# Convergence problem when ustructured mesh is used.

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 LinkBack Thread Tools Display Modes
 August 31, 2000, 00:46 Convergence problem when ustructured mesh is used. #1 SangJin Ryu Guest   Posts: n/a 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?

 August 31, 2000, 20:06 Re: Convergence problem when ustructured mesh is u #2 John C. Chien Guest   Posts: n/a (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.

 September 25, 2000, 22:38 Re: Convergence problem when ustructured mesh is u #3 Chung Kuo Guest   Posts: n/a 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.

 October 5, 2000, 20:26 Re: Convergence problem when ustructured mesh is u #4 SangJin Ryu Guest   Posts: n/a 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.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post TB CFX 31 August 31, 2016 19:02 lr103476 OpenFOAM Running, Solving & CFD 30 November 19, 2007 15:09 hoochie OpenFOAM Running, Solving & CFD 2 July 28, 2007 07:07 Ed Schaub FLUENT 2 April 24, 2006 10:55 ales FLUENT 5 April 18, 2001 09:00

All times are GMT -4. The time now is 16:17.

 Contact Us - CFD Online - Top