A joint experimental study of the flow evolution and thrust characteristics of underwater supersonic gas jets

The flow pattern of underwater jet under co-flow is complex, with oscillations reflected in back pressure and nozzle thrust, impacting vehicle performance. This paper presents an experimental study on jet evolution under co-flow conditions at a Froude number of 2.86, conducted in a closed-loop cavitation water tunnel. It analyzes the multiphase flow structure, back pressure Pb and nozzle thrust under various nozzle pressure ratios (NPRs) and area ratios, revealing their interrelationship. It was discovered that decreasing NPR or increasing the area ratio diminishes the jet's ability to counter buoyancy and enhances plume accumulation at the nozzle exit. The intensity of frequency shift of nozzle back pressure is positively correlated with NPR but shows limited sensitivity to the area ratio. The drift range of peak frequency is greatly affected by the area ratio. Furthermore, the back-attack always characterized as a high amplitude signal, whereas jet breaking exhibits low amplitude. A narrow bubble structure at the nozzle exit induces peak back pressure and x-direction thrust, but bubble expansion reduces these effects.