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

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