Airflow characteristics induced by obstacle-generated backwater in partially filled circular pipes

Obstacles at the bottom of sewer pipes can generate pronounced backwater effects and substantially modify airflow in partially filled conduits. This study experimentally investigates the airflow characteristics induced by obstacle-generated backwater in circular pipes under horizontal (S0 = 0) and sloping (S0 = 0.015) conditions. Laboratory experiments were conducted with different obstacle geometries and inflow discharges. Airflow rate, air pressure gradient, and water surface profile were measured, and representative flow observations were used qualitatively to support the analysis. Within the tested hydraulic ranges, the relative air demand decreases with increasing inflow Froude number in horizontal pipes, whereas the opposite trend was observed in sloping pipes. In horizontal pipes, the smaller head loss and smoother free surface weakened air–water interaction. In sloping pipes, larger head loss and stronger free-surface fluctuations enhanced interfacial drag and increased airflow in the pipe headspace. Pressure measurements further indicated pronounced air-pressure gradients within the backwater region. Force analysis showed that the airflow was governed mainly by the balance between air-pressure gradient and interfacial drag, whereas air inertia was negligible under the present conditions. These results clarify distinct airflow regimes associated with obstacle-induced backwater and provide a hydraulic basis for understanding ventilation and gas transport in partially filled pipes with internal obstacles.