// Solve the Momentum equation

MRF.correctBoundaryVelocity(U);

fvVectorMatrix UEqn
(
    fvm::ddt(U) + fvm::div(phi, U)
  + MRF.DDt(U)
  + turbulence->divDevReff(U)
 ==
    fvOptions(U)
);

UEqn.relax();

fvOptions.constrain(UEqn);

if (piso.momentumPredictor())
{
    solve(UEqn == -fvc::grad(p));

    fvOptions.correct(U);
}