Roads to turbulence for an internal MHD buoyancy-driven flow due to a horizontal temperature gradient

Transition to chaos for a magnetohydrodynamic flow driven by a horizontal temperature gradient is experimentally studied. In the core of a horizontal cylinder filled with mercury, the buoyancy-driven flow destabilises when decreasing the applied vertical magnetic field B-o (Hartmann number steps: delta Ha similar to 0.021). For a moderate Grashof number (Gr Delta(T)similar to 10(4)-10(5)), transition is characterised by time-dependent temperature oscillations whose features are made evident from spatial correlations on temperature measurements. The destabilisation of the flow gives simultaneously rise to two waves. One horizontal transversal wave is detected within the central cross-sections whereas the other is travelling along the axis of the enclosure. Since temperature and velocity fields are non-linearly coupled, these two waves, which arise from a subcritical bifurcation, exhibit consistently the same frequency f(1). Decreasing more B-o, a Hopf supercritical bifurcation yields a third and last axial standing wave (low frequency f(2)) before the arising of chaos. For a larger Grashof number (Gr Delta(T)>10(6)), transition is dramatically modified by the presence of stabilisation or oscillating windows whose occurence is dependent both on space and on the control parameter Ha.