Air Entrainment by a Plunging Jet Translating over a Free Surface

Abstract

The photographs above illustrate the mechanism by which a vertical laminar jet (Djet'6 mm, velocity Vjet 5303 cm/s! induces air entrainment as it translates horizontally over a quiescent pool ~velocity Vt544 cm/s from right to left!. In the above surface view ~top photograph!, the laminar jet appears as a smooth cylindrical rod. Capillary waves are seen propagating upstream of the impact site, while a steep cavity and turbulent free-surface wake are found downstream. The below-surface view shows the free-surface cavity with air entrainment occurring from the cusp at the bottom of the cavity. The jet entrains no air when it is stationary or translating at speeds below a critical inception value ~38 cm/s in the present case!. For all observed cases, the inception point followed a Froude scaling given by Fr 5Vt/AgDjet'1.5. Visualizations of the flow not shown here indicate that the leading edge of the cavity forms on the downstream surface of the jet. It is believed that the cusp in the near wake of the jet is formed as a result of the low pressure created by the jet’s wake combined with the shear induced by the jet’s impact velocity. High-speed movies of the entrainment process indicate that the vortex rings of the evolving subsurface jet induce instabilities on the leading edge of the cavity. Above the critical conditions for air entrainment, the length of the cavity/jet interface is sufficient to allow the disturbances to grow into distended pockets of air. These pockets are subsequently drawn into the vortex ring of the jet and eventually broken up into clusters of bubbles by the turbulent jet.