The motion of entrapped air cavities in inclined ducts

The paper reports a study of the water surface profile of an entrapped air cavity while emptying water in an initially filled inclined duct. A one-dimensional (1D) model, which consists of the continuity and momentum equations applicable for open channel flow, pipe flow and air–water interface flow, is developed based on the finite volume method. A pressure drop model is proposed to reproduce a better profile around the cavity front, with a particular focus on air pressure changes inside the confined cavity to simulate a kind of transient flow with an entrapped air cavity. In contrast to the previous studies, the application of the model shows that when the pressure drop is not considered and the air pressure is not changed, the confined cavity soon vanishes. A comparison between the simulated and experimental results shows that the model is able to accurately reproduce the water surface profile of an entrapped air cavity while emptying inclined ducts.