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January 30, 2013, 07:48 
[Solved]Wall treatment with geometrical restriction

#1 
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Maxime Perelli
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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 (LauderSharma) for resolving the BL, but I get awful convergence. And I am doubtfully using LowRe model for my applications (incompressible, simpleFoam with Reynoldsrange between 20006000 Despite the y+ theory, I get acceptable results with realizableke 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) Sans titre1.jpg
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In memory of my friend Hervé: CFD engineer & freerider Last edited by mAx; February 4, 2013 at 03:56. 

February 4, 2013, 03:56 

#2 
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Maxime Perelli
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Problem solved by using kOmega SST model with nutUSpaldingWallFunction on fine grid.
Then k and omega were set with uniform value 1e10 instead of zeroGradient Once model converged yPlusRAS utility gave me 0.005 and 2.19 for min and max on walls.
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February 4, 2013, 05:49 

#3 
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Ehsan
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hi.congratiolation for solving the problem
How much should yplus be at walls for kOmegaSST? Could you introduce me an article about this model? 

February 4, 2013, 06:45 

#4 
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Maxime Perelli
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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.cfdonline.com/Forums/ope...megasst3.html
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February 4, 2013, 07:58 

#5 
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Ehsan
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thanks.then how you are sure about domain of y+ that is suitable or not?is there any appropriate values near wall?


February 4, 2013, 09:29 

#6 
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Maxime Perelli
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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
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February 4, 2013, 10:41 

#7 
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Ehsan
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thanks.could you send me your turbulency folders?


February 5, 2013, 06:51 

#8 
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Maxime Perelli
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/constant/RASProperties
RASModel kOmegaSST; turbulence on; /0/k inlet { type fixedValue; value uniform 0.006; } walls { type fixedValue; value uniform 1e10; } outlet { type zeroGradient; } /0/omega inlet { type fixedValue; value uniform 27.2; } outlet { type zeroGradient; } walls { type fixedValue; value uniform 1e10; } /0/nut inlet { type calculated; value uniform 0; } outlet { type calculated; value uniform 0; } walls { type nutUSpaldingWallFunction; value uniform 0; }
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February 11, 2013, 04:42 

#9 
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Gwenael Hauet
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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
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February 11, 2013, 05:59 

#10 
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Maxime Perelli
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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 1e10 at walls (instead of zeroGradient) Bonne journee
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March 9, 2013, 23:54 
B.C. for omega @wall

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Fumiya Nozaki
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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(1e10) at the walls instead of using the wall function? Best regards, Fumiya 

March 11, 2013, 07:14 

#12 
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Maxime Perelli
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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
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March 13, 2013, 09:25 

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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 

March 13, 2013, 09:59 

#14 
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Maxime Perelli
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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
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March 13, 2013, 10:58 

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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. 

March 14, 2013, 02:59 

#16 
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Maxime Perelli
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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)
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January 7, 2014, 09:24 

#17 
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Philipp
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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 !?
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January 7, 2014, 10:19 

#18 
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Maxime Perelli
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I am pretty sure I have mixed settings for epsilon in LowRe Model with settings for omega in komega SST.
Thanks for pointing me this out!
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