Dr. Alban Potherat
University of Coventry

Just like flows in a strong background rotation, flows of electrically conducting fluids in a strong background magnetic field have a tendency to become two-dimensional. The question of how why and when this happens has been the subject of a number of studies, in particular numerical ones where the transitional behaviour was analysed in configuration involving periodic or slip-free boundary conditions. Yet, in most realistic cases, bounding walls are present that have a direct influence on the process.

We report the results of an experiment where we studied the 2D-3D transition in a cubic container filled with liquid metal. We show that unlike the case with non-dissipative boundaries, three-dimensionality does not appear at a bifurcation when the flow intensity increases, but rather progressively. We also distinguish between two types of three-dimensionalities: a strong and a weak one. The weak one features moredate variations of physical quantities along the magnetic field direction, akin to the "Barrel effect" previously identified theoretically. It can also drive rather strong recirculating flows. Strong three-dimensionality, on the other hand, is a scale-selective breakdown mechanism that affects the smaller scales first and progressively contaminates the whole spectrum when the flow intensity increases.