On the mechanism of air entrainment by liquid jets at a free surface

The process by which a liquid jet falling into a liquid pool entrains air i s studied experimentally and theoretically. It is shown that, provided the nozzle from which the jet issues is properly contoured, an undisturbed jet does not entrap air even at relatively high Reynolds numbers. When surface disturbances are generated on the jet by a rapid increase of the liquid flo w rate, on the other hand, large air cavities are formed. Their collapse un der the action of gravity causes the entrapment of bubbles in the liquid. T his sequence of events is recorded with a CCD and a high-speed camera. A bo undary-integral method is used to simulate the process numerically with res ults in good agreement with the observations. An unexpected finding is that the role of the jet is not simply that of conveying the disturbance to the pool surface. Rather, both the observed energy budget and the simulations imply the presence of a mechanism by which part of the jet energy is used i n creating the cavity. A hypothesis on the nature of this mechanism is pres ented.