Destabilizing actions of steady magnetic fields on electrically conduction flows

The application of an external magnetic field to the flow of any electrically conducting fluid can drastically change the flow characteristics, particularly for highly conducting fluids like liquid metals or semiconductor melts. If the magnetic field is steady there is the general expection that the magnetic field influence is a damping one: suppression of instabilities, turbulence damping up to relaminarization, suppression of vortex shedding, etc. Indeed, several metallurgical applications of magnetic fields are based on the assumption that this action always consists of a strong damping. The primary action of a steady magnetic field is an anisotropic redistribution of vorticity. If this results in flow damping depends strongly on the question how the induced electric currents can close in the liquid volume. In the present lecture we give some examples from our research projects in the field where a destabi-lizing action of a steady magnetic field was found, even resulting in a serious increase of turbulence intensity for increasing magnetic field. First example is a simple liquid metal duct flow. Experimental results will be presented showing that by a suitable promotion of vorticity parallel to the magnetic field lines the turbulence intensity (and correspondingly the heat & mass transfer) is increased by factors 4 to 10 compared to both the non-magnetic case and the case without turbulence promoters. The turbulence intensity can even increase with growing distance from the turbulence promoter. A physical interpretation will be given. Second example is the standard hydrodynamic problem of a flow around a circular cylinder. 2-D and 3-D numerical simulations will be presented showing clearly the suppression of the Karman vortex street due to the magnetic field influence, but also the destabilizing magnetic field action: 3-D instability is found for Re=180, a Reynolds-number where the pure hydrodynamic flow is known to be 3-D stable. Due to the magnetic field action the Squire Theorem is no more valid and, indeed, 3-D instability is found at parameters where the flow is 2-D stable. Physical interpretation and comparison with experimental results will be given. Further examples for a destabilizing magnetic field action will be briefly mentioned: flows driven by thermoelectricity, steady magnetic field influence on an inductively stirred melt. These experimental results will mainly be shown by video sequences.