Shock Waves最新文献

The addition of materials such as polysulfone as a modifier to epoxy binders has the potential to significantly improve the crack resistance of the material. In this work, the effect of the polyphenylene sulfone additive on the epoxy resin strength has been studied experimentally. Epoxy samples were cured at 160 and (200,^{circ }hbox {C}) temperature. Under static conditions, the addition of 5% polyphenylene sulfone to epoxy cured at (200,^{circ }hbox {C}) leads to strength increase. Under dynamic loading, the spall strength decreased with increased polyphenylene sulfone addition in all epoxy resin samples. When the incoming pulse pressure is low, the spall strength of the samples cured at both temperatures is similar. The curing temperature effect begins to manifest itself at higher incoming pulse pressures in samples with 3–5% of polyphenylene sulfone. Curing temperature affects the characteristics of the spall fracture kinetics as well.

The study of explosions at the laboratory scale is difficult and limited due to the pyrotechnic safety and regulation. The laser-induced breakdown is a good alternative to overcome these constraints, but is inherently limited to very small scales (wave propagation distance of the order of a few centimetres). One of the key issues at such small scales is pressure measurement, which is one of the main experimental outputs at larger scales. Polyvinylidene fluoride (PVDF) pressure sensors offer suitable characteristics to address this problem. In this project, we performed laser breakdown experiments in air, using a pulsed, 1064-nm, Nd:YAG laser to induce a blast wave propagation. The laser delivers a maximum energy of 2.3 J over 7.5 ns. The beam was focused on to a millimetric spot to ensure a favourable power density leading to breakdown. The pressure field was measured with a face-on PVDF pressure gauge at various distances from the source. The signal was well fitted by a Friedlander-like profile. High-speed shadowgraph was used to image the shock wave propagation. By matching the laser breakdown energy to a TNT equivalent for chemical explosions, we evidenced a quantitative agreement with free-field TNT explosion results and showed that the classical laws for such explosions are relevant to describe the evolution of all the characteristics of the pressure wave (maximum overpressure, time of arrival, positive impulse, and positive phase duration). Those features confirm that face-on PVDF pressure gauges are well suited to measure blast effects at the laboratory scale.

This paper describes the generation and control of underwater microshock waves and microbubbles by a femtosecond pulse laser for regenerative medicine applications. To achieve local stimulation of individual cells in this field, it is essential to generate and control microshock waves at the same scale as cells. Consequently, the use of femtosecond pulse lasers has been suggested by researchers due to their noninvasive nature when generating microshock waves. However, the characteristics and control methods of microshock waves and microbubbles have not been sufficiently investigated. In this research, the laser-induced microshock waves were generated by a femtosecond laser with a pulse duration of 260 fs and a pulse energy of (2.1,upmu hbox {J}). First, pressure measurements of the shock waves were carried out, and their overpressure was found to exceed 0.3 MPa at a distance of (300,upmu hbox {m}) from the laser focal area. Second, the generation and behavior of microbubbles were successfully observed by optical measurements. A single bubble was generated when the femtosecond pulse laser was focused into water, and it subsequently expanded and contracted according to the Rayleigh–Plesset equation. In addition, its initial behavior was observed, and a comparison between optical measurements and high-speed images revealed that the shock waves were generated 200 ns after the laser has focused.

This study investigates the transient phase of the morphing shock control bump (SCB) over a flat plate using various velocities and accelerations. Specifically, five morphing profiles are tested, namely linear, parabolic, half-parabolic, reversed parabolic, and half-reversed parabolic morphing. The objective of this research is to numerically determine the optimal velocity profile, out of the tested ones, that reduces entropy losses, lag effect, and response time while presenting a dynamic shock system map. The simulations were conducted to solve the 2D supersonic unsteady flow with different free-stream Mach numbers ((M_{infty })). The Reynolds number (textrm{Re}_{infty }=6.6 times 10^7) based on the bump’s length is used. The investigation is achieved by comparing the lag effect, entropy losses, and time response. The study results indicate that the optimal speed to morph with is the one that results in neither a remarkable lag effect in the shock system nor high losses in the entropy deviation from the stationary steady-state case. Additionally, the reversed parabolic motion is the most suitable profile due to its short response time, small lag effect, and low losses. This is because the generated shock system from the appearance of SCB is initially weak, allowing for relatively fast motion. However, near the end of the morphing process, the opposite occurs, requiring relatively slow motion.

The interaction between a shock wave and an object causes an impulsive force on the object that lasts for a very short time on the order of milliseconds. This study proposes impulsive force measurements using anodized aluminum pressure-sensitive paint (AA-PSP), which is an optical pressure measurement technique. The response time of AA-PSP is on the order of microseconds, indicating the potential for fast pressure drag measurements by integrating the pressure distribution on the surface. In this study, at first, the response time of the fabricated AA-PSP is measured and determined to be (3.3~upmu hbox {s}). A cylindrical model coated with the AA-PSP is installed at the outlet of a shock tube, and the surface pressure is measured in order to calculate the pressure drag with a time resolution of (10~upmu hbox {s}). The experimental results are compared with an Euler CFD simulation computed using an in-house code. The maximum pressure drag in the numerical result exceeds that in the experimental result; however, the maximum pressure drag timings are in good quantitative agreement. In addition to measuring the impulsive force, the AA-PSP results allow for the visualization of complex flow phenomena, such as diffraction and decay in the shock wave strength. This simultaneous measurement of the flow field and impulsive forces can deepen the understanding of the relationship between them.

The supersonic impact of cylindrical reactive metal (magnesium, aluminum, titanium, or zirconium) projectiles with an inert aluminum oxide target is investigated experimentally with impact velocities ranging from 1.1 to 1.3 km/s. The focus of the present investigation is on elucidating the key processes that occur during the first 100 µs after impact and particularly on the blast wave that emerges shortly after impact. The peak overpressure of the blast wave is influenced by the jet of metal fragments that emerges from the interface between the projectile and the target through the fragment-flow hydrodynamic interactions, as well as by the prompt chemical energy release during oxidation of the fine metal fragments. The motion of the blast waves is tracked using high-speed imaging, and the shock wave overpressure is determined using the Rankine–Hugoniot equations. An energy-scaling method is used to infer an estimate of the kinetic and chemical energies that are released on a sufficiently rapid timescale to support the motion of the impact blast wave. It is found that only a small fraction (less than 0.5%) of the projectile chemical energy is released sufficiently promptly to enhance the peak blast wave overpressure. The energy release mechanism and blast wave enhancement are influenced by the type of reactive metal used. Furthermore, increasing the oxygen concentration increases the peak blast wave overpressure, suggesting that the use of a composite reactive material projectile comprised of both reactive metal and an oxidizer may be effective at enhancing the amount of energy release during the early-impact stage.

In case of an explosion event, secondary risks arise from component debris ejected from affected structures. In particular, concrete and masonry wall elements pose a potential hazard due to their high fragmentation when exposed to blast load. Adequate structural design and blast mitigation measures are crucial to ensure the integrity and robustness of the structure. A polyurea-based coating can increase the strength of the load-bearing structure and reduce the amount and velocity of debris. The general effectiveness of this approach has already been widely demonstrated. However, comprehensive research for this specific application in terms of blast loading is lacking. The paper investigates the factors that influence the behavior of polyurea-coated concrete walls with both experimental and numerical approaches. Experiments are performed with a shock tube representing a far-field detonation scenario. Additionally, a numerical model is implemented in LS-DYNA considering the dynamic, strain rate-dependent material properties of reinforced concrete and polyurea as well as the adhesive bond of the polyurea–concrete interface. The test results are used to validate the adopted simulation model. By comparing the results to uncoated concrete wall panels for both experiment and simulation, the blast mitigation effect of polyurea coating for concrete wall panels is assessed: the coating proved to be effective in preventing the generation of high-speed ejecta and also in limiting, to some extent, the crack propagation in the concrete panel. The results set a ground for further development of recommendations or guidelines for structural protection using polyurea coating.

The paper presents the experimental proof of shock wave amplification and shock-to-detonation transition (SDT) in a two-phase mixture of liquid triethylaluminum (TEA, (hbox {Al}(hbox {C}_{2}hbox {H}_{5})_{3}))—a pyrophoric material reacting with water—and superheated steam in a shock tube. Fine synchronization of TEA injection in the flow of superheated steam with the arrival of a decaying shock wave is shown to change the shock wave dynamics from attenuation to amplification followed by propagation with a nearly constant velocity of 1500–1700 m/s (at a small dose of TEA injection) and 2000–2300 m/s (at a large dose of TEA injection) in a tube during a certain time interval. These speed levels are consistent with the thermodynamic calculations for the detonation speed in the fuel-lean and near-stoichiometric TEA–superheated steam mixtures, respectively. With the small dose of TEA injection, the pressure profiles recorded in the experiments do not generally correspond to the pressure profiles relevant to detonation waves, whereas with the large dose of TEA injection, the pressure profiles resemble those for detonation waves.

Shock propagation at microscales has been an area of utmost interest in recent years due to the recent developments in the fields of micro-electro-mechanical systems (MEMS) and medical science. In the present investigation, post-shock boundary layer flow is numerically examined for shock wave propagation in micro-ducts of 1000 (upmu )m (times ) 150 (upmu )m, 1000 (upmu )m (times ) 300 (upmu )m, and 1000 (upmu )m (times ) 400 (upmu )m cross sections at incident shock Mach numbers ranging from 1.97 to 2.31, similar to the experimental investigations of Giordano et al. (Shock Waves 28:1251–1262, 2018). The shock is introduced using the stagnation properties corresponding to the Mach number of shock-induced flow. The shock position and the shock wave attenuation parameter are compared with the experimental findings of Giordano et al. Numerical results suggest the existence of a turbulent boundary layer behind the shock wave similar to the experimental findings.

In this study, we focus on the early stages of the Hartmann–Sprenger tube operation in the jet regurgitant mode to investigate three key relationships: (1) between the nozzle dynamic pressure and the start time of temperature rise at the resonance tube end, (2) between the nozzle dynamic pressure and the resonance tube end pressure, and (3) between the distance l from the resonance tube end to the contact surface and the rise in the end gas temperature of the resonance tube. The results showed that the condition for the rise in the resonance tube end gas temperature after the gas jet from the nozzle reached sonic flow was the stabilization of the average fluctuation pressure value at the resonance tube end in conjunction with the nozzle dynamic pressure. At the time of operation stabilization of the Hartmann–Sprenger tube, the average fluctuation pressure at the resonance tube end converged to a constant value almost equal to the nozzle dynamic pressure. This result means that the nozzle dynamic pressure is almost equivalent to the fluctuation pressure at the resonance tube end. Moreover, the distance l did not significantly increase with the resonance tube length L and remained nearly constant. The gas temperature at the resonance tube end obtained from the experimental results generally agreed with the temperature calculated from compression through an adiabatic and isentropic process from L to l.