Instability of the swirling flow of an electrolyte driven electromagnetically*

We present experimental results complemented with numerical simulation of a recently discovered instability observed in a free-surface flow driven by an azimuthal electromagnetic force in a thin layer of an electrolyte contained in an open annulus. The force is created by a radial current flowing between two concentric electrodes and a uniform magnetic field produced by a permanent magnet. The flow instability leads to the formation of travelling anticyclonic vortices close to the external electrode that exist for long times once they appear. Experimental characterization includes dye visualization, thermography and PIV. A small layer thickness and the circumferential direction of the driving force suggest that the flow should be essentially uni-directional and could be described by approximate Q2D equations. Surprisingly, we found that not only the flow is fully 3D, but also multiple axisymmetric flow solutions can exist for the same set of governing parameters. However, a linear stability analysis indicates that only one of such solutions can potentially lead to azimuthally periodic vortex patterns observed in our experiments. Finally, it is shown that it is possible to reproduce the emergence of the instability using a fully 3D hybrid finite volume-spectral numerical method.