Dimensionality, secondary flows and helicity in low-Rm MHD vortices

Abstract

In this paper, we examine the dimensionality of a single electrically driven vortex bounded by two no-slip and perfectly insulating horizontal walls a distance $h$ apart. The study was performed in the weakly inertial limit by means of an asymptotic expansion, which is valid for any Hartmann number. We show that the dimensionality of the leading order can be fully described using the single parameter $l_{z}^{{\it\nu}}/h$, where $l_{z}^{{\it\nu}}$ represents the distance over which the Lorentz force is able to act before being balanced by viscous dissipation. The base flow happens to introduce inertial recirculations in the meridional plane at the first order, which are shown to follow two radically different mechanisms: inverse Ekman pumping driven by a vertical pressure gradient along the axis of the vortex, or direct Ekman pumping driven by a radial pressure gradient in the Hartmann boundary layers. We demonstrate that when the base flow is quasi-2D, the relative importance of direct and inverse pumping is solely determined by the aspect ratio ${\it\eta}/h$, where ${\it\eta}$ refers to the width of the vortex. Of the two mechanisms, only inverse pumping appears to act as a significant source of helicity.