A comparative study of pre-aeration effects on hydraulic jump air–water flow properties at high Froude numbers

A hydraulic jump forming in the stilling basin at the base of a high-head dam spillway is often characterized by high Froude number and inflow pre-aeration conditions. Although the air entrainment in hydraulic jump has been studied extensively, the effects of the inflow pre-air-entrainment on the downstream air–water flow properties and energy dissipation performance have not been investigated systematically, especially for high-Froude-number cases. This paper presents a comparative study of the pre-aeration effects on the air–water flow characteristics of high-Froude-number hydraulic jumps. Two pre-aeration levels were achieved using a smooth sloping chute and a ramp aerator on the chute. Apart from the change in jump length required for the same amount of energy dissipation, the major impact of the variation in pre-aeration was observed for the void fraction and air entrainment flux within the jet-shear region, whereas the bubble count rate and interfacial velocity were less affected. Differences were also observed from the prediction of previous studies under smaller Froude number and negligible pre-air-entrainment conditions. The pre-air-entrainment was found to affect the scale effects on the bubble counts and the interfacial area between air and water phases. This study may facilitate a bridging between the laboratory findings and the cases closer to the real engineering application of large-scale hydraulic jumps.