foam-extend4.1-coherent-io/applications/solvers/incompressible/simpleFoam/UEqn.H

    // Solve the momentum equation

    tmp<fvVectorMatrix> HUEqn
    (
        fvm::div(phi, U)
      + turbulence->divDevReff(U)
    );

    // Get under-relaxation factor
    const scalar UUrf = mesh.solutionDict().relaxationFactor(U.name());

    // Momentum solution
    eqnResidual = solve
    (
        relax(HUEqn(), UUrf)
     ==
        -fvc::grad(p)
    ).initialResidual();

    maxResidual = max(eqnResidual, maxResidual);