Numerical simulations of an inline rising unequal-sized bubble pair in a liquid column

Abstract

We have studied the motion of a pair of unequal-sized bubbles rising inline in a quiescent liquid column, using the Volume of Fluid (VOF) method. In our work, a wide range of Archimedes numbers (400<Ar<5000) is used to study the bubbles’ shape, rise-velocities, and the wake region. We found that the leading bubble is unaffected by the proximity of the trailing bubble and moves with its terminal speed, whereas, the trailing bubble shows an acceleration with an increase in its speed as it approaches the leading bubble. We have also examined the wake behaviour behind the bubbles using the pressure distribution and extracted the vortex structures using the Q-criterion. Our work has unveiled two dominant vortex structures in the wake: (1) a toroidal ring on the bubble surface near the maximum diameter plane which occasionally detaches, and (2) a set of hairpin-like structures near the rear stagnation point, which grows by diffusion. The interaction between the toroidal ring and the hairpins determine the overall wake features.