Two-Dimensional Oscillation Of Convection Roll In A Finite Liquid Metal Layer Under A Horizontal Magnetic Field

We investigate the two-dimensional (2-D) oscillation of quasi-2-D convection rolls in a liquid metal layer confined by a vessel of aspect ratio five with an imposed horizontal magnetic field. Laboratory experiments were performed in the range of Rayleigh and Chandrasekhar Q numbers of 7.9 x 10(4) <= Ra = 1.8 x 10(5) and 2.5 x 10(4) <= Q <= 1.9 x 10(5) by decreasing Q at set Ra-number intervals to elucidate the features and mechanisms of oscillatory convection. Ultrasonic velocity profile measurements and supplemental numerical simulations show that the 2-D oscillations are caused by oscillations of recirculation vortex pairs between the main rolls, which are intensified by periodic vorticity entrainment from the vortex pair by the main rolls. The investigations also suggest that the oscillations occur at sufficiently large Reynolds Re numbers to induce instabilities on the vortex pair. The Re number is smaller for larger Q/Ra in the 2-D oscillation regime and the variations can be approximated by the effective Ra number; namely, the value reduced by the critical value for the onset of convection depending on Q. The variations steepen with further large Q/Ra and approach a scaling law of the velocity reduction as (Ra/Q)(1/2), which is established assuming that viscous dissipation is dominated by Hartmann braking at the walls perpendicular to the magnetic field. The results suggest that these phenomena are organized by the relationship between buoyancy and magnetic damping due to Hartmann braking.