$\frac{d}{dt}+\frac{1}{r}\frac{d}{dr}(r{v}_r)+\frac{1}{r}\frac{d}{d}(v_{})+\frac{d}{dz}(v_z)=0$

Spherical coordinates:

$\frac{d}{dt}+\frac{1}{r^2}\frac{d}{dr}(r^2{v}_r)+\frac{1}{rsen}\frac{d}{d}(v_{}sen)+\frac{1}{rsen}\frac{d}{d}(v_{})=0$