High-speed cine and video photographs were used to capture the flow patterns of a column of water jet impinging into a pool of water. The impact results in air entrainment into water in a form of a void and there are no mixing between the water in the jet and the surrounding water. Conservation of fluid momentum shows that the rate of increase of the height of the air void depends on the drag coefficient of the jet front. By neglecting the frictional losses, the application of energy conservation yields an expression that relates the maximum height of the air void with the properties of the water jet.
Soh, W., Khoo, B. and Yuen, W. (2005). The entrainment of air by water jet impinging on a free surface. Experiments in Fluids: experimental methods and their applications to fluid flow, 39 498-506.