Transition between turbulent magnetically driven flow states in a Rayleigh-Bénard cell

Transition between turbulent flow regimes is studied experimentally in a cylinder of liquid mercury heated from below under the influence of a rotating magnetic field. The latter creates a rotating flow which almost completely suppresses the temperature fluctuation near horizontal boundaries at a much lower angular velocity than a simple mechanical rotation. Our experiment confirms that this effect persists in the deep turbulent range to Grashof numbers as high as about 10⁹. An intermediate range is observed for Gr > 2 × 10⁸ with the temperature fluctuation suppressed in the core but near the sidewall. This is explained by turbulent friction replacing the Coriolis force as the leading retarding force. The linear instability of a simplified model is studied numerically. The model considers a base flow consisting of a uniform rotation and a formally independent uniform meridional flow in a cylinder with an adverse vertical temperature gradient. The model shows that the bulk meridional flow being itself much slower than the otation is able to delay the Rayleigh-Bénard instability.