Experimental and Numerical Investigation on the Motion of Discrete Air Pockets in Pressurized Water Flows

Entrapped air pockets are linked to various operational issues in closed pipe flows. Tracking of entrapped air pockets is a relevant task in numerical simulation of hydraulic systems subjected to air pocket entrapment. However, few studies have focused on the kinematics of discrete air pockets and the motion of these at various combinations of pipe flow velocities, air pocket volumes, and pipe slopes. Such information is relevant for the improvement of current numerical models for the simulation of transition between pressurized and free surface flows, and related air pocket entrapment. This work explores the link between pipe flow velocity, pipe slope, and entrapped air pocket celerities. Systematic experimental studies were performed and provided insights on the nature of entrapped air pocket motion in cases when buoyancy forces are either summed or opposed to flow drag. Results indicated that when flow drag and buoyancy forces are summed, the observed air pocket celerity can be approximated by the celerity in quiescent conditions plus the pipe velocity. This approximation fails in cases when drag force opposed buoyancy. This work also proposes a novel approach for the simulation of the air pocket motion as non-Boussinesq gravity currents. Modeling results compared favorably with experimental measurements in horizontal slope and pipe flow velocity cases. Such results are an indication of the feasibility of integrating this approach into more complex hydraulic models that are able to simulate flow regime transitions. (C) 2014 American Society of Civil Engineers.