[panda60]

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.

[panda60]

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 ?

[idrama]

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.

[aldo.iannetti]

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

[subhkirti]

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.

[AlanR]

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

[jms]

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!

[Gerard]

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.

[albet]

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