Extreme Lagrangian velocity fluctuations in bubble induced turbulence

Recent experimental studies on the Eulerian structure functions (ESF) in bubble induced turbulence (BIT) have highlighted striking similarities with single-phase homogeneous isotropic turbulence (HIT), with Kolmogorov scaling applying at scales below the bubble diameter for low-order ESF, and higher-order ESF indicating strong intermittency that decreases with increasing scale. Here we explore the Lagrangian structure functions (LSF) in BIT using a recently developed 3D Lagrangian bubble and tracer tracking technique. For short time scales, the LSF in BIT are remarkably similar to those in HIT, which we show is due to the acceleration probability distributions in BIT being very similar to those in HIT. However, while the intermittency associated with higher-order LSF in HIT monotonically decrease with increasing time scale, those in BIT grow in time, reaching values far exceeding those in HIT, before finally reducing and approaching those observed in HIT at long time scales. We show that these extreme fluctuations at intermediate times are associated with tracers being trapped in quasi-vertical and quasi-horizontal spiral vortices that are generated in the flow by the rising bubbles, with scales controlled by the bubble properties. These results have important implications for understanding the extent to which the small-scale properties of BIT resemble those in HIT, and for modeling mixing processes in BIT.