one can finally condense the whole source in one term, arriving to the incompressible Navier–Stokes equation with conservative external field: ∂ ∂ u ∂ ∂ t + ( u ⋅ ⋅ ∇ ∇ ) u − − ν ν ∇ ∇ 2 u = − − ∇ ∇ h .