetrahedra, there might be a few additional precautions to take, and we have seen in some cases adding 2% upwind to the velocity solution (not too much to avoid loss of precision, just a bit for stability) may help. There may be other suggestions, but I need to check with the turbulence experts.

Best regards,

Yvan

[Cbarnaud]

Hello,

Currently, my mesh is pretty bad I think and I have 2% of bad cell offset. It is a mix of plenty of elements, I attached the salome report on my mesh.
I tried the blending option: spots with a great increase in velocity still appear and made my simulation diverge.
I will be happy to see the feedback from your colleagues.

Best regards,
Camille B.

[Yvan Fournier]

Hello,

In many cases, robustness is quite sensitive to the outlet condition and mesh, so if you have a tetrahedral mesh, extruding a layer or 2 at the outlet may help in many cases. Not a sure solution, but easy to try.

Best regards,

Yvan