Pressure Surges Following Sudden Air Pocket Entrapment in Storm-Water Tunnels

Deep storm-water storage tunnels may undergo pressurization during intense rain events. In the process, air pockets may become entrapped and pressurized, causing significant flow changes. Currently, the role of nearby surge relief structures is uncertain with respect to air-water interactions. This paper presents results from experimental and numerical investigations on air pocket entrapment and compression following reflection of inflow fronts on system boundaries. Steady flows were established in the pipe apparatus, with the upstream portion flowing in the pressurized regime while the downstream flowed in free surface conditions. Sudden flow obstruction caused by valve maneuvering at the downstream end generated unsteady conditions that were monitored by transducers and a MicroADV probe. Partial valve maneuver runs were performed, aiming to represent cases in which surge relief is provided during air pocket compression/expansion cycles. Whereas experiments performed without surge relief (complete valve obstruction) yielded an oscillatory pressure pattern upon air pocket entrapment, when relief was available, a single pressure pulse pattern was observed instead, and no subatmospheric pressures were recorded. A simple two-phase model that includes essential features of the problem was developed, and its pressure predictions compared well with the experimental data.