Experimental study of mixed flow transitions in pipes with air pocket effects

Transient mixed flows—marked by rapid free-surface/pressurized transitions and strong air–water interaction—frequently arise in urban drainage conduits during intense storms and can trigger damaging pressure excursions. This study examines unsteady hydraulics in a 20 m, 0.15 m-diameter sloping acrylic pipe over a systematic matrix of dimensionless inflow Q* and downstream submergence y*. Synchronized crown–invert pressure measurements and high-speed imaging were used to capture transient pressure signatures, front propagation, and air pocket migration. Six regimes (types 1–6) were identified and organized into a regime map in (Q*, y*). Initial peak positive pressures vary non-monotonically with y*: A slight increase at low submergence is followed by sharp amplification upon formation of a pressurized front, attenuation under outlet sealing without strong compression, and renewed amplification once y* > 1.0. Minimum negative pressure behavior is mechanism dependent: Type 3 exhibits moderate minima associated with long duration rarefaction, whereas types 4 and 5 produce much deeper but short-lived minima. At type boundaries, minima attenuate as the dominant mechanism shifts and energy is dissipated during early sealing–unsealing. These results provide a mechanistic basis for surge prediction and mitigation (ventilation layout, inflow control, pressure relief) and supply well-instrumented data for validating mixed flow models.