Air Pocket Dynamics under Bridging of Stratified Flow during Rapid Filling of a Horizontal Pipe

This paper focuses on air–water interactions during pipe filling. The air–water mixed flow is related to a mobile hydraulic jump in the horizontal pipe section. The trapped water–air front at the crown of the pipe bridge determines the upstream boundary conditions for the stratified flow, suppressed hydraulic jump, and slug flow regions in the horizontal pipe section. The velocity field, the water level, and the pressure were measured at different stages of the transitional flow process. A full-scale two-phase computational fluid dynamics model, with two-equation eddy viscosity turbulence to account for small scales of motion, was used to predict the air pocket dynamics. The paper suggests that in the case of a frozen upstream water–air front, the formation of a hydraulic jump downstream can be explained by long-wave-type instability, which is confirmed by the free-surface supercritical flow Froude number condition. The hydraulic jump that merges stratified flow and slug flow is characterized by the speed of the surge wave, which is determined by the pressure, cross-sectional area of water flow, and density changes. A mixing efficiency parameter was introduced to explain the dissipation of turbulent energy due to the air–water mixing front of the suppressed hydraulic jump.