Gravity Current and Air Pocket Entrapments Caused by Shear-Flow Instabilities in Rapid-Filling Pipes

Abstract

Understanding the behavior of air-water interactions in closed conduits is very important for urban water systems that are subject to rapid filling conditions, such as stormwater systems during intense rain events. Among different mechanisms for air pocket appearance in closed conduits, shear flow instability is one that has significant capability to capture large volumes of air. Upon capture, air phase influences surging and, upon uncontrolled release, lead to issues such as manhole cover displacement and/or geysering. This work 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 and creates a cavity flow. After some advance within the pipe, a second valve was maneuvered at the upstream end, enabling pressurized flows from the upstream end. The pipe-filling bore that was created pushed air in high velocity over the air cavity. In some cases, air pocket entrapment followed leading to pressure peaks appear to be characteristic of this type of entrapment. In the numerical part, an CFD model with the same scale of the experimental apparatus was implemented alongside with a larger-scale model ten times larger than the experimental scale. Results comparison showed that CFD predicts air pocket formation more frequently than comparable experiments. CFD results were also compared to and matched well with the theoretical instability threshold formulas. Overall, useful results for the future investigations of air-pocket entrapments caused by shear flow instabilities in the stormwater systems was obtained to help to solve many operational problems.