Fei Ma

On a stepped channel, three distinct flow regimes, nappe, transition, and skimming, occur successively with increasing discharge. This work proposes a criterion of the transition flow occurrence: a critical slope of the channel exists between mild and steep slopes, and for mild slopes, air pocket disappearance and nappe parallel to channel slope are defined as the lower and upper limits of transition flow, whereas for steep slopes, the two phenomena oppositely represent the upper and lower limits. The analytical approaches based on the criterion demonstrate the critical slope of 30° and predict the lower and upper limits of transition flow. The experimental data verify the existence of the critical slope and its prediction accuracy, and indicate that the prediction of the lower and upper limits of transition flow works well for both mild (30°) slopes. During stepped channel design, employing the critical slope can control the transition flow occurrence, therefore reducing the risks caused by structure vibration and excessive splashing.

Under large unit discharges, cavitation damage may occur and energy dissipation may remarkably decline in stepped chutes due to absence of air entrainment. Herein, a pre-aeration device, entraining air by contracting the flow locally by walls without special air vent, is proposed for increasing the maximum available unit discharges. The test results demonstrate that the aerator proposed in this paper generates a ski-jump and a local hydraulic jump to entrain air into flow. It provides fully aerated flows for the stepped chutes, and significantly increases the energy dissipation with large unit discharges.

Flip bucket is a common element used to dissipate energy for release works. For purpose of avoiding excessive scour and flow choking, the slot-type flip bucket was developed. In this paper, a flow-separating slot-type flip bucket (FSSFB) is proposed on this basis, which can divide the approach flow into three branches by dividing walls, and thus generate two small completely separated jets resulting in better energy dissipation performance and reduced scour. Based on model tests, the jet trajectory of FSSFB is investigated. Considering the local head loss from the flow passing the dividing walls, the take-off velocity is amended for calculating jet trajectory using projectile method. Based on fitting analysis, the head loss coefficient is a function of the relative width b/B, the relative angle θ/β of the slot and the Froude number Fr_o of approach flow. Finally, an empirical relationship of head loss coefficient is provided, and the error of calculation of jet trajectory is less than 10% for FSSFB.

Considering the limitations of unit discharge for stepped chutes, a ski-jump-step energy dissipater, which is a new type of energy dissipater, is first proposed for increasing the maximum available unit discharges. In this system, preaerations are achieved through jets generated by ski-jumps and further aerations are achieved via aeration basins. It provides fully aerated flows for the stepped chutes with large unit discharges, and the problems caused by high-speed flows like energy dissipation, cavitation, and atomization are well solved. Experimental investigations about hydraulic characteristics of the ski-jump-step energy dissipater were conducted, including the flow regimes, the principle and effectiveness of energy dissipation and the characteristics of the air entrainment and the pressure distribution. Even under conditions of large unit discharges (q=118.00 m2/s on the prototype), results obtained demonstrate energy dissipation as high as 75.8% and air concentrations of at least 3.6% alon...

The Discussers have read the paper with interest and congratulate the Authors for their remarkable contribution. Their work gives a comprehensive overview of several physics-based criteria for air ...