I am getting stuck in a detail in a paper. It's about the axi symmetric Navier Stokes equations
$$u\_t - \nu\,\Delta u + u\cdot \nabla u + \nabla p=0$$
We consider in cylindrical coordinates $u=(u^r, u^\theta,u^z)$. And we have the following vorticity equation in cylindrical form.
$$\omega^r = \frac{1}{r}\frac{\partial u^z}{\partial \theta} - \frac{\partial u^\theta}{\partial z} = - \frac{\partial u^\theta}{\partial z}, \\ \omega^\theta = \frac{\partial u^r}{\partial z} - \frac{\partial u^z}{\partial r}, \\ \omega^z = \frac{1}{r}\frac{\partial}{\partial r}(r u^\theta) - \frac{1}{r} \frac{\partial u^r}{\partial \theta} = \frac{1}{r}\frac{\partial}{\partial r}(r u^\theta).$$
My question goes as follows: Suppose $J=\frac{w^r}{r}$, then how to derive the following steps?


\begin{align}
& \int J(\omega^r \partial\_r+\omega^z \partial\_z)\frac{u^r}{r} r \, dr \, dz \\[8pt]
= {} & \int [\nabla\times (u^\theta e\_\theta)] \left(J \, \nabla \frac{u^r}{r}\right) r \, dr\,dz \\[8pt]
= {} & \int (u^\theta e\_\theta) \left(\nabla J\times\nabla \frac{u^r}{r}\right) r \, dr \, dz
\end{align}


I'm assuming some integration by parts is involved but couldn't derive them; besides, where does the cross product come from? The equality above that bugs me is from the bottom of page 11 of [this](https://arxiv.org/pdf/1505.02628.pdf) paper.



Cylindrical coordinates in axis symmetric flow
There's a typo in the last two expressions you write: $u^{\theta } /r$ should be $u^r /r$.


Then, it seems to check out (for any well-behaved $J$): In the first expression, just insert the expressions for the $\omega $ in terms of the $u$; in the second expression, note
$$
\nabla \times (u^{\theta } e\_{\theta } ) = e\_z \partial\_{r} u^{\theta } + e\_z u^{\theta } /r - e\_r \partial\_{z} u^{\theta }
$$
and
$$
\nabla (u^r /r) = e\_r \left( \frac{1}{r} \partial\_{r} u^r -\frac{1}{r^2 } u^r \right) + e\_{\theta } \frac{1}{r^2 }\partial\_{\theta } u^r +e\_z \frac{1}{r} \partial\_{z} u^r
$$
and multiply that out - no integration needed, already the integrands are identical. To get to the third expression, consider the volume integral of
$$
\nabla \cdot \left[ (u^{\theta } e\_{\theta } ) \times (J\nabla(u^r /r)) \right]
$$
You don't say anything about the boundary conditions, but I suppose the field in the square brackets doesn't have net flux out of the volume - then this vanishes by Gauss' theorem. Also, the $u$ have to be regular enough so that different components of $\nabla $ commute on them ...


