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Convergence problem when ustructured mesh is used.

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Old   August 31, 2000, 00:46
Default Convergence problem when ustructured mesh is used.
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SangJin Ryu
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I want to predict pressure drop of the silencer using STAR-CD. Shape of baffles and pipes in the silencer required unstructured mesh. But I don't have any mesh-generation program. So I used PROSTAR for hexahedral cell generation and used IDEAS for tetrahedral cell gerenration. And then I joined two meshes by "arbitrary matching" in STAR-CD. There is a problem not in the mesh generation but in the iteration process. I made three cases with the same fluid cells. Only the difference between them is the baffle cells and boundary conditions. It is interesting that two cases converged very well but one case did not. If the mesh had a problem, I thought, all three cases should had not converged. What is the problem? I suspect that there is a problem in mingling of PROSTAR mesh and IDEAS mesh. What is your opinion?
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Old   August 31, 2000, 20:06
Default Re: Convergence problem when ustructured mesh is u
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John C. Chien
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(1). Congratulation! Your success rate is very high, two good cases out of three. (2). The geometry and the boundary conditions will have influence on the iteration processes also. (3). It is a little bit too early to make any comment, before you can show that the trend remains the same when the mesh is refined systematically.
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Old   September 25, 2000, 22:38
Default Re: Convergence problem when ustructured mesh is u
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Chung Kuo
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When you say one case did not converge, it means diverged or just fluctuating within some range without reaching criteria. If it diverged, load the divergent result and look into the flow field to see where it blown up. If it fluctuated, then you might have to live with it because that could be right for your solution. Sometims, the turbulence model is not so good to completely shut off the "turbulence", so there is no convergent stedy solution. Or, the two mesh patterns do not match well, then look into the flow field, you should be able to see something strange, or .... good luck.
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Old   October 5, 2000, 20:26
Default Re: Convergence problem when ustructured mesh is u
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SangJin Ryu
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Thank you for your advice.

Residual did not diverge but fluctuate. But the overall tendency of convergece was very very slow. So I will look into the flow field.

I refined part of the mesh using adpative refinement. But there is no effect of adaptive refinement. I think that I did not refine mesh to the reguired level, whick I could not guess. And there is restriction on the number of cells due to machine capability, so I could not refine till converce criterion was met.

I divided the model into two submodels, one of which is made up of only hexahedral cells and the other include tetrahedral cells as its major part. And I changed baffle boundary condition in each submodel. But there was no fluctuation. Convergence criterion was met without any difficulty. So I think that there is convergence problem when two major parts of model are facing each other, which are made up of different cell types, and there is baffle cell between them as wall boundary condition. I should verify this by making change in my model.
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