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-   -   what is correct boundary condition for K-w sst model at y+<1? (https://www.cfd-online.com/Forums/openfoam/121743-what-correct-boundary-condition-k-w-sst-model-y-1-a.html)

mechy August 4, 2013 14:35
what is correct boundary condition for K-w sst model at y+<1?
 
Hi

I found some thread about k-w sst boundary conditions
unfortunately they are not clear in low y+

please let me know what is correct boundary conditions for k, w and nut
for y+<5

Best Regards

andre.weiner August 4, 2013 15:42
Hello!

omega:
Menter's automatic wall funcion works for all parts of the momentum boundary layer,
in openfoam it's the omegaWallFunction http://www.cfd-online.com/Wiki/Near-...k-omega_models
or you use the solution for the viscos sublayer to set the boundary value
http://turbmodels.larc.nasa.gov/sst.html (there is an equation for omega_wall)
for y^+<5 both give the same results

k:
fixed value 0 or
zero gradient
if you look at same results for k near the wall (from other simulations for example), you will see that both are valid (again for y^+<5)

nut:
calculated or
fixed value 0 or
nutLowReWallFunction (sets the value to zero and provides a function for calculating y^+ which is needed for the yPlusRAS tool)

This is what i tried so far and it worked for mi :-)

Best regards, Andre

mechy August 5, 2013 03:51
Quote:
Originally Posted by andre.weiner (Post 443776)
Hello!

nut:
calculated or
fixed value 0 or
nutLowReWallFunction (sets the value to zero and provides a function for calculating y^+ which is needed for the yPlusRAS tool)

This is what i tried so far and it worked for mi :-)

Best regards, Andre
thanks so much for your answer

what function should be set for calculating y+ ? and how set it ?
can you attached an example for k,w,nut (initial file at folder 0)?

Best Regards

Joachim August 5, 2013 10:29
Actually, I think that nutLowReWallFunction is a wall function for low-Re turbulence models. If you use LaunderSharmaKE and have y+ ~ 60, you can use it for example.

Also, try using 1e-9 (or any small value) rather than 0 when imposing the BC. :)

Good luck!

andre.weiner August 5, 2013 14:17
for k:

Code:
    myWall
    {
        type            zeroGradient;
    }
// or
    myWall
    {
        type            fixedValue;
        value          uniform 0;
    }
for omega:

Code:
    myWall
    {
        type            omegaWallFunction;
    }
for nut:

Code:
    myWall
    {
        type            fixedValue;
        value          uniform 0;
    }
// or
    myWall
    {
        type            calculated;
        value          uniform 0;    // optional value entry
    }
// what i prefer
    myPatch
    {
        type            nutLowReWallFunction;
    }
I don't know for what the nutLowReWallFunction schould or schloud not be, but if you take little look at the code:

it sets nut to zero:
Code:
tmp<scalarField> nutLowReWallFunctionFvPatchScalarField::calcNut() const
{
    return tmp<scalarField>(new scalarField(patch().size(), 0.0));
}
and provides a function for y+:

Code:
tmp<scalarField> nutLowReWallFunctionFvPatchScalarField::yPlus() const
{
....
    return y*sqrt(nuw*mag(Uw.snGrad()))/nuw;
}
Regards, Andre


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