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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 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 in the frequency, and the shock wave transmission coefficient decreases with the increase in the angle. [ABSTRACT FROM AUTHOR]

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 the air is conducted to characterize the influence of UDP, and the effects of dimensional parameters of the SDT are also investigated. [ABSTRACT FROM AUTHOR]

In order to research the operation synchronous of dual-tube pulse detonation engine, the operation performance and acoustic behavior of dual-tube pulse detonation engine under different fill fractions are studied experimentally. The results show that the instability of deflagration to detonation transition is the main reason for the non-synchronous work of the two tubes. With the increase of fill fraction, the detonation sound pressure increases gradually. In the region near the tube exit, the pressure and velocity of the shock wave attenuate rapidly, and the attenuation speed gradually slows down while the propagation distance increases. The time interval of detonation waves arriving at the tube exit of the two tubes can significantly affect the peak sound pressure and the duration of the sound wave outside the tube. With the increase of time interval, the peak sound pressure decreases and the total duration as well as the duration of the positive pressure increase. The synchronization of the working process of the dual-tube pulse detonation engine can be diagnosed by analyzing the pressure and duration of the sound wave outside the tube. The research results in this paper have certain reference significance for improving the synchronous of multi-tube pulse detonation engine and will be useful for the application of multi-tube pulse detonation engine in aircraft power system. [ABSTRACT FROM AUTHOR]

Propagation of detonation waves crossing the gas–liquid interface is a basic phenomenon worth studying for underwater detonation engines. In this work, the pressure evolution of detonation waves penetrating into water is theoretically and experimentally investigated. The one-dimensional shock wave theory is adopted to solve the pressure–velocity relations of the reflected and transmitted shock wave in different mediums. Experiments under different filling pressure are performed based on a two-phase shock tube system. Theoretical results show that the range of pressure rise ratios between the detonation and transmitted wave is 2.40–2.50. Its trend is determined by the total atoms number of fuel under low filling pressure, but dominated by the ratio of C/H atoms under high filling pressure. Experimental results demonstrate that pressure rise ratios are in good agreement with the theoretical values. There are similar attenuation laws (decay to 50% in 0.3 ms) for subsequent pressure development after those two waves. Under the interface effect, the transmitted wave is stretched and the pressure zone becomes wider. The difference of acoustic impedance between two phases leads to wave property changes at the interface and exit. These changes result in the reciprocating cavitation zones and reformed shock waves in the water, greatly influencing the water pressure. [ABSTRACT FROM AUTHOR]

The detonation-powered underwater engine, with the advantages of high specific impulse, high speed, and simple structure, has very broad application prospects in the field of underwater propulsion, and dual-tube combination is an effective means to improve its propulsion performance. In this work, near-field pressure evolution of shock waves and high-pressure zones between two detonation tubes is numerically studied. The two-fluid model and three-dimensional conservation element and solution element method are adopted to reveal the formation, intersection, and interaction of shock waves. Detonation waves generated by two detonation tubes decouple into shock waves after penetrating into water and form a high-pressure zone near each tube exit. The two leading shock waves intersect with each other in the propagation, creating the second high-pressure zone between two tubes. Then, a propagating forward merged new shock wave covers the two original wave-fronts and maintains higher pressure. Pressure evolution under different tube intervals, ignition delays, and filling conditions is also presented to discuss their influence on the performance of dual-tube detonation. The intensity and directivity of shock waves are found to be sensitive to these factors, complexly affecting the thrust components, which provides a depth understanding of dual-tube combination in the application. [ABSTRACT FROM AUTHOR]

An underwater detonation tube (DT) experiment is carried out in a water tank to investigate the bubble dynamics and pressure field characteristics of an underwater detonation gas jet. In the experiment, a 0.78 liter DT filled with a 0.29 MPa methane–oxygen mixture (equivalent to 0.85 mg of TNT, trinitrotoluene) is detonated. By means of high-speed photography and pressure field measurements, the jet process is divided into four different stages. The evolution patterns and features of the four stages are characterized according to the morphology of the detonation gas bubble, and the dimensionless parameters of the bubble dynamics are defined and calculated using image post-processing. The transmitted shock wave and pressure pulsations of the bubble oscillations are extracted using a low-pass filter with a cutoff frequency of 1000 Hz. The time intervals between consecutive pressure peaks are compared with the oscillation periods obtained from parameter studies of bubble dynamics. The bubble dynamics generated by the sudden release of detonation products in the first oscillation are found to be similar to those of underwater explosions. An expansion-necking structure is observed, formed by the impulsive release of the remaining detonation gas from the DT. A numerical simulation is conducted under the same filling conditions as the experiment to supplement the experimental results. The experiment demonstrates the feasibility of underwater detonation gas jets, which could provide an alternative means of generating pulsation bubbles. [ABSTRACT FROM AUTHOR]

Pressure evolution in the process of an underwater-detonation gas jet is experimentally researched in this study. The pressure-test experimental system is established to analyze the propagation laws of shock waves and the morphological changes of bubble pulsation. The results show that the peak pressure attenuation of the shock wave gradually slows down as the propagation distance increases. Owing to the blockage of water, a high-pressure zone is formed near the tube exit (HPZ-NE). The shock wave continues to propagate downstream after separating from the HPZ-NE, with a high-pressure zone behind the wave (HPZ-BW). The HPZ-NE has a longer duration (approximately 3.7 ms) and higher initial pressure than the HPZ-BW, and its position does not move with the shock wave. After propagation of the shock wave, the gas jet in the detonation tube continuously penetrates the water, leading to the formation of a bubble. The bubble develops in an approximate spherical shape during the first pulsation period, which is divided into four stages: rapid expansion, slow expansion, slow contraction, and rapid contraction. The expansion and contraction processes account for half of the pulsation duration, respectively. The inertia and the pressure difference alternately become the main factors of morphological changes. When the bubble contracts to the minimum volume, a new shock wave is formed by the centroid. The bubble experiences several pulsation periods in the subsequent development, accompanied by a process whereby the centroid gradually moves away from the tube exit. • The pressure evolution in the process of an underwater-detonation gas jet is experimentally researched. • The propagation laws of shock waves and the morphological changes of bubble pulsation are analyzed experimentally. • The laws of shock wave propagation and interaction are researched, and the evolution mechanism of the bubble is revealed. [ABSTRACT FROM AUTHOR]

• The three pulsations of the gas bubble of the underwater detonation tube are captured by high speed photography. • The frequency spectrum and energy distribution of the signals in water and in the air are obtained by wavelet transform and fast Fourier transform. • The transmission characteristics of the pulsating signal and the shock wave degenerated by the detonation wave on the water–air interface have been preliminarily analyzed, which proves that the low-frequency pulsating signal has a stronger transmission ability. 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. [ABSTRACT FROM AUTHOR]