Abstract
Stepped spillways are frequently limited to specific discharges under around 30 m2/s due to concerns about potential cavitation damages. A small air concentration can prevent such damages and the design of bottom aerators is well established for smooth chutes. The purpose of this study is to systematically investigate the performance of a deflector aerator at the beginning of stepped chutes. Six parameters (chute angle, step height, approach flow depth, approach flow Froude number, deflector angle and deflector height) are varied in a physical model. The spatial air concentration distribution downstream of the aerator, the cavity sub-pressure, water discharge and air discharges are measured. The results describe the commonly used air entrainment coefficient, the jet length, as well as the average and bottom air concentration development to design an aerator. The lowest bottom air concentration measured in all tests is higher than the air concentration recommended in literature to protect against cavitation damages. And, unlike smooth chutes, there appears to be no significant air detrainment downstream of the jet impact. One deflector aerator seems therefore sufficient to provide protection of a stepped spillway.
Highlights
IntroductionStepped spillways are commonly used, owing to their efficient energy dissipation and the emergence of roller compacted concrete (RCC) dams in the 1980s
The performance of a stepped spillway aerator using a deflector was for the first time systematically investigated by varying the governing parameters on a large-scale physical model
They are similar to the zones observed on smooth chutes; The air entrainment coefficient β is mainly influenced by the chute angle φ, the approach flow Froude number Fo and the deflector angle α
Summary
Stepped spillways are commonly used, owing to their efficient energy dissipation and the emergence of roller compacted concrete (RCC) dams in the 1980s. Steep step spillways (chute angle φ ≈ 50◦ ) are found on the downstream face of RCC or gravity dams, while moderate slope stepped spillways (φ ≈ 30◦ ) are found on embankments dams. Stepped spillways have been widely studied in the past decades in terms of flow regime, energy dissipation, self-aeration and pressure distribution. General summaries of researches are given in [1,2,3]. One major drawback of stepped spillways is that the specific discharge q has been limited to q ≤ 30 m3 /s/m [4,5,6,7] to avoid cavitation damages upstream of the inception point
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