Bubble statistics in shallow angle plunging liquid jets

The statistical characteristics of bubble plumes generated by inclined liquid jets remain inadequately understood, particularly regarding their spatial evolution. This study employs an optical fiber probe for sequential multi-point measurements to investigate the evolution of bubble plumes produced by shallow-angle inclined jets impacting both quiescent fluid and crossflow. Measurements of air volume fractions reveal that crossflow significantly attenuates the jet's penetration capability. Under both conditions, the bubble size distributions exhibit power-law behavior, with variations in the exponents observed at different spatial locations. In quiescent water, a power-law exponent of -2 is observed within the jet impact region, where the strong shear effect of the oblique jet introduces more large bubbles and extends their survival time. This exponent gradually decreases with increasing distance from the jet's influence. In the presence of crossflow, the bubble size spectrum closely resembles that observed under breaking waves. This similarity arises because the jet induces intense surface overturning and fragmentation, becoming the primary source of bubble entrainment. Multi-point measurements further indicate that bubble fragmentation dominates near the free surface, while bubble coalescence becomes more prevalent at greater depths. The results for the bubble velocity distribution indicate that, under both conditions, the distribution of high-speed bubbles follows a power law with an exponent of -6. However, the physical mechanisms driving this behavior remain unclear.