Flow structures of over-expanded supersonic gaseous jets for deep-water propulsion

Abstract

Supersonic gaseous jets with large ambient pressure are typically found in deep-water propulsion systems. The interactions between gas and water are turbulent, and the plume exhibits unstable bubbly interfaces. This work employed both experimental and numerical methods to obtain the flow structures of over-expanded gaseous jets driven by different pressure ratios in water. Direct measurements of the interfacial behaviour of submerged gaseous jets with pressurised water, were performed using high-speed digital photography. Numerical simulations were also conducted to characterise flow structures by the interplay of inertia and buoyancy forces. It was found that the interface of injection into water showed continuous fluctuations and strong mixing in the conical regions. The intermittent expansion-necking characteristics just beyond the nozzle exit, induced the pressure oscillation of the flow field near the wall, localised liquid water attack of the exit wall, and aggravated the over-expansion of nozzle working conditions. These periodic phenomena were attributed to Kelvin-Helmholtz (K–H) instabilities, driven by the large gradients of velocity and density in the shearing layers. The oscillation characteristics were assessed at different ambient pressures, providing theoretical support for the design of engineering devices working over a wide range of water depths.