Instabilities in swirling liquid metal flows driven by rotating permanent magnets

J. C. Domínguez-Lozoya¹ - S. Cuevas¹ - M. Rivero² - E. Ramos¹

¹ Instituto de Energ\'ias Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos, 62580 MEXICO
² CONACYT -- Instituto Tecnológico de\,La\,Laguna, Torreón, Coahuila, 27000 MEXICO

Abstract
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 under the bottom of an electrically insulated cylindrical vessel. For a layer thickness smaller than a threshold, the swirling vortex flow created by induced Lorentz forces undergoes a spontaneous instability for specific magnet rotation frequencies, leading to the opening of the free surface and to the formation of regular structures which rotate stably with a speed of about one order of magnitude lower than that of the magnets. An ellipse and a triangle are observed for one and three rotating magnets, respectively. 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; for longer times, the resulting shape tends to a triangular form. The effect of the conducting plate at the bottom of the vessel on the occurrence of the instability is also explored. Figs 9, Refs 15.