Page - 832 - in Book of Full Papers - Symposium Hydro Engineering

COMMISSION INTERNATIONALE DES GRANDS BARRAGES ------- VINGT-SIXIÈME CONGRÈS DES GRANDS BARRAGES Autriche, juillet 2018 ------- AIR DEMAND OF BOTTOM OUTLETS: INSIGHTS FROM SCALE MODEL TESTS AND PROTOTYPE MEASUREMENTS * Benjamin, HOHERMUTH1, Lukas SCHMOCKER2, Robert M. BOES3 1 Doctoral Student, 3 Director of Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH ZURICH, 2 Research Scientist, Swiss Competence Center for Energy Research – Supply of Electricity SCCER-SoE SWITZERLAND 1. INTRODUCTION Bottom outlets (BOs) are a key safety feature of high-head dams. Their main purpose is the regulation and – if required - a fast drawdown of the reservoir water level in case of floods, structural damage of the dam or maintenance works. Additional purposes include residual flow release, flood diversion or sediment flushing. The large energy heads at the gate lead to a high-speed free-surface flow in the BO tunnel with flow velocities up to some 50 m/s for high dams. These large flow velocities and the high turbulence levels lead to considerable air entrainment and air transport, possibly resulting in negative pressures in the BO. This can cause problems with gate vibrations, cavitation and flow chocking. Sufficient air supply via an aeration chamber can mitigate these problems. Available design equations for the required air demand to prevent excessive negative pressures are based on scale model tests and prototype measurements. Most of these equations express the air demand β as a function of the Froude number at the vena contracta Fc. However, measured and predicted β-values show a large scatter. Some of the scatter can be explained by the different flow patterns. The remaining scatter can be explained with the neglect of crucial parameters in the design equation like the air vent loss coefficient ζ or the tunnel length L. This project aims to investigate the * Aération des vidanges de fond: Résultats des essais sur modèle à échelle réduite et des mesures en prototype 832