Tornados and cyclones driven by Magneto-hydrodynamic forces

Abstract

A numerical analysis is carried out to explore the variety of flow structures induced inside a conducting fluid emerging from the interaction of an electric current source with an external axial magnetic field. Results are obtained for Shercliff (S) numbers up to 1010 and Hartmann (Ha) numbers up to 115, for a single magnetic Prandtl number of Pm=8.6× 10−7. Depending on these numbers, very different swirling flow patterns are predicted. Transitions seem to be solely controlled by two dimensionless ratios Γ=Ha/(PmS)1/2 and N=Ha6Pm/S.For low Γ, vortex-breakdown, tornados and cyclones are obtained. These MHD tornados are found to be very similar in structure to atmospheric tornados. A slight increase in Γ results in the transformation of the tornado into a cyclone which rotates at relatively constant angular velocity. Surprisingly for further increase in Ha, the cyclone is restructured into an inverted tornado. The induced currents are found to be at the origin of this phenomenon. For large N, a Hartmann swirling flow occurs which is associated with almost vertical electric current lines. In this regime, both poloidal and azimuthal flows are damped, and remain mainly present in the forcing region at the edge of the electric current column. These results may be of relevance for the understanding of flow structures in the atmosphere, in aluminum reduction cells, electro-metallurgical processes, and in liquid metal batteries.