Experimental and numerical investigations of free surface and mixed flow transitions in urban drainage pipes under complex operating conditions

The stability and functionality of urban drainage networks are critical for flood mitigation. Transient mixed flows frequently arise during intense rainfall events or abrupt operational disturbances, generating substantial pressure surges that threaten pipeline integrity. In this study, an integrated experimental–numerical approach was employed to systematically investigate the transient behaviour of mixed flows. A transparent pipe system, 20 m in length and 0.15 m in diameter, was developed to examine three representative scenarios: sudden upstream inflow surge with free outflow, sudden upstream inflow surge with submerged outflow, and sudden downstream closure under steady inflow. To capture short-lived air compression and interface evolution rarely reported in previous studies, synchronous pressure measurements at both the pipe crown and invert were combined with high-speed visualisation. On the numerical side, an one-dimensional hybrid Random Choice Method–Godunov-Type Scheme model was developed. The model incorporates an adaptive switching strategy, applying Godunov-Type Scheme in smooth regions and Random Choice Method in zones of strong discontinuity. To prevent spurious switching and ensure numerical stability, a dual-threshold switching criterion was introduced. The proposed hybrid model enables stable resolution of sharp hydraulic discontinuities while limiting numerical errors in smooth regions during rapid flow regime transitions. Furthermore, comparison with experimentally observed surge front propagation and pressure evolution under various boundary conditions shows that the model reproduces the measured results with deviations within 5%. These results demonstrate that the proposed framework is robust and reliable, providing a validated and practical tool for simulating transient mixed flows in urban drainage systems.