Effect of hydrostatic pressure on air layer morphology in axisymmetric underwater vehicles

Air layer drag reduction is a promising drag reduction technique, whereas the ventilation process demonstrates dependence on hydrostatic pressure. In this study, using a combination of experimental and numerical simulations, a numerical model was developed for air layer drag reduction on axisymmetric underwater vehicles. A systematic investigation was conducted on air layer evolution and flow field characteristics under hydrostatic pressure ranging from 0.1 to 3 MPa. The results indicate that when the volumetric flow rate is used to characterize ventilation rate, hydrostatic pressure exerts a weak influence on the air layer shape, while a distinct linear relationship is observed between the mass flow rate and the absolute pressure at the jet nozzle. It is noteworthy that an increase in hydrostatic pressure alters air density, thereby affecting momentum exchange between the air layer and the external flow. This causes the re-entrant flow that severs the continuous gas layer to shift upstream, resulting in a reduction in the coverage area of the gas layer. This research contributes to the design of air layer drag reduction systems for axisymmetric underwater vehicles operating at varying submersion depths.