D N s y s D t = ∫ c . v . ∂ ∂ t ( ρ η ) d V + ∫ c . s . ρ η v b ⋅ n ^ d A + ∫ c . s . ρ η v r ⋅ n ^ d A {\displaystyle {\frac {DN_{sys}}{Dt}}=\int _{c.v.}^{}{\frac {\partial }{\partial t}}(\rho \eta )dV+\int _{c.s.}^{}\rho \eta \mathbf {v_{b}} \cdot \mathbf {\widehat {n}} dA+\int _{c.s.}^{}\rho \eta \mathbf {v_{r}} \cdot \mathbf {\widehat {n}} dA}
