Pressure field characteristics of underwater detonation gas jet below and above free water surface

Abstract

An underwater detonation tube (DT) experiment is carried out to investigate the pressure field characteristics of the detonation gas jet propagating under and above a free surface. In the experiment, a 0.78 L DT filled with 0.12 MPa methane-oxygen mixture is detonated. The evolution of jet shape and pressure field are recorded using high-speed photography, pressure sensors, hydrophones and microphones. There are mainly three bubble pulsation periods determined from the bubble evolution and field pressure evolution, and the bubble shape tends to be stable after the pulsation ends. The fast Fourier Transform and Wavelet Transform method are adopted to analyze the frequency characteristics and energy distribution of the pressure signals. The energy of the bubble pulsation and the energy of the shock wave have different contributions in different media. Specifically, the high-frequency energy formed by reverberation contributes more in water, while the low-frequency energy formed by bubble pulsation is the main part of the energy in the air. The shock wave formed by the decoupling of the detonation wave attenuates sharply during its propagation. The distribution characteristics of the detonation acoustic signal in the air are analyzed using the band-pass filter pair, and the transmission coefficient is defined to study the ability of the detonation acoustic signal to transmit through the water–air interface. The result indicates that the low-frequency signals, such as bubble pulsations, can transmit out of the water–air interface at a larger transmission angle more easily than the high-frequency signals. The signal of bubble pulsation has a stronger transmission ability through the water–air interface compared to the shock wave. Combining the bubble evolution and pressure evolution is helpful to analyze the different signal components in the water and air, which plays an important role in the analysis of the pressure field characteristics.