Visualisation of the Ludford column

When a liquid metal flows around a truncated cylinder in the presence of a magnetic field which is parallel to the axis of the cylinder, a stagnant region develops above the cylinder. This region is called Ludford column. The Ludford column represents the magnetohydrodynamic (MHD) analogue to the well-known Taylor columns in rotating flows. Whereas Taylor columns can be easily visualised using dye, the visualisation of Ludford columns remained elusive up to now because liquid metals are opaque. We demonstrate that this fundamental limitation of experimental MHD can be overcome by using a superconducting 5-Tesla magnet. This facility permits us to perform MHD experiments in which the opaque liquid metals are replaced with a transparent electrolyte while maintaining the key MHD-effects. We report results of a series of flow experiments in which an aqueous solution of sulphuric acid flows around a bar with square cross section. We vary the Reynolds number in the range 5 < Re < 100 and the Hartmann number in the range 0 < Ha < 14. The experimental procedure involves flow visualisations using tracer particles as well as velocity measurements using particle image velocimetry (PIV). Our experiments provide direct access to the Ludford columns for the first time and reveal the spatial structure of this basic feature of MHD flows.