Hydraulic jumps influenced airflow in a partially filled circular pipe

A hydraulic jump is one of the most common hydraulic features in open channel flow, inducing extra turbulence. The generation and release of hazardous gases, such as H2S and CH4, can be largely affected by the air movement induced by hydraulic jumps, posing corrosion and health risks in urban drainage systems. Despite extensive research on hydraulic jumps in open channels, gaps persist in understanding air–water interactions in partially filled circular pipes. This study experimentally examines the characteristics of hydraulic jumps and the associated air motion in a partially filled pipe. Experiments were performed in a 0.3 m diameter pipe, with water flow rates ranging from 8 to 21 l/s. Air flowrates and air pressure gradients along the pipe were measured to quantify relative air demand, drag coefficients, and momentum transfer. These results show that the relative air demand β, defined as a ratio of air to water flow rate, increases with the Froude number (Fr) of water flow, especially for Fr < 2, reaching 0.015–0.058 under partially filled conditions, substantially higher than that one for downstream full pipe flow cases. Across the hydraulic jump region, pressure gradients are 20%–80% greater than the ones at upstream or downstream sections, driven by rapid momentum flux changes and intensified turbulent dissipation. Drag forces and pressure gradients are identified as the dominant drivers of air motion, while wall friction is negligible. Drag coefficients decrease with increasing Fr. These findings improve the understanding of air–water interactions in partially filled circular pipes.