Air–water cavity in multi-plunging jet impacts

We report the formation of an original dome-shaped air–water cavity at the impact site of closely packed plunging water jets. The injector assembly consists of concentric rings of circular jets, similar to a shower-head configuration. We infer from high-speed imaging, laser-induced fluorescence and optical phase detection probes that the cavity consists of multiple stems and sheets of air that stretch, retract and pinch off to produce bubbles. We also illustrate various bubble production mechanisms in the cavity, along with coalescence and bubble breakup scenarios in the subsequent bubble cloud. Thereby, we describe the wide range of bubble sizes generated by such jets, from a few micrometres up to about 5 5 $5$ mm. Measurements of the cavity size are carried out for varying impact velocity, number of jets and jet spacing. We propose that this distinctive air–water cavity is generated by liquid entrainment when successive rings of plunging jets progressively mix with the pool water.