Experimental study of underwater pulse detonation gas jets: Bubble velocity field and time–frequency characteristics of pressure field

Underwater pulse detonation gas jets generated by a detonation tube are experimentally investigated in this study utilizing detonations in explosive gas mixtures to generate pulsating bubbles under water. Three stoichiometric gaseous fuels (methane, hydrogen, and acetylene) are detonated with oxygen under the same filling conditions. Digital particle image velocimetry and wavelet transform techniques are introduced to analyze bubble dynamics and pressure field characteristics by means of which the velocity field of the bubble interface and the time–frequency distributions of the pressure response under water are elucidated, respectively. Motions of the bubble interface, which can now be clearly seen with the oscillations, are indicated in high-speed photographic images. Three main frequency components and their duration are identified: reverberations of water tank, pulsations of the detonation gas bubble, and fluctuations of free water surface. Experimental results show that the reverberation concentrating in high frequencies is due to the detonation wave (DW) and reflected shock waves in the water tank; the pulsations are related to the bubble oscillating periods, which are stronger as the detonation pressure increases; and fluctuation occurs in both the bubble oscillating and floating stages. To explain the directional growth of the detonation gas bubble, an experiment of pulse detonation gas jet in air is conducted where the sudden release of detonation products behind the DW and the subsequent impulsive detonation gas jet are qualitatively presented. Results presented in this paper give in-depth analysis of pulse detonation gas jets and provide a new way to generate pulsating bubbles under water.