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

#1 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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.
__________________
In memory of my friend Hervé: CFD engineer & freerider 

February 4, 2013, 05:49 

#3 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,210
Rep Power: 19 
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
In memory of my friend Hervé: CFD engineer & freerider 

February 4, 2013, 07:58 

#5 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,210
Rep Power: 19 
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
In memory of my friend Hervé: CFD engineer & freerider 

February 4, 2013, 10:41 

#7 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,210
Rep Power: 19 
thanks.could you send me your turbulency folders?


February 5, 2013, 06:51 

#8 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
/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; }
__________________
In memory of my friend Hervé: CFD engineer & freerider 

February 11, 2013, 04:42 

#9 
New Member
Gwenael Hauet
Join Date: Jan 2010
Location: Grenoble, France
Posts: 4
Rep Power: 9 
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
__________________
Gwen 

February 11, 2013, 05:59 

#10 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
In memory of my friend Hervé: CFD engineer & freerider 

March 9, 2013, 23:54 
B.C. for omega @wall

#11 
Senior Member
Fumiya Nozaki
Join Date: Jun 2010
Location: Yokohama, Japan
Posts: 213
Blog Entries: 1
Rep Power: 11 
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
In memory of my friend Hervé: CFD engineer & freerider 

March 13, 2013, 09:25 

#13 
New Member
Join Date: Feb 2012
Posts: 6
Rep Power: 7 
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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
__________________
In memory of my friend Hervé: CFD engineer & freerider 

March 13, 2013, 10:58 

#15 
New Member
Join Date: Feb 2012
Posts: 6
Rep Power: 7 
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 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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)
__________________
In memory of my friend Hervé: CFD engineer & freerider 

January 7, 2014, 09:24 

#17 
Senior Member
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 20 
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 !?
__________________
The skeleton ran out of shampoo in the shower. 

January 7, 2014, 10:19 

#18 
Super Moderator
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 3,210
Rep Power: 34 
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!
__________________
In memory of my friend Hervé: CFD engineer & freerider 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Automatic wall treatment in CFX  Chander  CFX  13  May 6, 2017 00:45 
Wall treatment with OpenFOAM  roby  OpenFOAM Running, Solving & CFD  46  March 2, 2017 08:38 
Water subcooled boiling  Attesz  CFX  7  January 5, 2013 04:32 
large y+ with enhanced wall treatment  keryfluid  FLUENT  4  May 7, 2012 05:19 
UDF for wall slipping  HFLUENT  Fluent UDF and Scheme Programming  0  April 27, 2011 12:03 