CFD Online Discussion Forums - Heat transfer in a channel: solving for T; questions about Pe and solving

Hi,

I'm doing CFD for a wavy channel, using simpleFoam and the k-omega-SST model to solve the velocity field. I'd now like to solve for T, and did some intermediate processing (post-U, pre-T):

For my first attempt at solving T (scalarTransportFoam) I used

Code:

div(phi,T) Gauss upwind;

which worked well and converged quickly. However, when switching to Gauss linear, the solution diverged. A colleague suggested to check the Peclet number first, which I calculated with the following code snippet:

Code:

    // characteristic length of each cell: cbrt(V)

    volScalarField CLen

    (

      IOobject (

        "CLen",

        runTime.timeName(),

        mesh,

        IOobject::NO_READ,

        IOobject::AUTO_WRITE

      ),

      mesh,

      dimensionedScalar(

        "CLen",

        dimLength,

        0.

      )

    );

    forAll (CLen, i)

    {

      CLen.internalField()[i] = Foam::cbrt(mesh.V()[i]);

    }

    

    // Pe number

    volScalarField Pe

    (

      IOobject (

        "Pe",

        runTime.timeName(),

        mesh,

        IOobject::NO_READ,

        IOobject::AUTO_WRITE

      ),

      mesh,

      dimensionedScalar(

        "Pe",

        dimless,

        0.

      )

    );

    Pe = CLen*mag(U)/(alpha + turbulence->nut()/Prt);

The results are somewhere between 6e-5 and 300. The books say that I'd need Pe < 2 for more accurate schemes.

My main question now is: what can I do to get a usable temperature field? While Pe > 2 does not necessarily mean that the solution will be wrong, I could increase alpha and nu (in order to maintain Pr = 7), but that would mean that I have to increase U or the domain size as well to adjust Re. Wouldn't that just lead to the same problem, but on a different scale?

Another possibility is of course that my code for Pe is just plain wrong. Maybe someone could comment on this; OF's Pe tool just calculates Pe at the boundary, using a different equation.

I can not refine the mesh by a factor of 150, as I already have about 2 million cells.

Regards