High-Velocity Air-Water Flow Measurements in a Prototype Tunnel Chute: Scaling of Void Fraction and Interfacial Velocity

Aeration occurs in many natural and human-made flows and must be considered in engineering design. In water infrastructure, air-water flows can be violent and of very high velocity. To date, most fundamental research and engineering design guidelines involving air-water flows have been based upon laboratory scale measurements with limited validation at prototype scale with larger Reynolds numbers. Herein, unique measurements were conducted in high-velocity air-water flows in the tunnel chute of the 225-m-high Luzzone arch Dam in Switzerland. For each of the two test series, an array of 16 double-tip conductivity probes was installed in the circular tunnel chute of 3 m diameter and slope of approximate to 37 degrees measuring void fraction, bubble count rate, interfacial velocity, and droplet sizes for four different discharges of up to 15.9 m(3)/s corresponding to Reynolds numbers of up to 2.4 x 10(7) and mean flow velocities of up to 38 m/s. Void fraction and interfacial velocity distributions, as well as design parameters such as depth-averaged void fractions and flow resistance, compared well with previous laboratory studies and empirical equations. The droplet chord sizes exhibited scale effects, and care must be taken if air-water mass transfer and droplet momentum exchange processes are assessed at the laboratory scale. (C) 2021 American Society of Civil Engineers.