The detonation engine, which can produce high specific impulse during the underwater detonation process (UDP), has become the forefront of underwater propulsion. In this paper, the thrust mechanism conducted in UDP and the propagation characteristics of the complex pressure waves are numerically studied, and the correlation between those two features is analyzed. The thrust from UDP is generated in a submerged detonation tube (SDT) and driven by the stoichiometric methane-oxygen mixture. The results show that detonation of the pre-filled combustible gas mixture gives rise to complex pressure waves and delivers several force impulses to the SDT. The impulses present different effects on the thrust performance, which is divided into two stages. In the first stage, before the detonation wave collides with the exterior water, the thrust is provided by the persistent back pressure effect of the detonation product. When the detonation wave propagates through the SDT exit and strikes the gas-water interface, a transmitted shock wave and a reflected shock wave are formed, which produce the impulses dominating the second stage. The reflected shock wave eventually impinges on the inner wall, imposing a force impulse on it. The pressure disturbance on the annular wall caused by the transmitted shock wave and subsequent detonation gas jet leads to another two thrust impulses. Finally, a comparison between the thrust of the SDT and its counterpart in air is conducted to characterize the influence of UDP, and the effects of dimensional parameters of the SDT are also investigated.

To study the transmission characteristics of near-field detonation noise into water, the detonation noise transmission system is built on a laboratory-scale water tank using a detonation tube with a diameter of 30 mm. The interaction of the detonation gas jet with the air-water interface, the development of the cavity and the growth of the liquid column are experimentally observed by a high-speed camera. The spectral distribution characteristics of detonation noise above and below the interface are recorded by a microphone, a hydrophone and an underwater blast sensor. Analysis of the experimental images shows that the size of the cavity increases with increasing filling pressure and decreases with increasing nozzle height. By normalizing the evolution time of the cavity with the cavity lifetime, it is concluded that the time for the cavity to develop to the deepest is about 0.27, independent of the filling pressure. The pressure field data analysis results show that the main frequencies of the detonation sound waves are 100 Hz and 400 Hz, and the frequency distribution has nothing to do with the filling pressure. Through the defined acoustic wave energy transmission coefficient, it is demonstrated that the detonation acoustic wave transmission coefficient decreases with the increase of the frequency, and the shock wave transmission coefficient decreases with the increase of the angle.