Air-Water Flow Patterns and Shockwave Formation in Low-Level Outlets

Reservoir dams play a significant role in society and the economy. Low-level outlets (LLOs) are key safety devices providing reservoir drawdown for maintenance and emergency purposes, sediment flushing, and release of environmental flow. High velocities and turbulence levels of the free-surface flow lead to air entrainment and air transport along the LLO tunnel, resulting in subatmospheric air pressures. In addition, shockwaves formed downstream of the gate may lead to a complete filling of the tunnel cross section, possibly resulting in slug flow and flow pulsations, which should be avoided for operational safety. In this study, physical model tests were performed to investigate the effects of gate opening and hydraulic head on shockwave patterns. Especially for large contraction Froude numbers, the shockwaves were strongly aerated, resulting in a complex air-water flow pattern. The results provide insights into the formation and propagation of shockwaves, contributing to an improved design of LLO tunnels.