[Ale1542]

Hi everyone, I'm currently working on a simulation involving liquid metals usign rhoSimpleFoam as a solver (and OpenFOAM v9). I need to change the Prt from the default value to 1.5. I did it by setting this value in constant/thermophysicalTransport.

FoamFile
{
    format      ascii;
    class       dictionary;
    location    "constant";
    object      thermophysicalTransport;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

RAS
{
    model   eddyDiffusivity;

    Prt     1.5;
}

I've searched through the tutorials and, apart the from the definition of Prt in the alphatWallFunction boundary condition, this is the only way I've found up to now.
Hence, my question is: Is it correct ro set a value for the Prt in the way I did? Or it's wrong and there is another way?
Thank you

[zd7s18533]

dear Ale1542

you are using eddyDiffusivity for RANS model, thus the diffusion term in energy equation is as following:

thermophysicalTransport->divq(he);

the calculation of divq(he) in eddyDiffusivity.C is as following:

-correction(fvm::laplacian(this->alpha()*this->alphaEff(), he))
-fvc::laplacian(this->alpha()*this->kappaEff(), this->thermo().T());

where alpha is phase fraction used in multiphase(for rhoSimpleFoam this term equals to unity), alphaEff is Effective thermal diffusivity of enthalpy, kappaEff is Effective Effective thermal turbulent diffusivity

the calculation of kappaEff and alphaEff in heThermo.C is shown as follows(assuming that you are solving enthalpy equation):

alphaEff = this->kappa_/Cp_ + alphat;
kappaEff = this->kappa_ + Cp_*alphat;

the calculation of alphat in eddyDiffusivity.C is shown as follows:
alphat_ =
this->momentumTransport().rho()
*this->momentumTransport().nut()/Prt_;

in short, you did the right thing, you can see that Prt_ is used to calculate alphat_

[zd7s18533]

dear Ale1542

you did the right thing and Prt_ is used to calculate alphat_: turbulent thermal diffusivity of energy of mixture.

you can inspect the code in heThermo.H, heThermo.C, eddyDiffusivity.H and eddyDiffusivity.C to find more information

[Ale1542]

Thank you very much for your answer!
One last question if you don't mind: the "compressible::alphatWallFunction" can be used in a low-Re approach? My y+ is lower than 1 and I'm not sure if this formulation can handle a wall resolving approach (for example, referring to the kOmegaSST model, I'm sure that the "omegaWallFunction" can handle both low and high-Re approaches).

Thank you again for your time

[zd7s18533]

I guess it depends on which nutwallfunction is used, you can inspect the code in alphatWallFunctionFvPatchScalarField.H:

operator==(rhow*tnutw/Prt_);

obviously the wall alphat is calculated from the density on the wall, turbulent viscosity on the wall and Prandtl number, thus if you use lowRe nut wall function, then alphat is calculated from low-Re approach