In the general case and not only in the incompressible case, the energy equation means that for an inviscid thermodynamic fluid the specific entropy is constant along the flow lines, also in a time-dependent flow. Basing on the mass conservation equation, one can put this equation in the conservation form: ∂ ∂ ρ ρ s ∂ ∂ t + ∇ ∇ ⋅ ⋅ ( ρ ρ s u ) = 0 meaning that for an inviscid nonconductive flow a continuity equation holds for the entropy.