AbstractThe emptying process of initially stagnant full-pipe flows in sewer systems and water transmission lines was extensively studied in the literature. The following air intrusion and depressurization wavefronts can cause several issues such as the collapse of pipes, damage to joints, and valves. However, there is a lack of study on the emptying process when the flow has initial velocity. Thus, the present study focuses on the emptying process of a full-pipe flow with an initial flow rate, which decreases during the emptying process. For this purpose, two experiments without and with initial flow rates were performed in a reservoir-circular pipe system. In both experiments, an air cavity intrusion starts downstream of the pipe at the top of the free-surface flow and propagates upstream. It was found that when there is no initial flow rate, the pressure inside the cavity is atmospheric. However, when there is an initial flow rate, at the onset of the air intrusion, a subatmospheric pressure develops inside the air cavity. It was also found that the magnitude of this subatmospheric pressure decreases when the cavity further propagates upstream. In addition, when there is an initial flow rate, the cavity appears at a certain driving pressure.