An experimental study of kicked thermal turbulence

We present an experimental study of turbulent Rayleigh-Benard convection (RBC) in which the input energy that drives the turbulent flow is in the form of periodical pulses. A surprising finding of the study is that in this 'kicked' thermal turbulence the heat transfer efficiency is enhanced compared to both constant and sinusoidally modulated energy inputs. For the apparatus used in the present study, an enhancement of 7% of the dimensionless Nusselt number Nu has been achieved. The enhancement is found to depend on two factors. One is the synchronization of the kicking period of energy input with the intrinsic time scale of the turbulent flow. When the repetition period of the input energy pulse equals half of the large-scale flow turnover time, a resonance or optimization of the enhancement is achieved. The other factor is the pulse shape (the inverse square of the energy input duty cycle). We find that a spiky pulse is more efficient for heat transfer than a flatter one of the same energy. It is found that in this kicked thermal turbulence there exist appropriate ranges of the kicking strength A and the kicking frequency f in which the Rayleigh number Ra grows to a saturation level and that the saturated Ra fluctuates between a lower saturation level Ra(l)(sat) sat and an upper saturation level Ra(u)(sat). For large enough saturated Ra, power-law dependences on f and A are found: Ras(l)(sat) proportional to (Af)(0.80+/-0.02) and Ra(u)(sat) proportional to f(0.70+/-0.01) A(0.84+/-0.02). The scaling law for Ra sat is found to agree quantitatively U I with the prediction of a mean-field theory of kicked turbulence (Lohse, Phys. Rev. E vol. 62, 2000, p. 4946) when the latter is appropriately extended to the case of kicked thermal turbulence. It is further found that a large-scale circulatory flow (LSC) still exists in the kicked RBC, and that its Reynolds number has the same scaling with Ra as in the steadily driven case, i.e. Re(f) proportional to Ra(0.46+/-0.01). The present study provides an example of achieving enhanced heat transfer in a convective system by first triggering the emission of clustered thermal plumes via an active control and then synchronizing the transport of the plume clusters with an internal time scale.