Experimental study on the evolution and oscillation mechanisms of underwater supersonic gas jets

Underwater high-speed gas jets exhibit highly complex nonlinear and unsteady flow fields due to the significant physical property contrasts between gas and water. This study experimentally investigates the evolution of such jets under varying nozzle pressure ratios (NPR) and area ratios. Synchronized measurements of jet structure, pressure fluctuations, and thrust oscillations are performed to systematically examine their intrinsic relationships. Spectral proper orthogonal decomposition (SPOD) is further employed to extract dominant spatiotemporal coherent structures from the interfacial dynamics. Results show that the jet development proceeds through three distinct stages, characterized by periodic necking-bulging cycles with an average period of approximately 10 ms and intermittent back-attack phenomena. SPOD identifies a dominant mode at St = 1.35 × 10^-3, accounting for 46.78 % of the total fluctuation energy, which embodies the necking-bulging cycle as a traveling wave structure along the jet shear layer, and a low-frequency mode at St = 1.35 × 10^-4 associated with back-attack events. The peak frequency remains stable within St = 0.0011 ∼ 0.0014 across all tested conditions. A strong correspondence is established between back-attack events and paired pressure peaks at the nozzle exit, as well as between thrust oscillations and jet bulging dynamics, with the maximum thrust reaching 66.76 N, a value approximately 68 times the mean. Furthermore, the quantitative effects of NPR and area ratio on jet morphology, SPOD modal energy distribution, and load characteristics are elucidated. The resulting spatiotemporal evolution and parameter correlation models provide a theoretical foundation and experimental reference for flow prediction and stability control in engineering applications such as underwater propulsion.