Air-Pocket Entrapment Caused by Shear Flow Instabilities in Rapid-Filling Pipes
Publication: Journal of Hydraulic Engineering
Volume 146, Issue 4
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.
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Data Availability Statement
Some or all data, models, or code generated or used during the study are available from the corresponding author by request, including analytical, numerical, and experimental test data.
Acknowledgments
The first and second authors acknowledge the support of the General Directorate of State Hydraulic Works (DSI), under the Republic of Turkey Ministry of Forestry and Water Affairs, which has provided fellowships through Grants 33070922-772.02-736534 and 33070922-772.02-826712.
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Published In
Journal of Hydraulic Engineering
Volume 146 • Issue 4 • April 2020
Copyright
©2020 American Society of Civil Engineers.
History
Received: Dec 21, 2018
Accepted: Aug 23, 2019
Published online: Jan 29, 2020
Published in print: Apr 1, 2020
Discussion open until: Jun 29, 2020
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