Lorentz force effects in magneto-turbulence

We experimentally characterize magnetic field fluctuations in a strongly turbulent flow of liquid sodium in the presence of a large externally applied field. We reach high interaction parameter (up to N=17) for moderate magnetic Reynolds number (up to Rem=18), a previously unexplored parameter range for liquid metal flows. As the interaction parameter (i.e. the ratio of Lorentz to inertial forces) is increased, the system passes through distinct regimes, which we classify. We find that for certain ranges of the applied magnetic field, particularly at high values, the induced magnetic field exhibits large, coherent oscillations. Spatial structure in these induced field oscillations suggests the formation of non-axisymmetric vortices that precess at a fraction of the impeller rotation rate. We also investigate the effect of rough versus smooth boundaries and relate these results to topographic core–mantle coupling in the Earth.