Volume of fluid-based CFD study of triggering and eruption in urban storm sewers

Geyser eruption in urban drainage is a high-velocity discharge of an air–water mixture driven by intense air–water interactions. It is sudden, destructive and strongly non-linear, and is often associated with air entrapment, surcharge flows during intense rainfall and transient hydraulic processes, posing risks to drainage assets and surface infrastructure. A representative horizontal main pipe–vertical riser configuration is modelled using a volume of fluid (VOF) two-phase framework and a systematic numerical study is conducted of the inlet pressure head, dimensionless initial entrapped air index, Ia0, and the initial water-column height. Results identify a distinct eruption threshold: under the fixed geometry and boundary conditions considered in this study and within the tested parameter range, geyser eruption is triggered when Hin/H0 ≥ 1.33 andIa0 ≥ 3.20. As the free surface enters the upper riser, pronounced pressure spikes at PT4 and PT5 can serve as precursors. For a given inlet pressure head and the initial entrapped air condition, increasing the initial water-column height leads to higher eruption velocities, highlighting its control over eruption intensity.