[AMR96]

Hello,

For my thesis, I have to simulate the aerodynamics of a cornering vehicle but to get used to doing external aerodynamics I have decided to first simulate the aerodynamics of an Ahmed body on a straight line in a wind tunnel. I am using the SST k-omega model for this simulation.

I have tried to do this by using the wind tunnel dimensions given in the following paper (15 m x 1.87 m x 1.4 m): https://www.researchgate.net/publica...the_Ahmed_body

The problem I have right now is that either the residuals become constant after a number of iterations-specifically the continuity residual does not fall below the 1e-3 threshold- or the simulation converges but I don't obtain the correct drag and lift coefficients. Additionally, during the simulation the lift and drag coefficients have small oscillations.

Below are the specifics of the simulation setup and the mesh and also some of my attempts at fixing the problem:
For the mesh
Method: Tetrahedral
General element size: 60 mm
Body sizing using body of influence (size): 20 mm
Face sizing (the Ahemd body): 5 mm
Inflation layer: First layer thickness is 9e-3 mm with 20 layers in total.

Boundary conditions
Inlet: velocity inlet at 40 m/s. Turbulent intensity 0.1% and Turbulent length scale 0.073 m
Outlet: pressure outlet
Wind tunnel walls: symmetry
Ground: no-slip wall
Car: no-slip wall

Reference values
https://imgur.com/a/evpnJBl

URF
Please see the attachments

I have tried refining the mesh to what is below but I get a similar behavior for the residuals.

Method: Tetrahedral
General element size: 30 mm
Body sizing using body of influence (size): 10 mm
Face sizing (the Ahemd body): 5 mm
Inflation layer: First layer thickness is 9e-3 mm with 20 layers in total.

I have also tried to reduce the momentum equation URF which helps with the convergence but leads to incorrect drag and lift coefficients.

One thing that I observed was the high value of the y+ value on the ground segment in the area where the body of influence is not present, the value was in the order of magnitude 1e2 which is very high. I have tried to further refine the mesh on the ground but the number of elements becomes very large, so far I am observing the same residual behavior as before and the lift and drag coefficients are still oscillating.

At this point I am not sure what is causing the problem. Any help and guidance is greatly appreciated.

Best,

Amir

[AMR96]

In case anyone comes across a case like this, please follow this tutorial. It helped me with fixing the mesh and setting up the simulation.

https://www.youtube.com/watch?v=91bonD180ls