On the Dynamics of air bubbles in Rayleigh-Benard Convection at various aspects ratios*

A laboratory investigation was carried out to uncover the distinct dynamics of air bubbles in Rayleigh-Benard (RB) convection in a rectangular cell at aspect ratios of 1.25, 1.5, 2.0, and 2.5. The experiments were performed at Rayleigh numbers on the order of Ra$\sim $ 10$^{\mathrm{10}}$. Streams of 1-mm bubbles were released from the bottom of the RB tank along the path of the roll structure at a fixed location of s/D$=$1/2, where s is the distance along the diagonal with respect to the center of the tank and D is the half diagonal distance. Three-dimensional particle tracking velocimetry was used to track simultaneously a relatively large number of bubbles, and to quantify the associated pair dispersion for various initial separations in the range of 20$\le \eta \le $200, where $\eta $ is the local Kolmogorov length scale. We will discuss distinct effects of the aspect-ratio dependency on the dynamics of the bubbles, path instability, pair dispersion, and the relation with the large-scale roll structures.