Shock Waves最新文献

Viscoelastic materials have extensive military applications due to their energy absorption capabilities, with the potential to reduce blast energy imposed on buildings, vehicles, and personnel. Based on current literature, limited information is available regarding the mitigation of blast energy related to these uses. The impact of thickness, nanoparticle addition, and layering variation was assessed in this study using commercially available viscoelastic materials in open-air blasts of Composition C4 to determine shock energy mitigation capabilities. Time-pressure waveforms were recorded to identify optimal changes in shock wave characteristics: reduced peak pressure, positive phase duration, and impulse, with increased rise times. Results were analyzed through trend and linear regression analysis to evaluate factors possibly influencing the behavior of the materials. Polyurethane-based materials reduced peak pressures by extending the positive phase duration, whereas silicone rubber maintained a similar duration with reduced peak pressures, suggesting differing energy dissipation mechanisms. Polyurethane was more effective due to its pressure reduction regardless of thickness, enabling thinner layers to be used to achieve similar results. Overall, thinner layers were more efficient, as diminished returns were evident by asymptotic points once reaching a 7-mm thickness. Incorporating graphene nanoplatelets increased energy transfer with peak pressure increases up to 16% in the polyurethane-based samples and impulse increases of 7.5% in the silicone rubber-based samples, making the baseline samples more effective. Layers alternating in material type reduced peak pressures up to 16% relative to baseline samples, with the most reduction occurring in the thicker layers. The alternate layering patterns proved pivotal in the results, those starting with silicone rubber being correlated to increases of 21% in positive phase duration and 6.5% in decay time.

This paper provides a comprehensive discussion on the flow characterization and design of the Mach 5 shock tunnel facility at the Hypersonic Research Center of New Mexico State University (NMSU). It reviews the operational principles of low-enthalpy shock tunnels as well as the measurement techniques employed for the facility characterization. The material and thickness of the secondary diaphragm are shown to significantly affect the stability of stagnation properties. Stagnation conditions are determined through an analysis of pressure–time history data measured in the driven tube. Schlieren flow visualizations over a 10(^circ ) half-angle straight cone and a sphere are used to estimate the freestream Mach number. Additionally, femtosecond laser electronic excitation tagging (FLEET) velocimetry is conducted to measure instantaneous velocities in the freestream and turbulent boundary layer flows within the test section. The shock tunnel has a total temperature ranging between 610 and 630 K, with a freestream Mach number of 5.1. The steady test time, as indicated by pitot pressure measurements, ranges from 2 to 2.5 ms, while velocimetry and wall-static pressure data suggest that driver gas arrival in the test section occurs approximately 30 ms after flow stabilization. The facility was made available for use in undergraduate courses in Fall 2022.

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.