J. Fluid Mech. (2016), vol. 794, pp. 639–654. c  Cambridge University Press 2016 doi:10.1017/jfm.2016.181 639 Effects of geometric confinement in quasi-2-D turbulent Rayleigh–Bénard convection Shi-Di Huang 1 and Ke-Qing Xia 1, † 1 Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China (Received 9 November 2015; revised 26 January 2016; accepted 3 March 2016) We report an experimental study of confinement effects in quasi-2-D turbulent Rayleigh–Bénard 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 (Γ = 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 Γ 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 Γ 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 Γ 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. Key words: Bénard convection, plumes/thermals, turbulent flows 1. Introduction Thermal turbulence is ubiquitous in both nature and industrial applications. Turbulent Rayleigh–Bénard (RB) convection, a fluid layer heated from the bottom and cooled from the top has become a paradigm for the study of general convection phenomena. Over the years, there have been extensive studies addressing how the heat transport and flow dynamics of turbulent RB convection are determined by two control parameters, i.e. the Rayleigh number Ra = αgTH 3 /νκ and the Prandtl number Pr = ν/κ , where T is the temperature difference across the fluid layer of height H, g is the acceleration due to gravity and α, ν and κ are the thermal † Email address for correspondence: kxia@phy.cuhk.edu.hk