[PanPeter]

Hi together

I want to define a field in the object registry in the same manner as phi is defined for the rhoSimpleFoam solver, in which the density rho*U interpolated to the fv surfaces and multiplied with the respective fv surface size (line 11).

surfaceScalarField phi
(
    IOobject
    (
        "phi",
        runTime.timeName(),
        mesh,
        IOobject::READ_IF_PRESENT,
        IOobject::AUTO_WRITE
    ),
    linearInterpolate(rho*U) & mesh.Sf()
);

How can I define a new object phi_out that only interpolates the density flux rho*U on a specific mesh boundary, for example an outlet?
I.e. how should I change line 11 to apply the calculation to the mesh boundary only?