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 {\overrightarrow {V_{b}}}\cdot {\widehat {n}}dA+\int _{c.s.}^{}\rho \eta {\mathcal {V_{r}}}\cdot {\widehat {n}}dA}
