High-velocity air–water flows in rough-walled tunnels

High-velocity air–water flows are integral to a wide range of natural and engineered systems, particularly in hydraulic engineering and industrial processes. These flows frequently occur at hydraulic structures, such as tunnel chutes downstream of high-head gates, where they are characterized by significant air entrainment, air dragging, and complex interactions between air and water phases. While previous studies investigated the effects of invert roughness on high-velocity open-channel flows, the impact of wall roughness on the properties of high-velocity flows within closed conduits remains insufficiently understood. This study addresses this gap through large-scale experiments with Reynolds numbers up to 3 × 106 and Froude numbers up to 47, investigating the effects of wall roughness on key air–water flow properties, such as void fraction, interface frequency, chord size, interface velocity, and turbulence intensity. Results indicate that increased tunnel roughness enhances air entrainment, flow bulking, and turbulence intensities and reduces interface frequencies and velocities. Furthermore, the study demonstrates that vertical profiles of flow properties scale when uniform flow parameters are used. This emphasizes the importance of accounting for wall roughness and the associated hydraulic regime when dealing with high-velocity air–water flows in tunnels, for research and design purposes. Finally, the study provides several empirical equations, offering valuable design and verification tools to hydraulic engineers.