Non-intrusive detection of air–water surface roughness in self-aerated chute flows

Stepped spillways are known to enhance the energy dissipation potential when compared with common smooth invert chutes. In the skimming flow regime, the self-aerated flow becomes more chaotic and surface waves (referred to as air–water surface roughness) are enhanced when the discharge decreases. In this study, experiments on smooth invert and stepped chutes models with a slope of 1: 2 were conducted to characterize this surface roughness by the use of a high-speed camera and ultrasonic sensor. It was found that the amount of entrapped air – at the water level where the air concentration is 90% – is reduced when a stepped spillway is considered. With decrease in step height (and on smooth invert chutes), entrapped air became more relevant. Wave heights increased with increase in step height and exceeded significantly the characteristic water level with 90% air concentration which was detected by a conductivity probe. A wide range of wave frequencies indicated the turbulent structure of stepped spillway flows.

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