foam-extend4.1-coherent-io/applications/solvers/heatTransfer/buoyantPisoFoam/UEqn.H

    // Solve the Momentum equation

    fvVectorMatrix UEqn
    (
        fvm::ddt(rho, U)
      + fvm::div(phi, U)
      + turbulence->divDevRhoReff(U)
    );

    UEqn.relax();

    if (piso.momentumPredictor())
    {
        solve
        (
            UEqn
         ==
            fvc::reconstruct
            (
                fvc::interpolate(rho)*(g & mesh.Sf())
              - fvc::snGrad(p)*mesh.magSf()
            )
        );
    }