Shock Waves最新文献

Experiments were conducted to investigate detonation propagation in a curved tube filled with stoichiometric 2H(_{2}+)O(_{2}+)7Ar and CH(_{4}+)2O(_{2}). The test section of the experimental setup was a semicircular channel with an internal radius of 500 mm. Detonation velocities were calculated based on the arrival time of the wave front, monitored by pressure transducers. The detonation cellular evolution was recorded using smoked foils. The results revealed that after crossing the obstacle, the detonation wave failed and promptly re-initiated. It then decayed from an overdriven detonation to a steady-state detonation. The detonation development processes were divided into five regimes. The formation of the boundary behind the obstacle and the generation mechanism of the overdriven detonation were thoroughly analyzed. The formation of the boundary behind the obstacle is associated with the curved shock front and the non-uniform cellular structure. The re-initiation distance for an unstable mixture in a curved tube was significantly shorter than that in a straight channel. In the absence of the obstacle, the cell width decreased radially outward, a linear relationship was determined. The speed of the detonation wave initially decreased and then gradually increased.

This paper focuses on the blast resistance performance and protection mechanism of a polyurea-sprayed vehicle armor composite structure under blast impact. We study the blast resistance performance of a steel plate with composite structures in four different spray configurations, which depend on whether spraying occurs and the spray position on the steel plate. First, near-field airburst tests are conducted for four different sprayed types of composite structures for a 2-kg TNT equivalent, and then, the test conditions are simulated using LS-DYNA software. Based on the verification of the accuracy of the calculation model, the dynamic response of the back-sprayed structure at different standoff distances is compared and analyzed. The test and simulation results reveal that compared with other spraying configurations, the back-sprayed structure has better blast resistance, and the low impedance ratio of the front-sprayed structure is the major cause of the aggravation of the structure damage. With the decrease in the standoff distance, the deformation range and flatness factor of the structure are constantly reduced, and the damage mode and protection mechanism of the composite structure keep changing. The blast resistance performance of polyurea is mainly based on the energy absorption and storage during the tensile phase and the energy release and dissipation during the rebound phase. For the back-sprayed structure, the steel plate is always the main energy-absorbing structure. In a certain load range, the energy absorption ratio of polyurea is proportional to the strength of the blast load. Additionally, when the load strength exceeds the tolerance limit of the surface steel plate, the blast resistance of polyurea cannot be effectively exerted. In such a case, the damage modes of steel plate and polyurea tend to be similar.

The results of an experimental and kinetic modeling study of the ignition of (hbox {H}_{{2}}{-}hbox {CO}{-}hbox {O}_{{2}}{-}hbox {Ar}) mixtures behind the reflected shock wave are reported. The experiments were performed with test mixtures containing (0.75{-}3.0{%},hbox {H}_{{2}}), (0{%}{-}3.0{%},hbox {CO}), and (1.5{%},hbox {O}_{{2}}) in argon at temperatures from 950 to 1650 K and a total gas concentration of ({sim }10^{{-5}}~ hbox {mol}/hbox {cm}^{{3}}). The reaction was monitored by recording the time evolution of the pressure behind the reflected shock wave, intensity of the chemiluminescence of electronically excited OH* radicals at 308.0 ± 2.0 nm, and the absorption by ground-state OH radicals at a 306.772-nm bismuth atomic line. The measured parameters were the time (uptau _{{1}}) it took to reach a ground-state OH concentration of (2.0 times 10^{{-9}}~hbox {mol}/hbox {cm}^{{3}}) and the time (uptau _{{2}}) to reach the maximum OH* emission intensity. Kinetic simulations demonstrated that (uptau _{{1}}) corresponds to the beginning of fuel consumption, and (uptau _{{2}}) to the time for most of the fuel to be consumed. Therefore, the process of ignition was treated as consisting of two stages: the induction period (uptau _{{1}}) and the burnout time (uptau _{{2}}-uptau _{{1}}). These two time intervals demonstrate different sensitivity to the elementary reactions of the kinetic mechanism. A numerical model capable of predicting the effects of the presence of hydrocarbon impurity, oxygen vibrational relaxation, and pressure rise was used to simulate the experiment. The best agreement between experimental and theoretical results is achieved when these additional factors are taken into account. In addition to the sensitivity coefficient analysis for identifying the most important reactions, a new criterion, referred to as the relative integrated production, was proposed, which compliments the sensitivity coefficient analysis through its ability to identify the most productive reactions.

The factors that influence the symmetry of an imploding detonation are investigated experimentally and theoretically. Detonations in sub-atmospheric acetylene–oxygen were initiated and made to converge in an apparatus that followed that of Lee and Lee (Phys Fluids 8:2148–2152, 1965). The width of the test section was controlled with a wave-shaping insert, which formed the test section against the viewing window, creating an effectively two-dimensional problem with a channel width comparable to the detonation cell size. The convergence of the detonation was observed via self-luminous open-shutter photography and high-speed videography. The resulting videos were analyzed to quantify the wave speed, degree of asymmetry, and direction and magnitude of the offset in the center of convergence. To determine the experimental parameters that influence the symmetry of the imploding wave, the wave-shaping insert was intentionally canted by (0.3 ^{circ } {text {--}} 0.6^{circ }), accentuating the asymmetry of the imploding detonation. The experiment was modeled using a Huygens construction wherein the detonation is treated as a collection of wavelets, each assumed to propagate locally at a velocity determined by the channel width. The results of the model reproduced the observed offsets in detonation convergence from the center of the apparatus, confirming that velocity deficits resulting from the narrow channel width control the observed asymmetry.

High-speed imaging and digital signal processing are used to measure temperatures and pressures produced by explosions of solid chemical energetic materials. These measurements are used to enhance understanding of hazards faced by personnel working or training near explosions. The techniques described provide a complement to point measurements. Peak incident pressures studied are between 21 and 138 kPa, a region important for injury studies of personnel exposed to airborne shock.

Rotating detonation combustor (RDC) is a form of pressure gain combustion, which is thermodynamically more efficient than the traditional constant-pressure combustors. In most RDCs, the fuel–air mixture is not perfectly premixed and results in inhomogeneous mixing within the domain. Due to discrete fuel injection locations, local pockets of rich and lean mixtures are formed in the refill region. The objective of the present work is to gain an understanding of the effects of reactant mixture inhomogeneity on detonation wave structure, wave velocity, and pressure profile. To study the effect of mixture inhomogeneity, probability density functions of fuel mass fractions are generated with varying standard deviations. These distributions of fuel mass fractions are incorporated in 2D reacting simulations as a spatially/temporally varying inlet boundary condition. Using this methodology, the effect of mixture inhomogeneity is independently investigated to determine the effects on detonation wave propagation and RDC performance. As mixture inhomogeneity is increased, detonation wave speed, detonation efficiency, and potential for pressure gain all decrease, ultimately leading to the separation of the reaction zone from the shock wave.

Explosive breaching is a tactic operational professionals use to gain rapid entry and tactical advantage. This tactic exposes individuals to repeated low-level blasts (LLB), overpressure exposure generally occurring from user-directed munitions. The experimentation described in this paper highlights the need for further research into implementing explosives in tactical situations, specifically in confined areas, and the effects on individuals exposed. While current safety calculations predict peak pressures from an open-air detonation, this study incorporates the impulse of the total explosive event in a confined space. Sixteen explosive events were conducted to measure peak overpressures of the total duration of the event using pencil probes and flush mount-type sensors. These pressure sensors measured detonations at distances greater than or equal to the calculated minimum safe distances (MSD). The study compares these data with the Hopkinson–Cranz scaling law, the Weibull formula, and Kingery–Bulmash (KB) predictions. Additionally, a scaled mouse-to-human model for developing mild traumatic brain injury (mTBI) using pressure vs. impulse (P–I) graphs demonstrates areas of concern in the collected data. Results show that at distances exceeding the MSD, with personal protective equipment (PPE), and at pressures lower than those considered safe, mTBI is possible. Peak overpressures were measured to be 2.5 times higher than safety thresholds and impulses as high as 274 kPa ms. Confined area detonations produced 1.2–1.4 times greater pressures than open-air detonation measurements. Individuals who undergo breaching training will likely experience multiple exposures of this nature throughout their career, often occurring in rapid succession.

Application of microwave radar is a useful approach to gauge piston motion in a free-piston driver. One difficulty associated with conventional microwave technique is its spatial resolution during rapid velocity shifts at diaphragm rupture timings. This study, while departing from the standard practice of analyzing standing wave peaks, introduces an alternative by examining the phase shift of the microwave in-phase and quadrature signals. A compact free-piston-driven expansion tube, MX6.0, is used as the test bed for this technique. A microwave frequency of 4.2 GHz is used to take measurements in a compression tube with a diameter of 50 mm and a length of 2.0 m, tracking the motion of the piston. After arranging the microwave radar systems, the piston velocity and displacement trajectory are measured. Compared to the lower-resolution measurements using conventional microwave wavelength intervals, the use of microwave phase allowed for an exceptionally high spatial resolution in analyzing the piston motion.

The back pressure rise in a supersonic air intake could affect the engine performance and, in extreme conditions, result in a catastrophic unstart phenomenon. The present study compares different back pressure states that occur during an unstart of a mixed-compression air intake at Mach 3 using a fast-response pressure-sensitive paint, with an emphasis on the isolator flow. At low back pressure, the isolator dynamics is strongly correlated with the unsteadiness around the external compression corner. At high back pressure, a normal shock train dictates the isolator flowfield from its leading shock foot downstream. At the onset of unstart, an oblique shock train transpires involving large-scale flow separation, boundary layer thickening, and mitigated unsteadiness at the isolator floor. Like in previous studies, the prominence of low-frequency unsteadiness and upstream wave propagation is observed at high back pressure. However, in addition, the present study shows strong upstream communication of back pressure in a narrow frequency band through acoustic mechanisms, that eventually leads to the intake unstart. At the onset of unstart, the prominent frequency varies linearly along the isolator length, matching closely with the half-wave resonator model. Suppressing the oscillations at the preferred frequencies could be a promising control strategy to mitigate or delay intake unstart. When the intake unstarts, a 3D bifurcated shock stands at the inlet and the unsteady flow spillage takes place around oblique shocks off the sidewalls at low frequencies.