Plunging liquid jets and the role of subharmonic gravity-capillary waves in air entrainment*

As a jet of liquid falls into a pool, it can cause air bubbles to form at the plunge point and be drawn into the pool, a process known as air entrainment. This process is a useful analog for many two-phase flows including the ocean surface, metal casting, and aerated wastewater treatment and aquaculture ponds. Despite the ubiquity of air entrainment by plunging jets, it remains poorly understood. Prior works have tended to correlate air entrainment with factors that control it only indirectly, unconcerned with potentially important phenomena local to the plunge site. We attempt to remedy this by employing a plunging jet that does not entrain unless it is deliberately made to do so by the introduction of well-characterized harmonic disturbances. At lower jet forcing frequencies, the disturbance-driven air entrainment behaves similarly to past works with disturbed jets, but at higher frequencies, the jet disturbances cause subharmonic surface waves to arise and entrain air by interacting with the submerged jet flow. By capturing entrained bubbles to measure their flow rate, we explore this dramatic and novel interplay between turbulence and free-surface flow.