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Effects of Wall Roughness on Air-Water Flow Properties of Low-Level Outlets

Author(s): Simone Pagliara; Benjamin Hohermuth; Stefan Felder; Robert M. Boes

Linked Author(s): Stefan Felder, Robert Boes

Keywords: Erated flows; Dual-tip phase-detection probe; High-head gate; High-speed flows; Physical modelling

Abstract: Reservoir dams serve many purposes, including hydropower production, climate change mitigation, and flood control. Low-level outlets (LLOs) are key safety structures of high-head dams. Their main purpose is to control the water level in the reservoir during flood events or maintenance works, and to support a rapid drawdown of the reservoir in case of emergency situations. Alternative purposes include controlling the first impounding of the reservoir, sediment flushing, turbidity current venting, and releasing of environmental flow to preserve downstream river health. In the free-surface tunnel, flow velocities can reach values of several dozen meters per second, leading to significant air transport and sub-atmospheric pressures which may induce or aggravate problems with cavitation and gate vibration. Sufficient flow aeration is crucial to mitigate these issues and to achieve a safe and reliable operation of LLOs. Despite recent new insights on the influence of hydraulic and geometric tunnel parameters on air-water flow properties in LLOs, the effect of wall roughness on their performances is still unclear. Considering that wall roughness and Reynolds number are often the key differences between scale models and prototype structures, novel physical model tests were performed to investigate the influence of tunnel wall roughness on key air-water flow properties including void fraction and interfacial velocity. Different combinations of gate opening and energy head at the gate were tested, corresponding to Froude and Reynolds numbers at the vena contracta up to 46 and 8·105, respectively. Two wall roughness configurations representing unlined rock and finished concrete were tested, and measurements were taken by means of a dual-tip phase-detection conductivity probe. Results indicate that the tunnel roughness has a significant effect on the air-water flow properties, leading to larger flow depths associated with higher void fractions and smaller interfacial velocities, while the turbulence levels increased. This study contributes to a safer design of LLO tunnels.


Year: 2023

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