[Voulet]

Hello dear Foamers.

I'm still new to CFD and I hope this questions is not too stupid.

Considering this laplacian toy solver :

/*---------------------------------------------------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     |
    \\  /    A nd           | www.openfoam.com
     \\/     M anipulation  |
-------------------------------------------------------------------------------
    Copyright (C) 2011-2017 OpenFOAM Foundation
    Copyright (C) 2019 OpenCFD Ltd.
-------------------------------------------------------------------------------
\*---------------------------------------------------------------------------*/

#include "fvCFD.H"
#include "fvOptions.H"
#include "simpleControl.H"

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

int main(int argc, char *argv[])
{

    #include "postProcess.H"
    #include "addCheckCaseOptions.H"
    #include "setRootCaseLists.H"
    #include "createTime.H"
    #include "createMesh.H"

    simpleControl simple(mesh);

    #include "createFields.H"

    // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
    while (simple.loop())
    {
        Info<< "Time = " << runTime.timeName() << nl << endl;

        while (simple.correctNonOrthogonal())
        {
            fvScalarMatrix TEqn
            (
                fvm::laplacian(k, T) == fvOptions(T)
            );

            fvOptions.constrain(TEqn);
            TEqn.solve();
            fvOptions.correct(T);

        }

        #include "write.H"

        runTime.printExecutionTime(Info);
    }
    return 0;
}
// ************************************************************************* //

with k a dimensionedScalar defined in a dictionnary as :

k              k [0 2 0 0 0 0 0] 1e3;

I get (with not yet any fvOptions file in system):

incompatible dimensions for operation 
    [T[0 0 0 1 0 0 0] ] == [T[0 0 -1 1 0 0 0] ]

I agree that in our physical world the thermal diffusion k dimension should be [0 2 -1 0 0 0 0] but why does OpenFaom doesn't allow me to do some strange things ?


TEqn is an equation of T [0 0 0 1 0 0 0].
Laplacian of T should be [0 -2 0 1 0 0 0]
and then fvm::laplacian(k, T) should be [0 0 0 1 0 0 0] with my k being [0 2 0 0 0 0 0].
Thus I would have [0 0 0 1 0 0 0] to he left hand side and I was hopping fvOptions(T) to be [0 0 0 1 0 0 0] on the right hand side but its dimensions are [0 0 -1 1 0 0 0]

Thus my question is : how does the fvOptions dimensions are chosen ? Is it always the dimension of the solved field divided by dimTime ?

Thanks for having reading me and have a nice week.

[Voulet]

Up. Anyone could give any advice ?

[dlahaye]

The constant k could either be the thermal conductivity (units W/(mK)) see https://en.wikipedia.org/wiki/Thermal_conductivity

or the thermal diffusivity (units m^2/s) see https://en.wikipedia.org/wiki/Thermal_diffusivity

fvOptions operates on T and retrieves physical units from T.