∂ ρ ∂ t + ∇ ⋅ ( ρ u ) = 0 , ∂ ( ρ u ) ∂ t {\displaystyle {\frac {\partial \rho }{\partial t}}+\nabla \cdot \left(\rho \mathbf {u} \right)=0\,,{\frac {\partial (\rho \mathbf {u} )}{\partial t}}}
