The Navier–Stokes momentum equation can be derived as a particular form of the Cauchy momentum equation, whose general convective form is D u D t = 1 ρ ρ ∇ ∇ ⋅ ⋅ σ σ + g . By setting the Cauchy stress tensor σ σ to be the sum of a viscosity term τ τ (the deviatoric stress) and a pressure term − − p I (volumetric stress), we arrive at