Fluctuations in Turbulent Rayleigh-Bénard Convection: From Boundary Layer to Large-scale Flows

We report a combined experimental and theoretical study on four aspects of turbulent Rayleigh-Bénard convection (RBC). The effect of turbulent fluctuations gives rise to a variety of flow patterns and statistical properties in different flow regions. The heat transport in RBC is determined by the thermal boundary layer (BL), in which molecular diffusion is dominant. We first study how turbulent fluctuations affect the mean and variance temperature profiles in the thermal BL. Our understanding of the temperature variance profile is then extended to the mixing zone outside the BL, in which turbulent convection is dominant and convective flow forms a large-scale circulation (LSC). The probability density function of temperature fluctuations is also investigated, and is found to be highly non-Gaussian with an exponential tail in the bulk region of the flow. Finally, using the shadowgraph technique, we measure the heat accumulation rate of RBC in a closed horizontal thin disk convection cell, which is found to be closely connected to random reversals of the LSC. Theoretical modeling is conducted to understand the experimental findings. Our work thus provides a complete and self-consistent framework for understanding the effect of turbulent fluctuations on the temperature field evolution and dynamic properties of the flow patterns in RBC. Such an understanding is relevant to many geophysical flows and various practical applications of turbulent thermal convection.