Help with Ahmed body Cd

Hello,
I have been trying to recreate the simscale validation study on the Ahmed body, and I’m having some difficulty on getting my results to match with the ones provided by the article [Aerodynamics: Flow around the Ahmed Body | SimScale]. I tried to follow the same steps in the project that is provided in the article [SimScale Login]

I’m trying to recreate mesh #2. I got a drag coefficient of 0.376, but the validation case has a Cd of 0.330. Could anyone please help me in understanding why my results are off? Here is a link to my project [SimScale]

Any help would be greatly appreciated, thanks in advance.

Hello Polaris23,

thanks for testing and sharing your results.
from having a look at your results first I did not notice any different.

Did you change anything in the simulation setup, except the flow domain?

I will see if I can give the tutorial a try myself and see how the result correlates with the validation case.

Best regards
Sebastian

No not really, I tired to follow the same setup parameters as in the associated project

Hello Polaris23,

It looks like after further review of the Meshing Log for your project and the Validation Project from SimScale, that the meshes are not the same between the two projects. Given that external aerodynamic simulations are highly mesh sensitive problems, it is not uncommon to see results vary like this. The Meshing Log statistics for both of the mesh’s indicate that they have fairly different mesh qualities and characteristics.

To refine the mesh to be more alike the Mesh 2 highlighted in the Ahmed Body Validation Case page, I would try the following:

  1. I would review the selections for the Local Element Refinements once again and the Geometry Primitive sizing for the Region Refinements; for example, it looks like the “Body Layers” Boundary Layer Refinement has an additional selection from the external flow volume’s boundaries (Face30@FlowRegion).
  2. It looks like the base of the Ahmed Body surfaces (the cylinder supports) are each split into 2 faces in the validation project vs in your model where each cylinder support is simply 1 face. This may seem trivial, but this means the Standard Mesher will see these 2 geometries and mesh them differently. Try to use the same model so that discrepancies are minimized.
  3. Try to make the Flow Volume sizing the same if you want to further mitigate errors. This isn’t as vital, given that your flow volume has a larger clearance between the inlet and the front face of the Ahmed Body, but it will help you close the gap between why the results are different given the same mesh settings.

I hope this helps you get closer to a better quality mesh. You can use the Mesh Quality tab to view the poor-quality mesh elements by filtering the different elements using the Isovolume Filters. This SimScale Documentation Page can walk you through how to do so: Mesh Quality | Mesh Visualization Tips | SimScale
Finding the location of poor quality mesh elements can help you refine the mesh further based on these insights at targeted locations.

Cheers,
Omar

Great explanation @oabed !

@Polaris23 , since we are talking about drag counts now, it also may worth to mention that supports connecting the body to ground are slightly moved backwards in your simulation setup. It could be affecting the wake formation, ground-body interaction, and consequently pressure drag.

Best,
Kaan.

Thank you @oabed and @kaany for your inputs. I will go over my model again, with your inputs in mind.

Thanks for the help!