Regularization of flow reversals in a MHD Rayleigh-Benard convection

In this paper Rayleigh-Benard convection has been investigated under the influence of a DC magnetic field. Similar configurations can be found in geophysical or steel production. Our group reported recently that spontaneous flow reversals of quasi two-dimensional rolls randomly occur in Rayleigh-Benard convection of liquid metal exposed to a horizontal magnetic field (Yanagisawa, et al., PRE, 2011). In fluid layers with relatively large aspect ratios (see Fig.1 (a)) the convection roll pattern becomes isotropic. However, the rolls are aligned with the magnetic field direction if the Lorentz force becomes comparable to the buoyancy or larger. In our experiment, where the fluid layer has a dimension of 200 * 200 * 40 mm (corresponding to an aspect ratio of 5), the convection pattern can show 3, 4 or 5 rolls regimes depending on the Rayleigh number Ra and the Chandrasekhar number Q. Flow reversals occur spontaneously between these steady states in the Ra-Q parameter space. A new regime has been found in experiments conducted at Chandrasekhar numbers in a magnetic system at HZDR. In this regime the flow reversals occur regularly. The non-dimensional characteristic time of the reversal normalized by circulation time of the roll is around 100, being similar to the characteristic time of the ‘random’ flow reversals reported by Yanagisawa, et al. Detailed observations reveal that movements of each cell toward the reversal start just after the last reversal occurs unlike the random flow reversals.