Browsing by Author "Wright, Henry John"

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    Investigation of novel deflector shapes for uncontrolled spillways
    (Stellenbosch : Stellenbosch University, 2024-02) Wright, Henry John; Bosman, Adèle ; Brink, Isobel ; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The hydraulics of stepped spillways are generally well understood, although numerous fundamental hydraulic aspects remain inadequately explored. Critical knowledge gaps persist, including aerated flow hydraulics, hydraulics of embankment flows, hydraulics of stepped spillways for steep gravity dams, environmental hydraulics as well as turbulent interactions between cavity flow and skimming flow. Notably, the elusive safe unit discharge limits for stepped spillways remain undefined, with conflicting findings in the literature. The majority of stepped spillways have been designed for a maximum unit discharge of 25 to 30m3/s/m due to the risk of cavitation damage. It has further been reported that the critical velocity of approximately 20 m/s for the inception of cavitation on stepped spillways is obtained at a unit discharge of 25 m3/s/m. Further research in the field revealed that a bottom aerator becomes imperative for discharges greater than 30m3/s/m. However, discrepancies persist, with other researchers suggesting that the safe unit discharge is lower, quoting 11.5 m3/s/m to 14 m3/s/m for step heights of 0.6 m to 1.2m, respectively. Therefore, the exact limits of stepped spillways remain unquantified when water flows on the downstream slope. In China, the Flared Gate Pier (FGP) has been used on stepped spillways, particularly the X-type and Y-type piers. These piers support the crest gates and have been customised to contract the flow rapidly into a high-velocity jet. These piers have been used at, amongst others, the Dachaoshan Dam, a 111 m high Roller Compacted Concrete (RCC) gravity dam, with a maximum unit discharge designed of 193m3/s/m. These piers redirect flow into high-velocity jets, achieving efficient energy dissipation without relying on the stepped spillway face. Although historically utilised exclusively with gated spillways, FGPs hold potential as deflector-type energy dissipaters and were used as the basis for the novel deflector investigations in this research. To date, a variety of aerators have been fitted to improve spillway performance. Other aeration methods, such as the use of Roberts splitters, rectangular protrusions and triangular protrusions have been proposed, with some of these designs being successfully implemented. However, research has noted that these methods yield only marginal increases in the safe unit discharge of stepped spillways. The main concern regarding stepped spillways is the cavitation risk during high discharges, with a critical cavitation parameter of 0.5 compared to 0.2 for smooth chutes. This limits the maximum allowable unit discharge. While cavitation pitting has not been reported on prototype spillways, the exact conditions under which cavitation on stepped spillways may occur remain uncertain. The current research investigated the feasibility of a novel deflector form aimed at increasing the safe discharge capacity of spillways by deflecting the flow away from the spillway slope. The research incorporated a comprehensive approach, comprising a series of numerical models to simulate the hydrodynamic environment as well as four physical models. Numerical model simulations were undertaken with FLOW-3D HYDRO® and ANSYS FLUENT® computational fluid dynamics (CFD) software to optimise the deflector geometries before being tested with a physical model. A 1:50 scale physical model was constructed to investigate the influence of different deflector shapes. The investigation spans a range of prototype unit discharges ranging from 50 to 200m3/s/m and evaluates factors such as water surface profiles created by the deflector and pressure distribution on the deflector. A regression analysis was performed on the collected physical model data to develop equations that predict the jet's inner and outer trajectory and jet breakup length. The proposed novel deflectors developed in this study proved to be effective at various flow rates when the flow trajectory and threshold pressures were considered. These deflectors could be used for dams higher than 150 m and unit discharges ranging between 100 and 200 m3/s/m. Further research is required to improve, amongst others, deflector geometries, to study variables and to undertake additional measurements to conform and improve the efficiency of the novel deflectors, using this research as a basis.