A Study of Air Entrainment and Aeration Devices on a Spillway Model

Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices called aerators. These serve to introduce air in the layers close to the channel bottom in order to reduce the cavitation damage. The performances of a spillway aerator are studied on a steep spillway model and high velocities (from 4 m/s up to 15 m/s). A new method of calculation of the jet trajectory above the aerator is developed. The conditions at the impact point of the jet are discussed and a comparison with experimental data is presented for low Froude numbers. The behaviour of the air demand relationship between the air discharge and the subpressure in the cavity beneath the nappe is analysed with new experimental data, and an analogy with results obtained in the study of cavity behind hydrofoils is developed. Air concentration and velocity profiles at various cross-sections above and downstream of the aerator are presented and analysed. They provide a better understanding of the air entrainment process occurring above a bottom aerator. A comparison between the free-surface aeration and the flow downstream of an aerator is presented and this analysis contributes to a method for the determination of aerator spacing