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

In the present paper we experimentally study the effects of a rotating magn etic field (RMF) on the fluid flow in an electrically conducting melt (Gall ium), kept in a cylindrical container heated from below (Rayleigh-Bdnard co nfiguration). The experimental data are compared to results obtained from t hree-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 ca n be summarized in terms of the parameter N-rot, which is defined as the ra tio of magnetic Taylor number (infinity B-2 . Omega) to Grashof number (pro portional to Delta T). It is shown that for 0.003<N-rot<0.1 large-scale reg ular 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 amp litude decreases with increasing N-rot, whereas the mean frequency increase s from 0.001 Hz up to 0.1 IHz for 0.003<N-rot<0.1. For N-rot>0.1 temperatur e fluctuations with amplitudes smaller than 1%-2% of Delta T and frequencie s greater than 0.1 Hz are observed. These oscillations can be attributed to Taylor vortices generated at the vertical cylinder walls. The regions of t he different oscillation modes within the parameter space are shown in a st ability diagram. (C) 1999 American Institute of Physics. [S1070-6631(99)018 03-6].