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

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