// Time-derivative matrix
fvVectorMatrix ddtUEqn(fvm::ddt(U));
// Convection-diffusion matrix
fvVectorMatrix HUEqn
(
fvm::div(phi, U)
- fvm::laplacian(nu, U)
);
// Solve the relaxed momentum equation
solve(relax(ddtUEqn + HUEqn) == -fvc::grad(p));