On the dynamics of air bubbles in Rayleigh-Bénard convection

The dynamics of air bubbles in turbulent Rayleigh-Bénard (RB) convection is described for the first time using laboratory experiments and complementary numerical simulations. We performed experiments at Ra = 5:5 ± 10⁹ and 1:1 ± 10¹⁰, where streams of 1 mm bubbles were released at various locations from the bottom of the tank along the path of the roll structure. Using three-dimensional particle tracking velocimetry, we simultaneously tracked a large number of bubbles to inspect the pair dispersion, R²(t), for a range of initial separations, r, spanning one order of magnitude, namely 25ν >r> 6225ν; here ν is the local Kolmogorov length scale. Pair dispersion, R²(t), of the bubbles within a quiescent medium was also determined to assess the effect of inhomogeneity and anisotropy induced by the RB convection. Results show that R²(t) underwent a transition phase similar to the ballistic-to-diffusive (t²-to-t¹) regime in the vicinity of the cell centre; it approached a bulk behavior t³=2 in the diffusive regime as the distance away from the cell centre increased. At small r, R²(t)/t1 is shown in the diffusive regime with a lower magnitude compared to the quiescent case, indicating that the convective turbulence reduced the amplitude of the bubble's fluctuations. This phenomenon associated to the bubble path instability was further explored by the autocorrelation of the bubble's horizontal velocity. At large initial separations, R²(t)∝ t² was observed, showing the effect of the roll structure.