Dynamical and statistical phenomena of circulation and heat transfer in periodically forced rotating turbulent Rayleigh-Benard convection

In this paper, we present results from an experimental study into turbulent Rayleigh-Benard convection forced externally by periodically modulated unidirectional rotation rates. We find that the azimuthal rotation velocity $\dot{\theta}$(t) and thermal amplitude $\delta$(t) of the large-scale circulation (LSC) are modulated by the forcing, exhibiting a variety of dynamics including increasing phase delays and a resonant peak in the amplitude of $\dot{\theta}$(t). We also focus on the influence of modulated rotation rates on the frequency of occurrence $\eta$ of stochastic cessation/reorientation events, and on the interplay between such events and the periodically modulated response of $\dot{\theta}$(t). Here we identify a mechanism by which $\eta$ can be amplfied by the modulated response and these normally stochastic events can occur with high regularity. We provide a modeling framework that explains the observed amplitude and phase responses, and extend this approach to make predictions for the occurrence of cessation events and the probability distributions of $\dot{\theta}$(t) and $\delta$(t) during different phases of a modulation cycle, based on an adiabatic approach that treats each phase separately. Lastly, we show that such periodic forcing has consequences beyond influencing LSC dynamics, by investigating how it can modify the heat transport even under conditions where the Ekman pumping effect is predominant and strong enhancement of heat transport occurs. We identify phase and amplitude responses of the heat transport, and show how increased modulations influence the average Nusselt number.