| Air entrainment by planar liquid jets at a free surface is studied experimentally. A plunging wavemaker generates a highly repeatable surface disturbance which is convected into a receiving pool by a planar jet, creating a two-dimensional cavity which collapses to entrap air. The temporal and spatial evolution of the cavity is displayed as coherent cross-sectional images constructed from conductivity probe data. For all experiments, the disturbance profile is recorded by a high-speed video camera; individual video frames are digitized to allow analysis of disturbance profile features.; A two-dimensional estimate of air entrainment rate is calculated for each experiment. It is shown that, by increasing jet thickness or increasing the jet entry angle with respect to the horizontal, a more energetic jet entrains a greater amount of air. The cavities generated in each experiment are shown to advance in the streamwise direction at half the velocity of the steady planar jet, in agreement with classical theory.; The consistent resolution of a fluid shoot being ejected by the top surface of the fluid tongue leads to a proposed mechanism for the air entrainment event in the planar geometry. As was observed by Zhu et al. (2000) in the circular jet geometry, the event is initiated by the disturbance impact on the receiving pool free surface. However, in this study, the sustained free surface elevation of the planar disturbance is responsible for generating a fluid slab which curls upward through the fluid tongue and penetrates the tongue free surface. The impact initially causes a reattachment of streamlines on the cavity surface, but the reattachment cannot be sustained. |
