
[Sponsors] 
in kepsilon wall function approach high Re turbulence models: question of velocity 

LinkBack  Thread Tools  Display Modes 
June 27, 2013, 04:04 
in kepsilon wall function approach high Re turbulence models: question of velocity

#1 
Senior Member
Roman Thiele
Join Date: Aug 2009
Location: London, UK
Posts: 368
Rep Power: 13 
Hej,
I looked through the different wall function for kepsilon turbulence models, however I feel I am missing something. In Wilcox (Turbulence modeling for CFD 2006) as well as many other publications, in wall functions the velocity seems to be set in the first grid point. In OpenFOAM, the following properties are changed with wall functions in the example of the kepsilon turbulence model,
__________________
~roman 

July 2, 2013, 05:18 
took a while but here is a solution to the problem

#2 
Senior Member
Roman Thiele
Join Date: Aug 2009
Location: London, UK
Posts: 368
Rep Power: 13 
Hej,
I think I found the solution to this. I found it in Pope, Turbulent Flow, 2000 under the way that wall functions are created. The original question was why the velocity is not set by any of the wall function. The answer is, that wall functions are apparently more robust by setting the shear stress and not the velocity, which makes sense when looking at separation points and other nasty flow phenomena, in which the velocity can become essentially 0. The way this is for the kepsilon model done is nutkWallFunction
__________________
~roman 

July 2, 2013, 13:22 

#3 
Senior Member
Mohsen KiaMansouri
Join Date: Jan 2010
Location: CFD Lab
Posts: 118
Rep Power: 9 
Thanks roman
it was very helpful.
__________________
“If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.” 

July 2, 2013, 19:02 

#4 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,209
Rep Power: 19 
hi
Whould you explain more and Why stage 4 is done?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

July 3, 2013, 04:26 

#5 
Senior Member
Roman Thiele
Join Date: Aug 2009
Location: London, UK
Posts: 368
Rep Power: 13 
Step 4 is introduced in order to have a more stable solver. Instead of setting the velocity in the first point away from the wall, a force (the wall shear force) is introduced in the equation. This way, the solver won't have any 0 velocities introduced by accident (think of stagnation points), this could destabilize the solver as you would be introducing 0's into the matrix.
The easiest way to introduce this source term into the equation is by modifying the turbulent viscosity, see Pope, Turbulent Flows, 2000, pp 442443 and Versteeg and Malalasekera, An introduction to computational fluid dynamics: The finite volume method, pp. 276277
__________________
~roman 

March 15, 2015, 22:12 

#6  
Senior Member
Huang Xianbei
Join Date: Sep 2013
Location: CAU,China
Posts: 283
Rep Power: 6 
Quote:
Thank you for your explain. I have a question about the wall function for nonlinear models. As the nonlinear model can't be depicted by a simple turbulent viscosity, how can we set the shear stress? say we obtained the shear stress as in your form: u_shear * U_p,real / U_p,nominal, so we should set the Reynolds shear stress by u_shear * U_p,real / U_p,nominal  nu*dU/dy, here, y represents the wallnormal direction. However, the nonlinear Reynolds stress is not symmetric, it has 6 shear stress components, which one should be modified? I'm confused by this problem for some days. I'm looking forward for your reply. Thank you Xianbei 

May 26, 2016, 09:14 

#7  
New Member
karar
Join Date: Feb 2013
Posts: 13
Rep Power: 6 
Quote:
Did you solve your wall function problem. I have tried to implement a nonlinear model and I have stuck the same problem. What kind of wall function should I use. thank you 

Tags 
turbulence model, velocity, wall function 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
mesh airfoil NACA0012  anand_30  OpenFOAM Meshing & Mesh Conversion  12  December 12, 2011 05:16 
Discussion: Reason of Turbulence!!  Wen Long  Main CFD Forum  3  May 15, 2009 09:52 
Variables Definition in CFX Solver 5.6  R P  CFX  2  October 26, 2004 02:13 
Quick Question  Wall Function  D.Tandra  Main CFD Forum  2  March 16, 2004 05:29 