Lagrangian acceleration in Rayleigh-Bénard convection at various aspect ratios

A laboratory investigation using three-dimensional particle tracking velocimetry is performed to uncover the distinctive Lagrangian dynamics of turbulent Rayleigh-Bénard convection at aspect ratios Γ=1.2 and 2, where emphasis is placed on the Lagrangian statistics of thermal plumes. A laterally extended investigation volume allowed us to capture the dynamics of the relatively high vertical motion. Approximately 3.5×107 events were analyzed at Rayleigh numbers of Ra≈1.1×1010 for Γ=1.2 and Ra≈2.8×109 for Γ=2. The probability density functions (PDFs) of the vertical Lagrangian acceleration exhibited heavier tails compared with the lateral counterpart away from the near-wall boundary layer, which evidenced higher Lagrangian intermittency in the vertical direction. The second-order, conditional acceleration PDFs with various vertical-velocity magnitude thresholds indicate that the intense vortical motion promotes stronger vertical acceleration with respect to the lateral. A comparison with previous works reveals the influence of roll dynamics induced by Γ on the distribution of the vertical acceleration. The pair dispersion is explored and showed t2 behavior under the local Kolmogorov timescale (τη) for both aspect ratios, with some departure at scales sufficiently lower than τη.