convergence problem in using incompressible transient solvers.
Dear Foarmers,
Hello, I'd like to simulate the 3D half-wing which is mounted on the symmetry plane. Recently, I applied pisoFoam and pimpleFoam to get the unsteady solutions but the solutions have been diverged. I tried to apply those solvers with lower tolerance criteria, various solver types and schemes but they generally weren't helpful at all. Please give me an advice on using the transient solvers. Here I attached the control, fvSolution and fvSchemes files recently used. controlDict : Code:
1 /*--------------------------------*- C++ -*----------------------------------*\ Code:
1 /*--------------------------------*- C++ -*----------------------------------*\ Code:
1 /*--------------------------------*- C++ -*----------------------------------*\ Geon-Hong. |
Your maximum Courant number is 5, too high. Set it to something less than 1.
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Courant number
Dear alberto,
Thanks for your reply. Of course I tried lower value of Courant number, which was set to be 0.5 or unity. However the problem was the same. Regards, Geon-Hong. |
One possibility
One possible reason is a mistake in BC's,
To investigate this, I am looking for examples and using their BC's... Or I am looking in the log file what is the diverging variable and trying to find the error there. Another approach is to try to simplify the case. Start from a working example and add feature after feature... and when you will know the step which is causing your troubles you can deal with it. Martin. |
dear Holecek
Dear Holecek,
Thank you for your reply. I tried other boundary conditions which are used in tutorial cases. Still it doesn't work at all. By the way, what log file should I check? Thank you very much. Regards, Geon-Hong. |
Did you check the mesh for problems (checkMesh)?
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checkMesh
Dear Alberto,
Of course I did. However, there is a awkward thing that, High aspect ratio cells found and Failed 1 mesh checks. But I can't find what is problem. The checkMesh result is as below, Code:
/*---------------------------------------------------------------------------*\ |
It seems you have 260418 cells with high aspect ratio, and at least one of them has an aspect ratio of 13183.7, which is extremely high.
Code:
***High aspect ratio cells found, Max aspect ratio: 13183.7, number of cells 260418 <<Writing 260418 cells with high aspect ratio to set highAspectRatioCells |
checkMesh results
Dear Alberto,
Yes, I checked the high aspect ratio cells from the file highAspectRatioCells in constant/polyMesh/sets. What can I do with this cell-set? Is there any way to improve this problem? I guess those high-aspect-ratio cells are generated from the cells adjacent to the wall. The wing I want to simulate is tapered, which means that the wing tip has smaller chord length than the root. Furthermore, I generated the structured grid over the wing. Thus the wing has the same number of grid points along the chord at both wing tip and root. This might cause such problems, I guess. Regeneration of the grid is only hope? ToT |
Quote:
It seems that you would have to regenerate the grid. If you have done it with sHM - it will not take you too long ^^ |
about grid problems
I've noticed just before that the problems have been occurred far from the wing, not adjacent to the wing.
The problem is that the grid has been generated in structured manner and the grid size along in the downstream was stretched while thin layer of grid for resolving the boundary layer near the wall is maintained. Now I am considering a hybrid grid arrangement : structured near the wall and unstructured at far field. Rob / I used gambit to generate the grid. Now I'd like to try Ansys instead. I have no idea about sHM. What is it? |
Fluent
One more thing that I'd like to mention is that I ran the same case(mesh) with the FLUENT but it provided converged transient solution. At least, FLUENT may provide a transient solution though the mesh have extremely high aspect ratio cells which have order of 1e4 as my case.
From following thread, Mads Reck also referred that aspect ratios of more than 1000 is by no means insane - we have it in airfoil aerodynamics all the time - and on wind turbine blades. Aspect ratios of 1e5 or even higher is usual business when you want to have a first cell height of around 1e-6 chordLengths. http://www.cfd-online.com/Forums/ope...e-comment.html I hope to resolve this problem as soon as possible. |
Quote:
Fluent starccm+ etc apply lots of tricks (trade secrets) to keep solver stable. The main difficulty with high respect ratio is that it makes pressure correction equation very difficult to converge, which could make solver unstable. OpenFOAM does not apply such tricks so things should be more difficult with it. For this reason you would need good quality meshes with openFOAM. |
Hi,
You have some cells with high nonorthogonality (90 starts to be a high value imo). You should used "limited 0,333" schemes and not "corrected" schemes (snGrad). I don't know if it would solve your problem, but it should be a start. Aurelien |
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