Flow structure and acoustics of underwater imperfectly expanded supersonic gas jets

In this paper, the process of vertical downward underwater supersonic gas jets and its acoustic characterization is investigated. This is because the acoustic properties of these jets are not well characterized to date; numerical simulations are conducted for three different jet conditions, for one case with an underexpanded jet and for two cases with overexpanded jets. For numerical validation, experimental overall sound pressure level measurements are available, giving a good agreement between simulations and experiments. The numerical data provide information about the formation of the jet pattern depending on the depth of penetration of the gas jet into the water. Adaptive filtering technique and time–frequency analyses are applied to evaluate the numerical acoustic data. Combined with the study of evolution of flow structures, the spectrum and spectrogram indicate that the formation of shock waves and its destruction by vibrations of the jet elevate pressure and sound pressure fluctuations in the flowfield. As a result of these investigations, it follows that computational fluid dynamics could be applied to well understand the physics behind underwater gas jet noise, and it is proposed to use computational fluid dynamics for further studies of gas jets blowing into water.