[Solved]-Wall treatment with geometrical restriction
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Hello,
I am turrning around since a week for wall treatment. My geometries don't allow me keeping first cell's y+ >30. If I generate BL with first cell's height computed for y+ =30, then I don't have enough cells for resolving flowfield in smallest section I tried to switch on Low Re turb. model (Lauder-Sharma) for resolving the BL, but I get awful convergence. And I am doubtfully using Low-Re model for my applications (incompressible, simpleFoam with Reynolds-range between 2000-6000 Despite the y+ theory, I get acceptable results with realizable-ke and default wall function (with yPlusRAS giving me min & max y+ respectively 0.5 and 30) Results (drop pressure) are confirmed with test Any advice from anyone? PS: I post picture of my mesh in minimal section (y+~1) Attachment 18629 |
Problem solved by using k-Omega SST model with nutUSpaldingWallFunction on fine grid.
Then k and omega were set with uniform value 1e-10 instead of zeroGradient Once model converged yPlusRAS utility gave me 0.005 and 2.19 for min and max on walls. |
hi.congratiolation for solving the problem;)
How much should yplus be at walls for kOmegaSST? Could you introduce me an article about this model? |
in my case y+ goes from 0.005 till 2.2 (results from yPlusRAS)
I don't have article about this, but I searched a lot in the forum. For example: http://www.cfd-online.com/Forums/ope...megasst-3.html |
thanks.then how you are sure about domain of y+ that is suitable or not?is there any appropriate values near wall?
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what I understood: this wall treatment switchs automatically between lowRe and standard wall function dependantly on your local y+
In my case, I build my mesh with y+~1 |
thanks.could you send me your turbulency folders?
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/constant/RASProperties
RASModel kOmegaSST; turbulence on; /0/k inlet { type fixedValue; value uniform 0.006; } walls { type fixedValue; value uniform 1e-10; } outlet { type zeroGradient; } /0/omega inlet { type fixedValue; value uniform 27.2; } outlet { type zeroGradient; } walls { type fixedValue; value uniform 1e-10; } /0/nut inlet { type calculated; value uniform 0; } outlet { type calculated; value uniform 0; } walls { type nutUSpaldingWallFunction; value uniform 0; } |
Hi -mAx-,
I have just a question about the behaviour of your flow when the y+~0.005. Usually the y+ rules for SST is y+~1 but in practice, it is very difficult to have on every walls the y+~1. Did you see some strange behaviour of your flow when y+ goes down, y+~0.005 ? Thanks |
Bonjour Gwenael,
I don't think the problem may come from y+~0.005, since it is << 1. But the problem may occures if you have cells with y+~30. But as far as I read, then you can use nutUSpaldingWallFunction which selects automatically the right wall function depedantly on your y+. In my case, I know that I have always y+~1 (or <<1, but never ~30), so I enforced (I think) solving directly sublayer by setting k and omega as 1e-10 at walls (instead of zeroGradient) Bonne journee |
B.C. for omega @wall
Hi -mAx-,
I understand that the appropriate boundary condition for omega is the "omegaWallFunction". Did you get good results by fixing omega to very small values(1e-10) at the walls instead of using the wall function? Best regards, Fumiya |
Hello fumiya,
I set k and omega with very low value, since I know that my y+ are below 1. That's why I used nutUSpaldingWallFunction and not omegaWallFunction. And I got good results |
Hi -mAx-
so I understand this methods works only if you can ensure having y+~1 or y+<1 everywhere. Otherwise with higher y+ you would have to use continuous wallfunctions for k and omega in combination with the nutUSpaldingWallFunction. I am being correct? Best regards, Chris |
Hello Chris,
I would rather say, with y+ >>1 you may use nutUSpaldingWallFunction, but k and omaga at walls should be zeroGradient. Turbulence gurus may correct me |
Hi -mAx-
yes I agree wallfunctions really being for a higher y+ range. But lets consider your case where you have walls with y+~1 and y+<<1 and now in addition also walls with y+ going up to 50. I think using nutUSpaldingWallFunction with continueous k and omega wallfunctions could be beneficial with varying y+ at walls. |
Yes, if I have also walls with y+ up to 50, then I would use nutUSpaldingWallFunction with k and omega set as zeroGradient.
I set both them to very small value (not zeroGradient), if I am sure that max y+ at walls is O(1) |
Hi Maxime,
Are you still sure about your boundary conditions? As far as I know omega goes to infinity at the walls. But you set it to zero. Why did you do so? I see you also set a wall function for nut, which will overwrite the nut at the boundary anyways, but setting a wrong boundary condition for omega will give wrong results in the whole domain !? |
I am pretty sure I have mixed settings for epsilon in Low-Re Model with settings for omega in k-omega SST.
Thanks for pointing me this out! |
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