While studying a text on fluid mechanics, I came across the following:
The continuity equation for a plane incompressible flow in polar form is
$$\frac{1}{r}\frac{\partial}{\partial r}(rv_r)+\frac{1}{r}\frac{\partial v_\theta}{\partial \theta}=0$$
How did it come about? There was no clear explanation in the text.
I know the continuity equation for an incompressible flow in rectangular coordinates is $u_x+v_y=0$, where $u$ and $v$ are the horizontal and vertical velocity components, respectively.