Experimental and theoretical study of the dynamics of wakes generated by magnetic obstacles

D. R. Domínguez¹ - A. Beltrán² - J. J. Román¹ - S. Cuevas¹ - E. Ramos¹

¹ Renewable Energy Institute, Universidad Nacional Autónoma de México 62580, Temixco Mor. Mexico
² Materials Research Institute (Morelia Unit), Universidad Nacional Autónoma de México, 58190, Morelia, Mich., Mexico

Abstract
We present an experimental and theoretical study of the dynamics of wakes generated by magnetic obstacles. The experimental obstacle was realized by circulating a liquid metal inside a closed loop with a slender cross-section and imposing a fixed localized magnetic field in a specific spot of the loop. Experimental observations made with an Ultrasonic Doppler Velocimeter include records of the axial velocity of liquid metal as a function of the axial coordinate in the region where the wake of the obstacle is formed. This information reveals important features of the stability and dynamics of the wake of the magnetic obstacle. The theoretical study is based on a numerical solution of a quasi-two dimensional model of the MHD balance equations whose non-dimensional form indicates that the flow can be described in terms of two parameters, the Reynolds and the Hartmann numbers. The numerical model considers the induced magnetic field as an electromagnetic variable (B-formulation). Theoretical studies predict that for a given Hartmann number, the flow transits from a steady state to a time-dependent state as the Reynolds number is increased as in the wake of a rigid obstacle, but in sharp contrast to this case, when the Reynolds number is increased further, the flow becomes steady again. Our experimental observations confirm that this prediction is correct. Figs 7, Refs 18.