Propagation of an Air-Water Interface from Pressurized to Free-Surface Flow in a Circular Pipe

Hydraulic transients with the movement of an interface between pressurized flow and free surface flow can be observed in rapid water emptying in the pipes. This study focuses on air cavity intrusion into horizontal and inclined circular pipes. Laboratory experiments were carried out to observe the negative surge during depressurization in a circular pipe. In order to generate the undular bore in a circular pipe, in some cases a sharp-crested weir at the open end of the horizontal pipe was used. Various behaviors of air cavity were observed for a series of weir heights. A numerical model is proposed and applied to the cavity flow in a circular pipe. To reproduce the undular bore in circular pipes (when water depth is greater than pipe radius), this study derived the depth-averaged shallow water equations with the effects of vertical acceleration using the Boussinesq equation. In this study, the continuity and the momentum equations of free surface and pressurized flows with the hydrostatic and the Boussinesq pressure assumptions have been used. The results from the model showed good agreement with the experimental results. (C) 2016 American Society of Civil Engineers.