Experimental study of the suppression of Rayleigh-Bénard instability in a cylinder by combined rotating and static magnetic fields

We consider experimentally transitions in a liquid metal cylinder heated from below and subject to superimposed rotating and static magnetic fields. The applied static magnetic fields are too weak to influence the characteristic velocity of the rotating field driven basic flow. Being itself turbulent, a strong enough magnetically driven flow suppresses considerably the temperature fluctuations due to the thermogravitational convection. The remaining background fluctuations are caused by unsteady Taylor vortices generated near the sidewall. Our experiment shows that the superimposed static "cusp" magnetic field reduces the amplitude of these remaining temperature fluctuations by a factor of 4, compared to the case with a superimposed uniform axial field. The observed behavior agrees well with the static field effect on the amplitude of the additional unstable Taylor vortex-type solutions. These solutions bifurcate subcritically and represent the governing structures in the background turbulence. Thus, the observations are consistent with the description of the background turbulence as an irregular phase trajectory around the skeleton of the subcritical flow states. If this "skeleton" is compressed by an external influence (the cusp static field in our case), then also the amplitude of turbulent fluctuations decreases by the same factor. Another effect of the cusp field is to sharpen the transition between buoyancy and magnetic forcing dominated regimes. This allowed us to obtain an empirical expression for the conditions of this transition. We conclude that the rotating magnetic-field-driven flow suppresses the buoyant flow at a much lower angular velocity than a rigid-body mechanical rotation.