Air–water interactions during rapid filling of a closed horizontal pipe

Rapid filling processes in urban water infrastructure, involving open-channel to pressurized flow transitions with complex air–water interactions, require precise understanding of transient pressure surges, air entrapment and air discharge mechanisms. This study establishes the first experimental framework classifying rapid pipe filling into four distinct stages: advancing bore (stage I), reflection bore (stage II), air intrusion bore (stage III), and residual air release with subsequent inertial oscillations (stage IV). Quantified parameters including bore speeds, pressure and air discharge rate evolutions during each stage, are systematically correlated with gate opening and inlet head, establishing novel predictive relationships. The arrival of the reflection and pipe-filling bores at the shaft can induce significant pressure surges, with peak magnitudes reaching 2.3 times the inlet head through newly identified scaling law dependent on operational parameters. The advancing, reflection and air intrusion bores all cause significant air discharge through the shaft, and an expression for the maximum air discharge rate is obtained. Critically, over 90% of the initial air is expelled during stages I–III, with 28%–77% expelled in stage I, which accounts for less than 11% of the total filling duration. The air discharge volume in stage I increases as the gate opening and inlet head increase. These findings provide a critical foundation for the development of validated transient air–water flow models, predictive mitigation of pressure surge risks, and optimization of resilient urban water infrastructure.