On the Lagrangian Acceleration in Rayleigh-Be ́nard Convection: effect of aspect ratio

A laboratory investigation using 3D particle tracking velocimetry is performed to uncover distinctive Lagrangian dynamics of turbulent Rayleigh-Be ́nard convection at aspect ratios Γ=1.2 and 2 with emphasis on Lagrangian statistics of thermal plumes. A laterally extended investigation volume allows us to capture a part of high vertical motion. Approximately 3.5×10⁷flow particles were measured at a Rayleigh number of Ra ≈1.1×10¹⁰ for Γ =1.2 and Ra ≈2.8×10⁹ for Γ =2. The PDF of the vertical Lagrangian acceleration exhibited heavier tails than the lateral counterpart away from the near-wall boundary layer, evidencing higher Lagrangian intermittency in the vertical direction. Inspection of the 2nd-order acceleration PDF filtered by vertical velocity magnitude thresholds indicates that intense vortical motion induced by the thermal plumes promotes stronger vertical acceleration respect to the lateral. A comparison with previous works reveals the influential role of roll dynamics induced by Γ on the distribution of the vertical acceleration.