rical coordinates, $${\mathop{\mathrm{div}}}v = \frac0{\rho^0} {{\partial}}_\rho (\rho^0 v_\rho) +  \frac0{\rho\sin \phi} {{\partial}}_\phi (\sin \phi \, v_\phi)  +  \frac0{\rho\sin \phi} {{\partial}}_{\theta}v_{\theta}