Bubble formation mechanism and air entrainment in multi-plunging jets

Air entrainment by plunging jets often involves jet broken down into clusters of columns or droplets before impacting a liquid surface, such as in waterfalls and water treatment plants. The resulting bubble clouds differ from those formed by a single jet, as the spacing among liquid structures affects bubble size, maximum penetration depth (H), mixing and turbulence levels. Here, we conducted model experiments using up to 61 closely packed smaller jets to mimic large-scale fragmented jets. An inverted dome-shaped structure is observed just beneath the surface, leading to a single bubble cloud instead of multiple overlapping clouds. Air/liquid fraction (Φ) measurements using an optical probe reveal that the dome is a two-phase structure made up of a 3D array of liquid jets and air tubes. High-speed backlight imaging shows larger bubbles are formed at the dome base while smaller bubbles are created through the pinching and retraction of air fingers around the surface of the dome. Radial profiles of Φ just below the dome show sharp peaks at the locations of the liquid fingers, but at greater depths, Φ profiles become Gaussian with constant maxima, similar to single jet. By accurately predicting the cloud size H using Φ measurements for the buoyancy term in the momentum budget, we demonstrate that bubble clouds produced by multiple jets are dynamically similar to those generated by a single jet, provided the impact momentum is appropriately adjusted for the number of jets.