Prototype Air-Water Flow Measurements in a Tunnel Chute

Air-water flows commonly occur in hydraulic structures such as spillway chutes and low-level outlets. In high-velocity flows, air entrainment is a dominant feature and must be considered for a sound design of these safety-relevant appurtenant structures. Most applied research and engineering design recommendations for such structures have been based upon laboratory scale air-water flow experiments. However, scale effects remain a common and complex issue in air-water flow research as they possibly affect the extrapolation of laboratory scale mass, momentum, and heat transfer processes to prototype scale and a validation from observations at prototype scale is generally missing. To tackle this challenge, we conducted measurements in high-velocity tunnel chute flows at the 225 m high Luzzone Dam in Switzerland. The mean flow velocities reached up to 38 m/s corresponding to Reynolds numbers up to 2.4x10(7). The measured void fraction and flow velocity profiles, as well as the bulk parameters mean void fraction and flow resistance agreed well with existing laboratory data and empirical equations with Reynolds numbers in the order of 10(5) to 10(6). However, microscopic air-water flow properties such as the droplet size showed significant scale effects, which requires further research.