Dynamics of the large-scale circulation in turbulent Rayleigh-Bénard convection with modulated rotation

We present measurements of the azimuthal rotation velocity θ(t) and thermal amplitude δ(t) of the large-scale circulation in turbulent Rayleigh-Bénard convection with modulated rotation. Both θ(t) and δ;(t) exhibit clear oscillations at the modulation frequency ω. Fluid acceleration driven by oscillating Coriolis force causes an increasing phase lag in θ(t) when ω increases. The applied modulation produces oscillatory boundary layers and the resulting time-varying viscous drag modifies δ(t) periodically. Oscillation of θ(t) with maximum amplitude occurs at a finite modulation frequency ω∗. Such a resonance-like phenomenon is interpreted as a result of optimal coupling of δ(t) to the modulated rotation velocity. We show that an extended large-scale circulation model with a relaxation time for δ(t) in response to the modulated rotation provides predictions in close agreement with the experimental results.