Flow Mechanism and Characteristics of Pressure-Equalizing Film Along the Surface of a Moving Underwater Vehicle

Pressure-equalizing film is a slice of air layer attached to vehicle exterior with nearly uniform inside pressure, similar to ventilated cavity in composition; it is generated through exhaust process of the inside air chamber as vehicle emerges from deep water, and can reduce the lateral force and pitching moment that vertical launched underwater vehicle suffered. In this work, the emerging process of vehicle from water with pressure-equalizing exhaust was numerically calculated to investigate the evolution and flow characteristics of the generated pressure-equalizing film along its surface. Results indicated that during the whole exhaust process, the film can be obviously classified into different sections according to the distribution of phase volume fraction or pressure. The exhaust velocity ratio and flow rate from vehicle interior chamber were also found to increase as vehicle moves. In the analysis of flow structures, vortex structures such as the horseshoe vortex, "detour-separation" vortex, and counter-rotating vortex pair (CVP) can be figured out in the region of the exhaust hole. Under the effect of re-entrant jet, water around the film tail would be entrained upstream then enter the surface film to mix with the pressure-equalizing air. It leads to the happening of the three-dimensional (3D) wall vortex in the flow field.