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Spin-up of a magnetically driven tornado-like vortex

Published online by Cambridge University Press:  14 November 2013

Tobias Vogt*
Affiliation:
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, PO Box 510119, 01314 Dresden, Germany
Ilmārs Grants
Affiliation:
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, PO Box 510119, 01314 Dresden, Germany
Sven Eckert
Affiliation:
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, PO Box 510119, 01314 Dresden, Germany
Gunter Gerbeth
Affiliation:
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, PO Box 510119, 01314 Dresden, Germany
*
Email address for correspondence: t.vogt@hzdr.de

Abstract

The spin-up of a concentrated vortex in a liquid metal cylinder with a free surface is considered experimentally and numerically. The vortex is driven by two flow-independent magnetic body forces. A continuously applied rotating magnetic field provides the source of the angular momentum. A pulse of about one order of magnitude stronger travelling magnetic field drives a converging flow that temporarily focuses this angular momentum towards the axis of the container. A highly concentrated vortex forms that produces a funnel-shaped surface depression. We explore experimentally the duration, the depth and the conditions of formation of this funnel. Additionally, we measure the axial velocity and calculate the axisymmetric flow field of this transient vortex at a lower force magnitude. The spin-up vortex is similar to the corresponding developed time-averaged turbulent vortex driven by the same magnetic forces (Grants et al., J. Fluid Mech., vol. 616, 2008, pp. 135–152). There are two main differences. First, the maximum swirl concentration condition cannot be expressed as a constant ratio of the two driving forces. Second, a much higher degree of swirl concentration is feasible. We explain these differences as due to a much lower turbulence during the spin-up.

Type
Papers
Copyright
©2013 Cambridge University Press 

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Vogt et al. supplementary movie

Video sequences of the free surface at different force ratios corresponding to figure 4. The red LED inidcates the moment of the TMF spin-up.

Download Vogt et al. supplementary movie(Video)
Video 29.4 MB