Flow mechanism of the underwater supersonic gas jet formed by the beveled nozzle

The internal flow of underwater supersonic gas jets formed by beveled nozzles is highly complex. Such jets exhibit unstable characteristics, including deflection and swinging, which severely restrict the propulsion performance of underwater vehicles. In this paper, optical and Schlieren techniques are adopted to capture the flow field structures of underwater jets from quasi-two-dimensional beveled nozzles. The jet deflection characteristics and internal flow mechanism at different developmental stages are studied under various bevel angles and nozzle pressure ratios (NPRs). Results show that the development of underwater jets produced by beveled nozzles includes an initial bubble stage and a relatively stable conical stage, with continuous expansion and contraction of the jet interface. A quantitative correlation exists between the jet deflection angle, bevel angle, and NPR. An empirical formula is established in this paper, which can effectively predict the jet deflection angle. Significant swinging occurs in the jet during the initial bubble stage, and this phenomenon becomes more pronounced with a smaller bevel angle and a larger NPR. Swinging also exists in the jet during the conical stage. However, unlike the initial bubble stage, an increase in NPR gives rise to more obvious deflection but a weaker swing effect. In addition, during the conical stage, the variation of the upstream jet interface is mainly governed by the internal pressure of the jet, besides the influence of Kelvin–Helmholtz and Rayleigh–Taylor instabilities.