Experimental study of a vortex in a magnetic field
Résumé
It is well-known that MHD flows behave differently from conventional fluid flows in two ways: the magnetic field makes the flow field anisotropic in the sense that it becomes independent of the co-ordinate parallel to the field. Secondly, the flow of liquid across the field lines induces an electric current, leading to ohmic damping. Experimental results that quantify the decay of kinetic energy and the growth of anisotropy of fully-developed MHD turbulence, when the ratio of the electromagnetic Lorentz forces to the non-linear inertial forces (termed the interaction parameter, N0) assumed low to moderate values, are available to us. In this paper, an experimental study of the long-time decay of an initially-three-dimensional flow structure subject to a steady magnetic field, when the interaction parameter takes large as well as moderate values, is presented. This investigation is also markedly different from previous studies on quasi-2D MHD flows in thin layers of conducting fluids, where only Hartmann layer friction held the key to the dissipation of the flow. The initial "linear" phase of decay of an MHD flow, characterized by dominant Lorentz forces and modelled extensively in the literature, has been observed for the first time in a laboratory experiment. Further, when N0 is large compared to unity, a distinct regime of decay of a vortex follows this linear phase. This interesting trend can be explained in terms of the behaviour of the ratio of the actual magnitudes of the Lorentz to the non-linear inertial forces- the true interaction parameter-which decreases to a constant of order unity towards the end of the linear phase of decay, and remains invariant during a subsequent "non-linear" phase.
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