SimScale CAE Forum

Vague force and force coefficents plots


#1

Hey everyone,
I was trying to simulate the flow over this wing but the values for the plots I’m getting are way off track I guess, I’ve tried simulating other iteration also which seemed to be fine but for this geometry, I’ve tried everything. I even made complete new CAD (although there doesn’t seem to any error in the geometry).

Can anyone help me out with this?
Here is my project link for any reference:
https://www.simscale.com/projects/Deepank/rear_final_2/


#2

Hey @Deepank,

Your runs are diverging to unphysically high results. The (or one of the) culprits in this case is your mesh. Its quality could definitely be improved. Take a peek at the meshing log:

You can either stick to the standard mesher and try to get a better mesh quality by means of local refinements (keep an eye on the meshing log aswell!):

Or you can try to switch for a hex dominant (parametric, preferably), so you have extra control over your mesh/refinements. So as a first measure, I’d try to get a better mesh.

There are also some things you can change in numerics to try and get a more stable simulation. These would be things to try after you get a better mesh (in case divergence persists):

  • tweaking relax factors;
  • adding non orthogonal correctors;
  • using first order schemes for divergence terms - although it’s not recommended for “final results”, since 1st order schemes are diffusive in nature.
  • if everything fails, there could be issues with boundary conditions or some other user error.

For future reference, it’s a good idea to keep an eye on your result controls as the simulations are running. If you can keep an eye on result controls while the simulations are running. If you see something like this, just stop the run to save some time.

Let us know if you need a bit more help than this, so people can jump in :smile:

/Ric


#3

Thanks for the reply,
Yeah ill always try to keep an eye on the results control and the divergence curve to verify whether the analysis is going fine or not. Regarding the mesh issue , ya i noticed that the meshing quality is not up to the mark but I’ve applied similar mesh with similar geometries and the results are pretty much fine with a proper convergence curves.


#4

@Deepank: Please see my answer about problem with aspect ratio in TET mesh in that topic:

Strange CFD results with velocity inlet and outlet

Cheers,

Restam


#5

Thanks a lot
@Ricardopg and @Retsam for your help i did changed my mesh as you both said and followed your instructions while meshing and the results were looking fine this and were not diverging and the plots also seemed to be fine untill now.
But i had to stop the analysis due to reference values error else it the solution did helped me.

Cheers
Deepank


#6

Adjust your ‘Relax Factors’ as an extreme LAST resort with the knowledge that the quality of your results will likely deteriorate significantly.

In fact, you can easily adjust relax factors to get almost every simulation to converge but if you do, your results would likely be FAR from realistic.

Check out this topic to see what I really think of relax factor adjustments:

I will never adjust relax factors them from their DEFAULT values until the issues of that topic are fully addressed…