Experimental investigation on flow structure and pressure characteristics of submerged high-speed gaseous jets under co-flow conditions

High-speed gaseous jets are commonly found in deep-water propulsion systems. The submerged high-speed gaseous jet becomes turbulent due to the density difference between the gas and liquid phases. In experimental studies, the interaction between the gaseous jet and the liquid has not been well characterized under co-flow conditions. In this paper, the effects of co-flow velocity, Fr, and jet pressure ratio, p0/p∞, on the flow structure of the gaseous jet and pressure on the nozzle outlet cross-section were experimentally studied. It has been observed that the increase of Fr makes the nozzle outlet cross-section gradually wrap up with transparent bubbles under subsonic gaseous jets. However, the transparent bubbles will be broken entirely under supersonic gaseous jets, and the gaseous jet will intermittently expand into bubbles downstream away from the nozzle outlet cross-section. The flow mechanism of the underwater gas jet at larger Fr seems to be different from that at smaller Fr and still water. Keeping Fr constant, the dimensionless streamwise length of the jet, Ls/LQ, increases as a power law with p0/p∞. Fr further promotes the penetration of the jet downstream under the fixed p0/p∞. In addition, the pressure on the nozzle outlet cross-section is seriously affected by p0/p∞ and Fr. The increase of Fr will seriously weaken the intensity and frequency of pressure oscillations on the nozzle outlet cross-section. This finding can provide theoretical support for the design and control of jet-propelled vehicles.