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

// Convection-diffusion matrix
fvVectorMatrix HUEqn
(
    fvm::div(phi, U)
  + turbulence->divDevReff()
);

// Time derivative matrix
fvVectorMatrix ddtUEqn(fvm::ddt(U));

// Get under-relaxation factor
scalar UUrf =
    mesh.solutionDict().equationRelaxationFactor(U.select(pimple.finalIter()));

if (pimple.momentumPredictor())
{
    // Solve momentum predictor
    solve
    (
        ddtUEqn
      + relax(HUEqn, UUrf)
     ==
      - fvc::grad(p),
        mesh.solutionDict().solver((U.select(pimple.finalIter())))
    );
}
else
{
    U = (ddtUEqn.H() + HUEqn.H() - fvc::grad(p))/(HUEqn.A() + ddtUEqn.A());
    U.correctBoundaryConditions();
}