Experiment on a confined electrically driven vortex pair.

An experimental study of the transition to turbulence in a confined quasi-two-dimensional magnetohydrodynamic flow is presented. A pair of counterrotating vortice is electrically driven in the center of a thin horizontal liquid metal layer, enclosed in a cylindrical container and subject to a homogeneous vertical magnetic field. When the forcing is increased, the pair is displaced away from the center. Boundary layer separations from the circular wall appear that trigger a sequence of supercritical bifurcations. These are singled out in numerical calculations based on our previously developed shallow water model as well as in the experiment, and these bifurcations are shown to resemble those observed in flows past a cylindrical obstacle. For the highest forcing, the flow then ends up in a turbulent regime where the dissipation increases drastically, which we could relate to a possible transition from a laminar to a turbulent Hartmann boundary layer. Finally we show the first experimental evidence of a transition to three-dimensionality in liquid metal magnetohydrodynamics (MHD) by comparing velocity measurements on either horizontal sides of the layer as we find that columnar vortice wobble for a high enough forcing.