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Problem with meshing long, thin faces in CFX

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Old   January 7, 2009, 19:41
Default Problem with meshing long, thin faces in CFX
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I am a student using CFX (and CFX-Mesh) to mesh and then analyse fluid flow through a trachea and bronchi, using a part imported from Solidworks. I am currently having trouble with meshing this part. It is essentially a circular profile lofted along a profile that branches out several times. At the points where it branches out into two, the part has been smoothed at the join, using fillets. A by-product of this is that extremely long, thin faces are created at the top and bottom of the fillets.

This is quite a simple model. However, I am having a fair amount of trouble in creating a successful mesh for it. The errors I am getting in CFX-Mesh concern either overlapping elements, elements with negative parametric area, or elements with large aspect ratios. These occur at the aforementioned long thin faces. I have tried refining the mesh at these faces, and also, merging these faces with nearby ones using virtual topology . Neither of these seem to work. I can avoid the "negative parametric element area" message by using reasonably large element sizes. However, the "overlapping element" that it highlights when I click on that error always seems to be non-existent. As in, it highlights a very large element in the surface mesh when the surface mesh is actually comprised of elements that are very small. This has me baffled. As for the aspect ratio of the elements, I have tried to reduce this in the mesh controls, but it seems to be largely dictated by the geometry of the face itself.

I am currently able to mesh the part correctly when the fillets are suppressed, so I am fairly sure that the problems stem from these. I was wondering if anyone had some advice for meshing geometries with very long, thin faces? Or perhaps I should be doing something to the geometry in Solidworks to get around this problem. I am fairly new to CFX, but the error messages and help that it has been giving me has been confusing and somewhat unintuitive.

Many thanks in advance,

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Old   January 7, 2009, 22:47
Default Re: Problem with meshing long, thin faces in CFX
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Can you merge all those faces with the whole branch using virtual topology? I am you mentioned you merged a few faces that are close together, try merging them all together with the branch as well.

Would that work?

In workbench, there's another tab called advanced meshing (depending on your license obviously). You may want to try that which is a patch independant mesher

Hope that helps.
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Old   January 8, 2009, 17:18
Default Re: Problem with meshing long, thin faces in CFX
Glenn Horrocks
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If it does not mesh using virtual topology then I recommend you go to the ICEM mesher. It is much better at meshing difficult geometries than CFX-Mesh. There is some functionality from ICEM in workbench with the patch independent meshing option.

Glenn Horrocks
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Old   January 8, 2009, 21:51
Default Re: Problem with meshing long, thin faces in CFX
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Thanks for your responses.

I have indeed tried merging many of the faces, however, it does not appear to work. I still get the same errors. I have also tried using the patch independent mesher in CFX with similar results.

At the moment I think using the ICEM mesher sounds like the best idea. It seems like it will be able to handle the geometry better. I will try to find a computer with access to this and see how things pan out.

Thanks again, Martin
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