Hi @terkoiz, thanks for posting on the forum and welcome to the SimScale Community 
It’s a bit hard to pinpoint where the problem is exactly without having a look at your project in particular. Could you share it with us? Here’s a guide to learn how:
However, one very common issue is divergence in these sorts of simulations (even if your simulation completed, this does not mean it’s necessarily converged). Please check these three resources:
- How to Check Convergence of a CFD Simulation? | SimScale
- Divergence in Simulation: How to Tell When and Where? | SimScale
- How to Check and Improve Mesh Quality | SimScale
Another problem we also often see is the mesh being very coarse and not accurately capturing gradients. If that’s the case, meshing refinements will certainly help.
Cheers
Igor