Unravelling the structure of self-aerated flows

Self-aeration is a fascinating phenomenon that is commonly observed in high Froude-number open-channel flows. Since the late 1960s, it is known that the structure of aerated boundary-layer type flows is composed of different flow layers, which can be characterized using the concepts of bubbles, waves and droplets. Yet, a measurement of these different contributions has not been achieved to date. In this work, a novel processing technique for air–water flow signals is introduced, allowing the flow structure to be unravelled. For the first time, concentration profiles and interfacial frequencies of entrained air bubbles, surface waves, and water droplets are presented, followed by an evaluation of their contributions to the S-shaped concentration profile of total transported air. The proposed discrimination technique represents a step change in the physical description of self-aerated flows at a fundamental level, which is anticipated to be highly useful for practical computations.