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

Abstract

The process by which a liquid jet falling into a liquid pool entrains air is 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 flow rate, on the other hand, large air cavities are formed. Their collapse under the action of gravity causes the entrapment of bubbles in the liquid. This sequence of events is recorded with a CCD and a high-speed camera. A boundary-integral method is used to simulate the process numerically with results 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 in creating the cavity. A hypothesis on the nature of this mechanism is presented.

A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols

“…When creating plumes using plunging water, it is important that the falling sheet or jet have the appropriate scale of surface roughness before impacting the water surface in order to create the correct sized voids along the air-water interface (Zhu et al, 2000). The larger voids in particular are critical for producing the correct plume bubble size distribution that includes bubbles larger than the Hinze scale.…”

A miniature Marine Aerosol Reference Tank (miniMART) as a compact breaking wave analogue

“…When using plunging water to create bubble plumes, it is important that the falling sheet or jet has the appropriate scale of surface roughness before impacting the water surface in order to create the correctly sized voids along the air-water interface (Zhu et al, 2000). The larger voids are important for producing the correct plume bubble size distribution that includes bubbles larger than the Hinze scale.…”

Nonlinear travelling waves on a spiralling liquid jet

“…A suitable numerical method can be obtained by modifying the method of Zhu et al 29 , which was used to solve numerically a liquid jet falling into a liquid pool.…”