Experimental study of bubble clouds generated by multi-plunging jets

Industrial and natural processes frequently involve plunging jets that break into clusters of liquid columns or droplets, impacting the liquid surface. To simulate air entrainment by fragmented plunging jets, we first examine bubble clouds formed by twin and triple identical jets. Bubble clouds from individual jets merge after a certain pre-interaction depth and become a unified cloud, similar to the case of a single plunging jet. We then perform experiments with hexagonally packed multiple identical jets to mimic large fragmented jets. They generate a single bubble cloud without revealing a pre-interaction depth. Doppler optical probe measurements reveal that the radial profiles of air/water fraction, $\varphi$, are initially bimodal just after the jet impact, gradually transitioning into a single Gaussian distribution at further depth. For a given impact velocity $V_i$, an increase in the number of jets results in a slower decay of the mean bubble speed at the cloud axis. Furthermore, the net air flux does not scale with the number of jets but is proportional to the diameter of the multi-jet. In all cases, the mean bubble size in a bubble cloud increases with depth, which may be primarily driven by the coalescence of bubbles as suggested by high-speed images and Weber number analysis. The mean penetration depth of bubbles, $H$, increases with the number of jets. Our investigations indicate that the momentum flux balance in the presence of buoyancy can be successfully used to estimate bubble cloud depth and the centreline velocity in all multi-jet experiments.