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

    // Add MRF and porous sources
    mrfZones.addCoriolis(UEqn());
    pZones.addResistance(UEqn());

    UEqn().relax();

    UpEqn.insertEquation(0, UEqn());