A study of underwater high-speed gas jets through experimental flow visualization

This study explores the unsteady flow physics of underwater supersonic gas jets. A controlled experimental facility has been constructed for direct flow visualization of the jet followed by an advanced image processing technique to derive both qualitative and quantitative insight into the flow evolution. The experiments are conducted for two different nozzle Mach numbers and several expansion conditions. It is found that the temporal evolution of the jet has broadly been divided into two major stages. In the first stage, the jet emerges from the collapse of a gaseous cavity. Here, the growth and collapse of the cavity have been investigated and tested through a parametric study over different Mach numbers for various expansion conditions. Afterward, several well-known flow phenomena observed during the continuous jet flow are investigated from the perspective of the two major modes of jet interface instabilities, which resemble the well-known Kelvin–Helmholtz and Rayleigh–Taylor instabilities. The instabilities are quantified from a time varying image processing analysis of the flow images. The effects of the operational parameters on the growth and control of those instability modes are examined. It is found that of the two instability modes, the second mode can aggravate the overall unsteadiness of the jet flow field, and it can be controlled by operating the jet at higher pressure. As the detrimental effect of back-attack during the initial stages of jet propagation can be critical to the propulsion performance of an underwater vehicle, the parametric dependence of the onset of first back-attack is also studied here.