Investigation on the Kinematics of Entrapped Air Pockets in Stormwater Storage Tunnels

Abstract

Various mechanisms can lead to the entrapment of air pockets within stormwater storage tunnels when they undergo rapid filling during intense rain events (Vasconcelos, 2006). These entrapped air pockets are linked to operational issues within systems such as damaging surges, storage capacity loss, and severe geysering upon their release through water-filled ventilation shafts. Therefore, tracking entrapped air pockets and their celerity is important in the context of numerical simulation to assess the risk of the aforementioned operational issues. Previous studies focused on quantifying the magnitude of pressure surges associated with air pocket compression or in obtaining the minimum flow velocities required to expel the entrapped air pockets from water mains (hydraulic clearing). However, the conditions controlling the motion of these finite volume pockets following entrapment require further investigation. A balance between drag and buoyancy forces is expected to control the motion of discrete air pockets in closed conduits, yet there have been limited studies in terms of how factors such as varying pipeline slope, background flow, and air pocket volume affect air pocket motion. This research aims to explore a link between ambient flow velocity, pipeline slope, and the celerity of entrapped air pockets of various volumes. This work presents experimental results from an investigation on the kinematics of entrapped air pockets in pressurized water flows under various shallow slopes (up to 2% favorable and adverse). Results of pocket trajectories and celerities are systematically compared for various tested slopes, flow rates, and pocket volumes. These experimental results are useful for the future development of numerical models that can include the motion of entrapped air pockets in closed conduits.