Hi Herbert!

Indeed I want to edit the BC for p from the BC of U (in the case of a two-phase Darcy law)

Okay,

I was just confused because of your nomenclature...;)

Dear Firends!!!

I used refCast for temperture filed like this;

label patchID = mesh.boundaryMesh().findPatchID("right");
-------------------------------------------------------------------------------------------------------------
fixedGradientFvPatchScalarField& gradUPatch=refCast<fixedGradientFvPatchScalarField >(T.boundaryField()[patchID]);
scalarField& gradUField = gradUPatch.gradient();

but during compilation faced this error:
================================================== =========
solveEnergy.H:78: error: ‘fixedGradientFvPatchField’ was not declared in this scope
solveEnergy.H:78: error: ‘gradUPatch’ was not declared in this scope
solveEnergy.H:78: error: ‘fixedGradientFvPatchField’ cannot appear in a constant-expression
solveEnergy.H:78: error: no matching function for call to ‘refCast(Foam::fvPatchField<double>&)’
================================================== =========

I also add this to my header but still have error
# include "fixedGradientFvPatchFields.H"

I use fixedValue in the same way in my code but no error would occur and everything works fine!!!

what's the problem?!