Another possible form for the energy equation, being particularly useful for isobarics, is: ∂ ∂ H t ∂ ∂ t + ∇ ∇ ⋅ ⋅ ( H t u ) = u ⋅ ⋅ f − − ∂ ∂ p ∂ ∂ t where H t = E t + p = ρ ρ e + p + 1 2 ρ ρ u 2 is the total enthalpy density.