Experimental and numerical study of Rayleigh-Benard convection affected by a rotating magnetic field

摘要

In the present paper we experimentally study the effects of a rotating magnetic field (RMF) on the fluid flow in an electrically conducting melt (Gallium), kept in a cylindrical container heated from below (Rayleigh-Benard configuration). The experimental data are compared to results obtained from three-dimensional, time-dependent numerical calculations. The paper presents the influence of the magnetic induction B, the frequency of the RMF #OMEGA#, and the temperature difference #DELTA#T between the hot bottom and cold top of the melt on heat transport and fluid flow, respectively. The results can be summarized in terms of the parameter N_(rot), which is defined as the ratio of magnetic Taylor number (propor. to B~2 centre dot #OMEGA#) to Grashof number (propor. to #DELTA#T). It is shown that for 0.003 < N_(rot) < 0.1 large-scale regular thermal waves exist, which travel azimuthally in the same direction as the rotation direction of the RMF. These thermal waves are connected with large-scale temperature fluctuations (amplitude 6%-10% of #DELTA#T). The amplitude decreases with increasing N_(rot), whereas the mean frequency increases from 0.001 Hz up to 0.1 Hz for 0.003 < N_(rot) < 0.1 For N_(rot) > 0.1 temperature fluctuations with amplitudes smaller than 1%-2% of #DELTA#T and frequencies greater than 0.1 Hz are observed. These oscillations can be attributed to Taylor vortices generated at the vertical cylinder walls. The regions of the different oscillation modes within the parameter space are shown in a stability diagram.