[brucethemuce]

I am implementing marangoni convection in compressibleMultiphaseInterFoam. Modifying the source code directly works as expected by the same form does not work in with fvOptions.
Using just fvc::grad(T) works fine, but the equation needs the gradient tangential to the interface.

Any ideas or help is much appreciated.

FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvOptions;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

MarangoniSource
  {
    type            vectorCodedSource;
    active          true;
    selectionMode   all;
    fields (rho, U);
    name MarangoniSource;

  codeLibs
  #{

  #};

    codeInclude
    #{
      #include "fvcDiv.H"
      #include "fvcGrad.H"
      #include "fvcSnGrad.H"
      #include "fvcFlux.H"
      #include "fvcMeshPhi.H"
      #include "surfaceInterpolate.H"
      #include "fvCFD.H"
      #include "fvc.H"
    #};

codeAddSup
#{
    vectorField& tangent_source = eqn.source(); //source will be a vector field
    const volScalarField& T = mesh_.lookupObject<volScalarField>("T");  
    const volScalarField& alpha1 = mesh_.lookupObject<volScalarField>("alpha.vapor");
    const volScalarField& alpha2 = mesh_.lookupObject<volScalarField>("alpha.pentane");
    const volVectorField& n      =( alpha2*fvc::grad(alpha1) - alpha1*fvc::grad(alpha2)  ); //normal vector to interface
    const volVectorField& nHat   = 
    (n /(( mag(n) ) 
    + dimensionedScalar("tinyNumber", dimensionSet(0, -1, 0, 0, 0, 0, 0), 1e-8))); //unit normal


    const volVectorField& Tangent=
      (
        mag(n)*
        dimensionedScalar("dSigma_dT", dimensionSet(1, 0, -2, -1, 0, 0, 0),-1e-4) *
        (
            (
                fvc::grad(T) 
                - (nHat & fvc::grad(T)) * (nHat) //problem is here
            )
        )
      ); //end field

    forAll(tangent_source, i){
      tangent_source[i][0] += Tangent[i][0];
      tangent_source[i][1] += Tangent[i][1];
      tangent_source[i][2] += Tangent[i][2];
    }

Pout << "*adding the tangent surface tension force*" << endl;
#}; //end of addsup

  codeCorrect
  #{

  #};

  codeConstrain
  #{

  #};

}//end of source

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