Free surface vortex instability in magnetically driven liquid metal swirling flow

Abstract eng:
We describe experimental observations of a free surface vortex instability in a shallow liquid metal layer driven by different arrays of rotating magnetized bars located underneath the bottom of a cylindrical container. For layer thicknesses smaller than a certain threshold, the swirling vortex flow created by induced Lorentz forces reveals a spontaneous instability for specific magnet rotation frequencies, leading to the opening of the free surface and the formation of regular structures that rotate stably with a speed about one order of magnitude lower than the magnets. An ellipse and a triangle are observed for one and three rotating magnets. When four or five magnets are used, transient polygons with the same number of corners appear on the deformed surface at the initial stages of spin-up. It is suggested that the formation of the structures is a consequences of resonant interactions between gravity and centrifugal waves.

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