Taylor dispersion of bubble swarms rising in quiescent liquid

TL;DR

This study investigates how bubble swarms in quiescent water disperse, revealing that bubble interactions and turbulence significantly influence their horizontal and vertical dispersion behaviors compared to single bubbles.

Contribution

It provides detailed 3D Lagrangian analysis of bubble swarm dispersion, highlighting the effects of bubble interactions, wake-induced turbulence, and bubble size on dispersion dynamics.

Findings

Vertical MSD in swarms is nearly ten times faster than single bubbles.

Oscillations in horizontal MSD are damped due to bubble-bubble interactions.

Larger bubbles with higher gas void fractions transition earlier from ballistic to diffusive regimes.

Abstract

We study the dispersion of bubble swarms rising in initially quiescent water using 3D Lagrangian tracking of deformable bubbles and tracer particles in an octagonal bubble column. First, we compare the dispersion inside bubble swarms with that for single-bubble cases and find that the horizontal mean squared displacement (MSD) in the swarm cases exhibits oscillations around the asymptotic scaling predicted for a diffusive regime. This occurs due to wake-induced bubble motion, however, the oscillatory behaviour is heavily damped compared to the single-bubble cases due to the presence of bubble-induced turbulence (BIT) and bubble-bubble interactions in the swarm. The vertical MSD in bubble swarms is nearly an order of magnitude faster than the single-bubble cases, due to the much higher vertical fluctuating bubble velocities in the swarms. We also investigate tracer dispersion in BIT and find that concerning the time to transition away from the ballistic regime, larger bubbles with a higher gas void fraction transition earlier than tracers, consistent with Mathai et al. (\textit{Phys. Rev. Lett.} 121, 054501, 2018). However, for bubble swarms with smaller bubbles and a lower gas void fraction, they transition at the same time. This differing behavior is due to the turbulence being more well-mixed for the larger bubble case, whereas for the smaller bubble case the tracer dispersion is highly dependent on the wake fluctuations generated by the oscillating motion of nearby bubbles.