Flow dynamics in the closure region of an internal ship air cavity

Accurate prediction of air leakage is crucial for the design of ship air lubrication systems based on internal cavities. Currently, the flow dynamics that govern the leakage remain largely unexplored, and the goal of this work is to elucidate them by means of numerical simulation. A geometrically simple test cavity is considered, and a simulation of the flow is conducted using large-eddy simulation coupled with a Volume of Fluid interface capturing method. The flow in the closure region is shown to be highly unsteady and turbulent. The cause of this is identified to be the pressure gradient on the beach wall of the cavity, occurring due to the stagnation of the flow. This pressure gradient pushes the air-water interface upwards, making it steeply inclined. As a result, the flow separates from the interface and forms a recirculation zone, in which air and water are mixed by means of overturning waves and turbulent entrainment Swarms of air bubbles leak periodically. Upstream of the closure region, the phase and length of the wave are found to be well-predicted using existing approximations based on linear flow theory. However, for the corresponding prediction of the amplitude of the wave the agreement is worse.