Effects of geometric confinement in quasi-2-D turbulent Rayleigh-Benard convection

We report an experimental study of confinement effects in quasi-2-D turbulent Rayleigh-Benard convection. The experiments were conducted in five rectangular cells with their height H and length L being the same and fixed, while the width W was different for each cell to produce lateral aspect ratios (Gamma = W/H) of 0.6, 0.3, 0.2, 0.15 and 0.1. Direct flow field measurements reveal that the large-scale flow slows down as Gamma decreases and there are more plumes travelling through the bulk region. Moreover, the reversal frequency of the large-scale flow is found to increase drastically in smaller Gamma cells, by more than 1000-fold for the highest value of Rayleigh number reached in the experiment. The reversal frequency can be well described by a stochastic model developed by Ni et al. (J. Fluid Mech., vol. 778, 2015, R5) and the probability density functions (PDF) of the time interval between successive reversals are found to follow Poisson statistics as in the 3-D system. It is further observed that the bulk temperature fluctuation increases significantly and its PDF changes from exponential to Gaussian as Gamma decreases. The influences of geometric confinement on the global heat transport are also investigated. The measured Nu-Ra relationship suggests that, as the lateral aspect ratio decreases, the relative weight of the boundary layer contribution in the global heat transport increases compared to that from the bulk. These results demonstrate that in the quasi-2-D geometry, geometric confinement has strong effects on both the global and local properties in turbulent convective flows, which are very different from the previous findings in 3-D and true 2-D systems.