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April 21, 2012, 11:56 
LowRe turbulence Model without wallfunctions

#1 
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bruce
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Dear All,
I am using Realizable kepsilon low Re. turbulence model for a simple test case with y+~13. By default, the standard wall functions are selected in GUI. I want to disable all the wall functions because my grid resolves the boundary layer. I have tried fluent commands to disable by selection, but it seems it could not work as it is getting selected again automatically. Although, it is better to include wall functions even for LowRe simulations, I need to disable it for validation purpose. Does any body help? Thank You 

April 21, 2012, 17:14 

#2  
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Lucky Tran
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Quote:
You cannot run a simulation without some type of wall function, how else do you apply your wall boundary conditions? Last edited by LuckyTran; April 21, 2012 at 17:33. 

April 23, 2012, 12:01 

#3  
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bruce
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Quote:
Quote:
Thanks 

April 23, 2012, 12:44 

#4  
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Lucky Tran
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You just need to distinguish what types of variables you like to apply at cells near the wall and only change the ones that Fluent does not do exactly the way you want it to do. So ask yourself, exactly what type of wall functions are you trying to implement? And does Fluent already do that or no? 

April 26, 2012, 14:37 

#5 
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bruce
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Dear Lucky
now i understand better. I have one more question. As said in the user guide of fluent, is epsilon really computed at the near wall cells using a algebraic relation? I meant the equation no. (10.810) in the below link. http://jullio.pe.kr/fluent6.1/help/html/ug/node451.htm Because, when i calculate the value of epsilon near the wall i get different results, for example Cmu = 0.09 kP = 0.005 kappa = 0.41 yp = 1.92e05 epsilon = 7.38 But, i found fluent reports it as 0.88. I wonder where does this difference comes from. Do you have any idea for this? Thank You 

April 26, 2012, 15:00 

#6  
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Lucky Tran
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That formula you referenced is correct for the lowRe approach (twolayer approach or standard wall function approach). For enhanced, there is a damping term. Most importantly, in the realizableke model, Cmu is no longer a constant. Cmu is constant only for the standard ke model. Did you take the variation of Cmu into account? I noticed you took Cmu as 0.09, this is the constant Cmu from the standard ke approach. 

April 26, 2012, 15:12 

#7 
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bruce
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Dear Lucky,
Sorry i forgot to mention that i used standard wall function. Since i had validation problems with enhanced wall function, i change it to standard kepsilon model with standard wall treatment. And i switched off energy and make it constant density. And as i calculated before using the relation for epsilon, i do not know why fluent result is 0.88 instead of 7.38 (at one particular cell). Do fluent use any other relation to calculate epsilon near wall? Do you see any problem? Thank You 

April 26, 2012, 15:57 

#8  
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Did you also check the calculation of turbulent kinetic energy? It probably is wrong also if dissipation is wrong. What is fluent reporting as the turbulent kinetic energy? While you are at it, also compare the production term, the production term should be approximately the same as the dissipation. Additionally, there were no convergence problems correct? There are many other ways to solve for dissipation near the walls but I had thought that those were the equations used by Fluent. I will look into it more. 

April 26, 2012, 16:07 

#9 
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bruce
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Yes, yP is the cell center to boundary distance as by definition (half of the cell height in my case).
The turbulent kinetic energy calculation is correct indeed. turbulent kinetic energy = Cmu * k^2 / epsilon. When i calculate manually that i get correct value as done by fluent. I will check the production term soon. And, since it is a flow over a flat plate, the convergence is very fast (1e07 in 1500 iterations with second order upwind). Indeed it looks odd. Thanks 

April 26, 2012, 16:10 

#10  
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April 26, 2012, 16:12 

#11 
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bruce
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forgot to mention that epsilon was taken as 0.88 what was used by fluent.


April 27, 2012, 18:04 

#12 
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bruce
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Hi Lucky,
I was very unlucky. The epsilon relation was indeed correct. The real problem was i always used paraview to visual the fluent results, for some reason data translation went wrong (may be badly interpolated) So i had used wrong kinetic energy to calculate epsilon. Now it is fine. Thank You 

April 27, 2012, 21:04 

#13  
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Lucky Tran
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