Kinematics of Entrapped Air Pockets Within Stormwater Storage Tunnels

Jose G. Vasconcelos and Carmen D. Chosie


During the rapid filling of stormwater storage tunnels, which is anticipated to occur during intense rain events, a number of different mechanisms may lead to the entrapment of air pockets as pointed by Vasconcelos and Wright (2006). Entrapped air pockets have been linked to operational issues such as damaging surges, loss of storage capacity and severe geysering upon their release through water-filled vertical shafts. However, the mechanisms behind the motion of these finite-volume pockets following their entrapment still require further investigation. For discrete air pockets (as opposed to stratified air-water flow conditions) a balance between drag and buoyancy is expected to control the motion of large pockets in closed conduits. Previous related investigations approached the problem of pocket motion in the context of water mains, focusing in the determination of a minimum velocity to ensure air pockets removal. However, in the context of the rapid filling of stormwater tunnels, water flows cannot be pre-specified, and the pocket celerity thus become important parameter, particularly in the context of numerical simulation of such flows. This work presents preliminary results of an experimental investigation performed at Auburn University in which different air pocket volumes are released in pressurized closed-pipe flow at selected slopes and flow rates. Kinematics of air pockets is studied for various flow conditions with the goal of obtaining a better understanding between observed pocket celerity and its relation to pocket volume, to flow velocity and pipeline slope.

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