Air-Pocket Entrapment Caused by Shear Flow Instabilities in Rapid-Filling Pipes

Abstract

Understanding air-pocket formation in closed conduits is important in urban water systems subject to rapid-filling conditions, such as in the case of stormwater sewers and tunnels during intense rain events. Captured air pockets influence surging and, upon uncontrolled release, lead to issues such as manhole cover displacement and/or geysering. Different mechanisms for air-pocket formation have been identified, among which are shear flow instabilities that have the potential to capture large volumes of air. This paper presents experimental and numerical research on air-pocket entrapment based on shear flow instabilities. A fully filled horizontal water pipe was opened at the downstream end to create a cavity flow and air intrusion of varying thicknesses. After some time, a second valve was maneuvered near the upstream end, triggering flow pressurization through a pipe-filling bore. The bore pushed air in high velocity over the free surface, and in some cases air-pocket entrapment was observed. A computational fluid dynamics (CFD) model replicating experimental conditions was able to reproduce shear flow instabilities, as well as measured velocities and pressures obtained with experimental measurements. Other larger-scale CFD simulations with similar geometry quantified the air fraction initially in the conduit that became entrapped within pockets. It is hoped that this research can help practitioners in anticipating the risk of large air-pocket entrapment in existing and proposed stormwater systems that may undergo rapid filling.