The mass balance for a fluid with density ($\rho$) and velocity ($\overrightarrow{V}$) is
- $\frac{\partial\rho}{\partial{t}}$ + $\bigtriangledown$.($\rho\overrightarrow{V}$) = 0
- $\frac{\partial\rho}{\partial{t}}$ + $\overrightarrow{V}$ .($\bigtriangledown\rho$) = 0
- $\frac{\partial\rho}{\partial{t}}$ + $\rho$($\bigtriangledown\overrightarrow{V}$) = 0
- $\frac{\partial\rho}{\partial{t}}$ – $\overrightarrow{V}$.($\bigtriangledown\rho$) = 0