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Old   June 1, 2013, 17:23
Default Porous media and surface model (SOLVED)
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Hello guys,

I have a very annoying problem that I cannot solve. I have a model of a vehicle that I want to simulate with a porous media. My CAD model is in fact a surface model that I have exported as STL and imported in star-ccm. This works fine. I have then created the flow domain in star ccm, created the regions, but now I have problems with the porous media.

I am sure there is a leak (I am using wrapper, remesher, prism layer and trimmer) because the inner part of the vehicle shouldn't have this blue mesh:

When I look closer, there is indeed 2 surfaces so it looks like the mesh has been created because of a leak. Of course it is not possible to see leaks with 2 regions so I don't know where that leak is:

And here is the radiator with its sides in pink:

I have created an interface between the front of the rad (which is in a separate region) and the same surface but assigned to the car region (same for the back), but it doesn't work.

The only way I have managed to make that work (which means not having that mesh inside the vehicle) is to have the radiator sides also in the car region (although I am sure this is wrong). The thing then is that I have 2 times the radiator sides and when simulating there is a huge pressure on that part (of course because there are 2 surfaces at the same place without an interface). And using an interface between these 2 radiator sides doesn't solve the issue.

What am I doing wrong? I have problems to understand what star-ccm+ wants with surfaces and not solid models.

Last edited by MMatt; June 13, 2013 at 10:05.
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Old   June 2, 2013, 13:55
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Hi MMatt,

I'm kind of new in this cfd world but I was thinking that you maybe can use the surface repair tool before you generate your mesh to find any uncertainties that are exist in your imported mesh.

I would also try to refine the cad model if possible before the import but that is not always an easy thing to do.

I hope this will help you a little...

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Old   June 2, 2013, 18:53
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The problem you're experiencing is that most surface-based meshes literally follow ever surface feature, including gaps. There are several ways to fix this, in order of preference (from my experience):
  1. Equivalence (merge) all nodes in the model that are n units apart or closer, where n is a number slightly larger than the width of the gaps (leaks)
  2. Shrink-wrap mesh the domain
  3. Create new surfaces to fill the gaps, then remesh the domain
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Old   June 6, 2013, 16:24
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Have you tried running the "Leak Detection" tool on your model?

If not, right click on the surface wrapper node under your mesh continua (Continua->Your Mesh Continua->Models->Surface Wrapper) and choose "Run Leak Detection". Place one point where you think there should be no mesh and one point in your flow domain. The tool will show you where the leak is that connects the two points.
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Old   June 13, 2013, 10:04
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Problem solved! It was quite stupid. The interfaces were not selected correctly. You really have to pay attention at which one is the master and which one is the slave. Taking this in account solved the issue!
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