# Continuity equation

1. Apr 1, 2009

### lukaszh

To solve
$$\frac{\partial\varrho}{\partial t}+\mathrm{div}(\varrho\vec{v})=0$$

2. Apr 1, 2009

### isabelle

Re: solve

That is the continuity equation, it expresses the fact that the total time derivative of rho is zero. But if v is also an unknown vector function, then you have four unknowns and one equation so the system is underdetermined.