Effect of the aspect ratio on the dynamics of air bubbles within Rayleigh–Bénard convection

Abstract

Laboratory experiments and numerical simulations were performed to quantify the effect of the aspect ratio, Γ, in the dynamics of air bubbles within turbulent Rayleigh–Bénard (RB) convection. We explored four scenarios defined by Γ=1.25, 1.5, 2, and 2.5 under Rayleigh numbers ranging from 2.0×109 to 1.6×1010. Continuous 1-mm bubbles were released at two locations from the bottom along the roll path. Three-dimensional particle tracking velocimetry was used to track a large number of bubbles and determine features of the trajectories and pair dispersion, R2(t), for various initial separations, rp within H/10≤rp≤5H/10; here, H is the height. The R2(t) of the bubbles within a quiescent medium was included for reference. Characterization of the bubble streams, namely, the center of mass (Lc), mean deviation (Rc) to Lc, vertical (vz) and lateral (vL) velocities, and their ratios reveal the strong modulation of the roll structure and Γ. In particular, Lc exhibited an approximately symmetric distribution around the maximum, which occurred at the middle height only in the Γ=1.25 case. Maximum Lc was near the wall top with the highest aspect ratio. However, Rc did not vary substantially among the cases. Bubbles' lateral pair dispersion RL2 shows correlated trends with Γ, particularly at large initial separations and times, whereas the vertical pair dispersion is mainly dominated by buoyancy. The RL2 decreased as Γ increased. It indicates the effect of different-sized roll structures modulated by Γ. In general, R2 embodies distinct features of Γ-modulated bubble dynamics in RB convection.