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Kw-SST not converging on vehicle CFD

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Old   October 16, 2021, 05:38
Post Kw-SST not converging on vehicle CFD
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Javier
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Hello,
I'm a student trying to do a simulation of external aerodynamics of a car with Kw-SST in Star CCM 2021.1.1. However, the solution doesn't seem to converge.
Going over a Wall y+ scene over the car, I get that there is a lot of area with an y+ between 5 and 30. I suposse that is an error I need to fix. Also, the drag coeficcient keeps oscilating. I'll attach images of most of the results.
Here is my setup for the mesh (Surface Remesher, automatic repair, trimmed, prism layer mesher):
  • Base Size: 0.05m
  • Target: 400%
  • Minimum: 10%
  • Surface Curvature: 55 points
  • Surface growth rate: 1.05
  • Number of prims layers: 4
  • Prism stretching: 1.5
  • Prism thickness: 33.33%
  • Volume Growth: Slow
  • Max Cell Size: 800%

Customs controls:
  • Box around car: size 200%
  • Ground: Target surface 400%
  • Vehicle: Target surface 50% Prism layer:6
  • Wake: size 100%
  • Walls: Target surface 800%

Models:
  • constant density
  • gas
  • k-omega turulence
  • SST (Menter) komega
  • Segregated
  • Turbulent
  • Steady
  • Three dimensional
The inlet velocity is 33m/s and I also put that velocity to a moving ground. Reference Pressure is 0 Pa as also is in the pressure outlet. Any other configuration is as default.

I really don't know how to fix it. I've tried finer mesh, but seems to be pointless. ¿How, and would you try to get a Y+ lower than 5 or higher than 30? ¿Do you think that will be it? Also, have to say that with default K-epsilon, the same case converges getting most of the car with an y+ higher than 30.
Any help will be much appreciated. Thanks!

residuals.jpg

wall_y+.jpg

mesh1.jpg

velocity.jpg

CD.jpg
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Old   October 18, 2021, 07:10
Default Kw-SST not converging on vehicle CFD
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Matt
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I think your biggest issue is that you are expecting a steady result from an unsteady flow. Try switching to implicit unsteady and you should see better residual convergence.

Y+ between 5 and 30 isn’t great but it isn’t the end of the world either. Make that fix too and consider using wake refinement.
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Old   October 18, 2021, 08:41
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Lucky
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Something you can do is create a field mean and variance monitor. Collect statistics while the case is running. Then plot the variances of pressure and so on. You'll be able to see where the solution is oscillating this way and see where you need to make improvements.
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converge error, help me, k-omega sst, starccm+, vehicle aerodynamics

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