The impingement of a long water jet column into the free surface of a reservoir was visualized using high speed photography. The entrainment of air during the impact of the jet creates a void in the shape of a truncated cone that prevents the mixing of the water in the jet with the bulk of the water in the reservoir and thus allows the water in the jet to penetrate further into the reservoir. The observation is significant in the study of surface cooling by water jet.
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