Lagrangian tracking reveals competing influences of clustering and turbulence on the rise velocity of bubble swarms

The rise velocity of bubble swarms is a fundamental property in many natural and industrial flows. A central question is whether clustering of bubbles enhances or suppresses their rise velocity compared to isolated single bubbles in quiescent fluid. We address this issue using a recently developed three-dimensional Lagrangian bubble tracking technique, with accurate rise velocity measurements obtained by simultaneously tracking tracer particles around each bubble. For bubbles within swarms, we find that clustered bubbles rise faster than unclustered ones. A conditional analysis based on cluster orientation suggests that this enhancement results from a wake suction effect acting on trailing bubbles. However, even clustered bubbles rise considerably slower than isolated single bubbles due to their exposure to turbulence generated by other bubbles, which randomizes their trajectories and reduces rising efficiency.