{
    volScalarField divPhi
    (
        "divPhi",
        fvc::div(phi)
    );

    // Update boundary velocity for consistency with the flux
    mrfZones.correctBoundaryVelocity(U);

    // Momentum equation
    fvVectorMatrix UEqn
    (
        fvm::div(phi, U)
      + turbulence->divDevReff()
    );

    // Add MRF sources
    mrfZones.addCoriolis(UEqn);

    // Add porous sources
    tmp<volTensorField> tTU;

    if (addPorosity)
    {
        tTU = tmp<volTensorField>
        (
            new volTensorField
            (
                IOobject
                (
                    "TU",
                    runTime.timeName(),
                    mesh,
                    IOobject::NO_READ,
                    IOobject::NO_WRITE
                ),
                mesh,
                dimensionedTensor("zero", dimless/dimTime, tensor::zero)
            )
        );
        volTensorField& TU = tTU();

        pZones.addResistance(UEqn, TU);

        trTU = inv(TU + tensor(I)*UEqn.A());
        trTU().rename("rAU");
    }
    else
    {
        trAU = 1.0/UEqn.A();
        trAU().rename("rAU");
    }

    // Under-relax momentum. Note this will destroy the H and A
    UEqn.relax();

    // Insert momentum equation
    UpEqn.insertEquation(0, UEqn);

    if (addPorosity)
    {
        // Manually over-ride the 3x3 block to handle the off-diagonal
        // part of the Ap coefficient
        const tensorField& TUIn = tTU().internalField();

        CoeffField<vector4>::squareTypeField& DD = UpEqn.diag().asSquare();

        const scalarField& V = mesh.V().field();

        // Note: insertion should only happen in porous cell zones
        // HJ, 14/Mar/2016

        register label cellI;

        forAll (pZones, pZoneI)
        {
            const labelList& curZoneCells = pZones[pZoneI].zone();

            // Loop over all cells in the zone
            forAll (curZoneCells, zcI)
            {
                cellI = curZoneCells[zcI];

                const scalar& cellV = V[cellI];

                const tensor& cellTU = TUIn[cellI];

                CoeffField<vector4>::squareType& cellDD = DD[cellI];

                cellDD(0, 0) += cellV*cellTU.xx();
                cellDD(0, 1) += cellV*cellTU.xy();
                cellDD(0, 2) += cellV*cellTU.xz();

                cellDD(1, 0) += cellV*cellTU.yx();
                cellDD(1, 1) += cellV*cellTU.yy();
                cellDD(2, 2) += cellV*cellTU.yz();

                cellDD(2, 0) += cellV*cellTU.zx();
                cellDD(2, 1) += cellV*cellTU.zy();
                cellDD(2, 2) += cellV*cellTU.zz();
            }
        }
    }
}