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 $\approx 37°$ measuring void fraction, bubble count rate, interfacial velocity, and droplet sizes for four different discharges of up to $15.9{\mathrm{m}}^3/\mathrm{s}$ corresponding to Reynolds numbers of up to $2.4\times {10}^7$ and mean flow velocities of up to $38\mathrm{m}/\mathrm{s}$. 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.