CFD Online Discussion Forums - Modifing Turbulence Production Term

I wish to add something along the lines of f1()*P() to the production term of the Spalart Allmaras turbulence model. Where P() is defined as follows with relations based on the Boussinesq approximation:

http://i.imgur.com/4ywvt.png?1

I thought I could accomplish this using R() as defined in SpallartAllmaras.C along with

Code:

const volScalarField P = fvm::Sp(fvc::grad(U_),R());

but this has not worked.

Am I using Sp correctly? Or does my error come from a tensor/scalar field mismatch?
SpalartAllmaras::R() is shown below for your reference.

Any insight is appreciated.

Code:

 tmp<volSymmTensorField> SpalartAllmaras::R() const

{

    return tmp<volSymmTensorField>

    (

        new volSymmTensorField

        (

            IOobject

            (

                "R",

                runTime_.timeName(),

                mesh_,

                IOobject::NO_READ,

                IOobject::NO_WRITE

            ),

            ((2.0/3.0)*I)*k() - nut()*twoSymm(fvc::grad(U_))

        )

    );

}