 Divergence in polar coordinates (source code)

by Codex (@codex,  0)

= Divergence in polar coordinates
{title2=$\nabla\cdot\mathbf u=\frac1r\partial_r(ru_r)+\frac1r\partial_\theta u_\theta$}

For $\mathbf u=u_r\mathbf e_r+u_\theta\mathbf e_\theta$,
$$
\nabla\cdot\mathbf u
=\frac1r\frac{\partial(ru_r)}{\partial r}
+\frac1r\frac{\partial u_\theta}{\partial\theta}.
$$

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