  When a liquid jet plunges into a pool, it can generate a bubble-laden jet
flow underneath the surface. This common and simple phenomenon is investigated
experimentally for circular jets to illustrate and quantify the role played by
the net gas/liquid void fraction on the maximum bubble penetration depth. It is
first shown that an increase in either the impact diameter or the jet fall
height to diameter ratio at constant impact momentum leads to a reduction in
the bubble cloud size. By systematically measuring the local void fraction
using optical probes in the biphasic jet, it is then demonstrated that this
effect is a direct consequence of the increase in the air content within the
cloud. A simple momentum balance model, including only inertia and the buoyancy
force, is shown to predict the bubble cloud depth without any fitting
parameters. Finally, a Froude number based on the bubble terminal velocity, the
cloud depth, and also the net void fraction is introduced to propose a simple
criterion for the threshold between the inertia-dominated and
buoyancy-dominated regimes.
