# Convergence Question

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 May 7, 2003, 19:59 Convergence Question #1 Colin Guest   Posts: n/a Hi, there. If the residual of continuity doesn't fall below 0.001, what should I do to get convergence? Adjust relaxation factor? Or adjust the mesh? Anything else? What the "thumb of rules" of these adjustments?

 May 7, 2003, 21:51 Re: Convergence Question #2 ap Guest   Posts: n/a Hi Colin, I don't know the problem you're solving, so I'll try to give some general advice. - First of all you should check if your problem is well-posed. Verify all boundary conditions. - Check your grid and refine it, if necessary. - If you're doing unsteady calculations, try to reduce time step and/or increase the number of iteration per time step. - Try to reduce under-relaxation factors. - Try different discretization schemes. - Change solver settings in the Solve->Multigrid dialog box (read the manual for more info). Hi ap

 May 7, 2003, 22:13 Re: Convergence Question #3 raintung Guest   Posts: n/a I met the same problem, convergence approach steadily at first, but then the residuals will level off(explict solver). I don't know what should I do. I set the some of the parameters under FAS Multigrid Controls , but divergence appears.

 May 7, 2003, 22:47 Re: Convergence Question #4 Colin Guest   Posts: n/a I think AP has given us some good ideas. Just try, maybe it works. Thanks Ap.

 May 8, 2003, 10:34 Re: Convergence Question #5 ap Guest   Posts: n/a If you give some datail of the problem you're solving, maybe I can be more specific. Hi

 May 8, 2003, 22:13 Re: Convergence Question #6 raintung Guest   Posts: n/a Can you give me some advice? I am simulating the 3Dflow field in compressor. The mesh is semi-structured and the boundary conditions obtain from the experiment. Because there is a thin fence on the endwall, it is very difficult to get desirable mesh and convergence is not easy. I use k-eplison Realizable model. What should I do? Thanks.

 May 9, 2003, 09:59 Re: Convergence Question #7 ap Guest   Posts: n/a It seems a complex situation... I assume your boundary conditions are good, because you have them from experiments. So try to check mesh quality and, if possible, refine it where quality is really low. You can try to remesh your domain using unstructured grids, if it's too difficult to refine the semi-structured one. If this doesn't work, try to reduce under-relaxation factors and to increase the number of iteration on the internal cycle (Multigrid panel). However, changing parameters of the solver may help, but doesn't solve problems due to a poor quality mesh Good work ap

 May 9, 2003, 22:13 Re: Convergence Question #8 raintung Guest   Posts: n/a Thank you. I will try to do that.

 May 9, 2003, 22:47 Re: Convergence Question #9 raintung Guest   Posts: n/a Can I ask you a simple question? We often say ˇ°refining meshˇ±, but how many means can we do that effectively? Thank you.

 May 10, 2003, 19:21 Re: Convergence Question #10 ap Guest   Posts: n/a I'm not sure I understood your question. You want to know how you can refine a mesh? Personally, I don't like grid adaption tools which are integrated in FLUENT. Usually they increase too much the number of cells. I prefer to use Gambit and to remesh the system by hand, choosing the number of nodes on the edges of my domain where I want to improve mesh quality. It requires more time, but I think it can give better results without increasing the number of cells too much. Hi

 May 10, 2003, 22:02 Re: Convergence Question #11 raintung Guest   Posts: n/a Thank you! I think it is a good idea. I quite agree with you.

 May 16, 2003, 05:41 Re: Convergence Question #12 phil Guest   Posts: n/a when you remesh in gambit, is it possible to then effectively patch the data from the previous analysis even though the mesh has changed?

 May 16, 2003, 09:35 Re: Convergence Question #13 ap Guest   Posts: n/a If the geometry doesn't change, you could use the Grid-to-grid solution interpolation function. See the post: GRID TO GRID INTERPOLATION in FLENT for a link to the page of the manual. Hi ap

 May 16, 2003, 12:41 Re: Convergence Question #14 phil Guest   Posts: n/a cheers ap i was fairly sure it could be done but didnt know what it was called!