    // Construct the Momentum equation

    tmp<fvVectorMatrix> tUEqn
    (
        fvm::div(phi, U)
      + MRF.DDt(rho, U)
      + turbulence->divDevTau(U)
     ==
        fvModels.source(rho, U)
    );
    fvVectorMatrix& UEqn = tUEqn.ref();

    UEqn.relax();

    // Include the porous media resistance and solve the momentum equation
    // either implicit in the tensorial resistance or transport using by
    // including the spherical part of the resistance in the momentum diagonal

    tmp<volScalarField> trAU;
    tmp<volTensorField> trTU;

    if (pressureImplicitPorosity)
    {
        tmp<volTensorField> tTU = tensor(I)*UEqn.A();
        pZones.addResistance(UEqn, tTU.ref());
        trTU = inv(tTU());
        trTU.ref().rename("rAU");

        fvConstraints.constrain(UEqn);

        volVectorField gradp(fvc::grad(p));

        for (int UCorr=0; UCorr<nUCorr; UCorr++)
        {
            U = trTU() & (UEqn.H() - gradp);
        }
        U.correctBoundaryConditions();

        fvConstraints.constrain(U);
    }
    else
    {
        pZones.addResistance(UEqn);

        fvConstraints.constrain(UEqn);

        solve(UEqn == -fvc::grad(p));

        fvConstraints.constrain(U);

        trAU = 1.0/UEqn.A();
        trAU.ref().rename("rAU");
    }
