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

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 k-epsilon 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.

I have try, if use zeroGradient for k and epsilon in the wall, the result a little not good. why ?
I remember that in OpenFOAM-1.5 , 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 ?

Take a look at the file in your OpenFOAM dictionary:

OpenFOAM-1.6/src/turbulenceModels/incompressible/RAS/RASModel/RASModel.C

at line 148. Maby it can help you.

input kqRWallFuntion meaning
 

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 k-epsilon without wall function (corresponding to Enhanced wall treatment in fluent)?

thanks

kqRWall Functions
 

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.

Try Doxygen
 

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 K-Epsilon 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

meaning of the value of kqRWallFunction
 

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!

Hi Jose,

I had the same question and got an answer here:

http://www.cfd-online.com/Forums/ope...lon-omega.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.

kqRWallFunction
 

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