CFD Online Discussion Forums - Mismatch LES resolution index and PopeIndex

Hi everyone,

I am doing LES on OpenFoam (using the dynamicKEqn and buoyantBoussinesqPimpleFoam if it matters) and as an indicator for the resolution I am trying to compare the resolved and modeled part of the turbulent kinetic energy. As the formula by Pope: Gamma = kres/(kres+ksgs).

This index can directly be calculated in OpenFoam by using in controlDict:

Code:

PopeCrit 

 {        

     type        resolutionIndex; 

     libs        (fieldFunctionObjects); 

     model       PopeIndex; 

     writeControl        writeTime; 

     includeKnum false; 

}

An additionnal term knum can be included in the equation for Gamma.

Since I could not find a lot of information about the implementation of this index in OpenFoam I wanted to check it and calculate Gamma myself.

Looking at other threads (such as: https://www.cfd-online.com/Forums/op...ean-r-les.html) I believe I can calculate it in Paraview by doing:
Gamma = kres/(kres+k)
wherein, kres = 0.5*sqrt((U_X-UMean_X)^2+(U_Y-UMean_Y)^2+(U_Z-UMean_Z)^2)

Unfortunately, these two fields do not match. They look alike in terms of space distribution but the values are different.

So my question is: what do I misunderstand ? and where does this mismatch comes from ?
And also maybe if you some of have have already used this Pope index and checked its well implementation ?

Thank you !
Robin

-----------------------------
For reference here is how the PopeIndex is coded which seems to do the same thing I do in Paraview:

Code:

bool Foam::resolutionIndexModels::PopeIndex::read(const dictionary& dict)

{

    if (!resolutionIndexModel::read(dict))

    {

        return false;

    }



    includeKnum_ = dict.getOrDefault<bool>("includeKnum", true);

    if (includeKnum_)

    {

        Cn_ = dict.getOrDefault<scalar>("Cnu", 1.0);

    }

    UName_ = dict.getOrDefault<word>("U", "U");

    UMeanName_ = dict.getOrDefault<word>("UMean", "UMean");

    kName_ = dict.getOrDefault<word>("k", "k");

    deltaName_ = dict.getOrDefault<word>("delta", "delta");



    return true;

}





bool Foam::resolutionIndexModels::PopeIndex::execute()

{

    // Calculate resolved k field

    const auto& U = getOrReadField<volVectorField>(UName_);

    const auto& UMean = getOrReadField<volVectorField>(UMeanName_);

    const volScalarField kRes(0.5*magSqr(U - UMean));



    // Calculate subgrid-scale k field

    const auto& kSgs = getOrReadField<volScalarField>(kName_);



    // Calculate total k field

    tmp<volScalarField> tkTot = kRes + kSgs;

    if (includeKnum_)

    {

        tkTot.ref() += mag(kNum());

    }