Pressure Surges Following Sudden Air Pocket Entrapment in Stormwater Systems
Stormwater storage tunnels may undergo rapid pipe filling conditions during extreme rain events. Such conditions are relevant as adverse conditions may develop, such as surging due to entrapment and compression of entrapped air pockets. Operational issues such as structural damage, geysering and the return of conveyed water to grade, among others, have been linked to air pocket entrapment.
This chapter presents results from the experimental investigations of pressure surges caused by sudden air pocket entrapment. Steady flow was supplied in a pipeline in such a way that pressurized flows existed at the upstream end while the downstream end experienced free surface flow due to free discharge conditions at the downstream end. A combination of flow rates and slopes resulted in several gradually varied flow profiles at the discharge, with various volumes of atmospheric air at the discharge end.
By suddenly closing the downstream discharge valve, an air pocket was entrapped and surges were recorded. To emulate conditions in which surge relief is provided during air pocket compression, valve operation also included cases with partial valve obstruction. Among the obtained results, one can notice significantly different surges for cases with and without pressure relief (e.g. with total or partial obstruction), and that the larger the obstruction degree the larger are the surges. Such findings are useful in the development of numerical models to simulate the rapid filling of stormwater tunnels incorporating these findings.
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