Item

Abstract

Bubbles play an important role in the transport of chemicals and nutrients in many natural and industrial
flows. Their dispersion is crucial to understanding the mixing processes in these flows. Here we report on
the dispersion of millimetric air bubbles in a homogeneous and isotropic turbulent flow with a Taylor
Reynolds number from 110 to 310. We find that the mean squared displacement (MSD) of the bubbles far
exceeds that of fluid tracers in turbulence. The MSD shows two regimes. At short times, it grows
ballistically (∝ τ2), while at larger times, it approaches the diffusive regime where the MSD ∝ τ. Strikingly,
for the bubbles, the ballistic-to-diffusive transition occurs one decade earlier than for the fluid. We reveal
that both the enhanced dispersion and the early transition to the diffusive regime can be traced back to the
unsteady wake-induced motion of the bubbles. Further, the diffusion transition for bubbles is not set by the
integral timescale of the turbulence (as it is for fluid tracers and microbubbles), but instead, by a timescale
of eddy crossing of the rising bubbles. The present findings provide a Lagrangian perspective towards
understanding mixing in turbulent bubbly flows.