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standard kEpsilon, kqRWallFunction and epsilonWallFunction (why should give value ?) 

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December 9, 2009, 06:19 
standard kEpsilon, kqRWallFunction and epsilonWallFunction (why should give value ?)

#1 
Senior Member
Jiang
Join Date: Oct 2009
Location: Japan
Posts: 186
Rep Power: 10 
Dear foamers:
I have been confused by the k and epsilon wall boundary condition in tutorials/imcompressible/simpleFoam/pitzDaily/0. It write like this: upperWall { type kqRWallFunction; value uniform 0.375; } upperWall { type epsilonWallFunction; value uniform 14.855; } in general, standard kepsilon model, in the wall using wall function, that means k can be got by solving k equation(using dk/dn=0 in the wall), then epsilon or velocity can be got through empirical formular. But I don't understand the exact meaning of kqRWallFunction and epsilonWallFunction, why should give a value here. I want to know it is neglected automaticly when using kEpsilon model. Or if I use zeroGradient for k and epsilon in the wall will be better ? Thanks for your reply. 

December 10, 2009, 00:09 

#2 
Senior Member
Jiang
Join Date: Oct 2009
Location: Japan
Posts: 186
Rep Power: 10 
I have try, if use zeroGradient for k and epsilon in the wall, the result a little not good. why ?
I remember that in OpenFOAM1.5 , just use zeroGradient for k and epsilon in the wall. kqRWallFunction and epsilonWallFunction just begin to use in 1.6 version. I am confused. If OpenFOAM use standard wall function for kEpsilon model ? 

December 10, 2009, 06:06 

#3 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
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Take a look at the file in your OpenFOAM dictionary:
OpenFOAM1.6/src/turbulenceModels/incompressible/RAS/RASModel/RASModel.C at line 148. Maby it can help you. 

May 22, 2010, 07:29 
input kqRWallFuntion meaning

#4 
Member
Aldo Iannetti
Join Date: Feb 2010
Posts: 48
Rep Power: 10 
Hi,
Can you please explane me the meaning of the input I have to give to (for exemple turbulent kinetic energy) near walls: { type kqRWallFunction; value uniform 0.001; } how to use kepsilon without wall function (corresponding to Enhanced wall treatment in fluent)? thanks 

September 30, 2010, 07:10 
kqRWall Functions

#5 
New Member
CFD user
Join Date: Apr 2010
Location: Germany
Posts: 28
Rep Power: 9 
Hello Panda,
I am trying to use the same wall function but I am not able to understand what it stands for. Did you figure out the answer to the question you asked here? My function is defined as below. { type kqRWallFunction; value uniform 0.375; } } This is similar to Omega, { type omegaWallFunction; Cmu 0.09; kappa 0.41; E 9.8; beta1 0.075; value uniform 3.5; } Where do I find all these coefficients? How does changing them has an impact on the overall solution. I looked at the file name given above, but I didn't get it. Thanks, Subhkirti. 

October 1, 2010, 02:45 
Try Doxygen

#6 
Member
Alan Russell
Join Date: Aug 2009
Location: Boise, Idaho USA
Posts: 61
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You can search Doxygen to find where functions are defined. Here's the Doxygen definition of kqRWallFunction from Doxygen:
Detailed Description template<class Type> class Foam::incompressible::RASModels::kqRWallFunctionFv PatDetailed Description template<class Type> class Foam::incompressible::RASModels::kqRWallFunctionFv PatchField< Type > Boundary condition for turbulence k, Q, and R when using wall functions. Simply acts as a zero gradient condition. Source files kqRWallFunctionFvPatchField.H kqRWallFunctionFvPatchField.C Definition at line 50 of file kqRWallFunctionFvPatchField.H. chField< Type > Boundary condition for turbulence k, Q, and R when using wall functions. Simply acts as a zero gradient condition. Source files kqRWallFunctionFvPatchField.H kqRWallFunctionFvPatchField.C Definition at line 50 of file kqRWallFunctionFvPatchField.H. Turbulence model standard coefficients are defined in source code. For example, look in /src/turbulenceModels/incompressible/RAS/kEpsilon/kEpsilon.C. If you want to change the coefficients, it's easy to do in the RASProperties file in /constant directory of your simulation. To modify KEpsilon coefficients, insert the following lines after turbulence on: kEpsilonCoeffs { Cmu x.x; C1 y.y; C2 z.z; } This will change the three listed coefficient values and leave the default values for the others. Alan 

January 18, 2011, 06:55 
meaning of the value of kqRWallFunction

#7 
Member
José
Join Date: Jan 2011
Posts: 73
Rep Power: 9 
Hello,
Can anybody explain me which is the meaning of the "value uniform" one sets on the wallFunction for k? wall { type kqRWallFunction; value uniform 0.00735; } Thank you for your help! 

January 20, 2011, 11:42 

#8 
New Member
Join Date: Nov 2010
Posts: 6
Rep Power: 9 
Hi Jose,
I had the same question and got an answer here: http://www.cfdonline.com/Forums/ope...lonomega.html The value specified with a wall function is kind of an initial guess. Greetings Gerard P.S.: Sorry for the misspelling of you name, I am not used to the keys I use at this moment. 

August 12, 2017, 15:03 
kqRWallFunction

#9 
New Member
Amir
Join Date: Jan 2017
Posts: 13
Rep Power: 3 
Hi Foamers,
I have a question about the standard wall function. I read that at high Reynolds number in the closest cell to the wall we should calculate the k by this equation: k= sqr (utau)/Cmu25 but I have seen that everywhere it is recommended to use kqRWallFunction that in fact gives the zeroGradient boundary condition. Could anybody help me to understand this? Regards, Amir Last edited by albet; August 12, 2017 at 16:05. 

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