Shock Waves最新文献

In the current practice of blast-resistant design, blast loads are determined by the Kingery and Bulmash charts in accordance with a database of free-air blasts of spherical charges and surface bursts of hemispherical charges initiated at the center. However, most charges are closer to cylinders in geometry. In addition, charge shapes and initiation configurations significantly affect blast loads under a near-field blast scenario. Therefore, it is imperative to develop a near-field blast loading model for cylindrical charges that can account for the effects of both charge shape and initiation configuration in blast-resistant design. Compared with incident blast loads, reflected blast loads are more relevant because the latter can be directly used for blast-resistant design. Accordingly, in this paper, experimental and numerical studies were performed to develop a near-field blast loading model for cylindrical charges in terms of the peak reflected overpressure and the maximum reflected impulse. Two series of tests were conducted with either one-end-initiated or both-end-initiated cylindrical charges to obtain reflected blast loads with different scaled distances. It was found that the spatial distribution of blast loads along the axial direction of the charges was extremely non-uniform. Then, high-efficiency numerical models were built using 2D to 3D mapping techniques. After being validated against experimental results, numerical models were employed to simulate the blast loads generated by cylindrical charges with different length-to-diameter ratios and initiation configurations (one-end, center, and both-end initiations) with scaled distances ranging from 0.2 to 1.0 m/kg(^{mathrm {1/3}}). To develop the blast loading model, the peak reflected overpressure and the maximum reflected impulse at the center of a rigid reflection surface were firstly determined by curve fitting as the benchmark blast loads, which were expressed as functions of scaled distance and length-to-diameter ratio, and then the benchmark blast loads were used to normalize the blast loads at different locations. Accordingly, the spatial distribution of blast loads can be described with the benchmark blast loads and a spatial load distribution function, in which the latter is determined by surface fitting of extensive numerical results. The results indicate that the blast loading model developed is able to predict the blast load with considerable accuracy.

Soil-filled gabion systems can be used in many civil applications such as retaining walls against flooding and erosion or shoreline protection. In addition, the gabion systems provide good resistance in high dynamic loading scenarios such as blast events. These systems allow for a modular setup of easy-to-use perimeter walls with variable height and cross section, application as a gravity wall, and use of local filling material. The latter is the subject of the present paper. Depending on aggregate size and morphology, size distribution, and humidity, soil materials exhibit different material properties such as compaction parameters, cohesion, and the angle of friction among others. Each of these parameters directly affects the structure’s response under highly dynamic conditions. To understand the influence of varying soil parameters at varying loading conditions and thus to predict the structure’s behavior precisely, the authors investigated soil-filled perimeter walls experimentally and using hydrocode simulations. Since the soil’s properties primarily influence the wall’s behavior—at the resistance side—an extensive laboratory test campaign was required to characterize different soils. The experimental data serve for the derivation of dynamic material models and are complemented by numerical simulations. Furthermore, this paper describes the execution of near-field detonation and shock tube tests of soil-filled perimeter walls to analyze their load-bearing behavior under blast load. The experiments are evaluated with regard to the failure mechanism as well as the blast mitigation. Additionally, the blast mitigation effect is numerically investigated and the results are compared to the experiments.

Large-eddy simulations are conducted to investigate supersonic flow over a compression ramp at a free stream Mach number of 4.0 and a unit Reynolds number of (4.56 times 10^{6}) per meter. Two ramp angles of (15^circ ) and (18^circ ) are considered along with three different ramp positions (P1, P2, and P3) from the plate leading edge, with the plate length increasing progressively from P1 to P3. Simulations reveal that with an increase in the plate length and ramp angle, the separation point shifts downstream, accompanied by an extended separation length. Furthermore, with an increase in the ramp angle and plate length, a higher Görtler number is observed upstream of the reattachment indicating a greater likelihood of Görtler instability. In particular, no streamwise vortices were observed for the 15P1 and 15P2 cases, while for the 18P3 case, increased instability resulted in the breakdown of streamwise vortices, driving the transition to turbulence. The wavelength of streamwise streaks decreased by approximately (15%) as the plate length increased by (approx 80%) from 18P1 to 18P3. Unsteady analysis revealed the role of spanwise secondary instabilities over these vortices, that trigger turbulent spots that propagate at a speed of (approx 0.6 U_{infty }). The peak value of the Stanton number is found to be (approx ) 15–27% higher than the time and span-averaged value for the 15P3 and 18P3 cases, highlighting a strong effect of downwash due to streamwise vortices on the wall heating rate distribution. The unsteady data also reveal a negative correlation between the flow reattachment location and the Stanton number close to the reattachment point. An earlier reattachment is shown to increase the Stanton number and vice versa resulting in a (approx 40%) variation compared to the time-averaged value. The results from this study underscore the critical influence of plate length on the formation of streamwise vortices, with significant implications for wall heating rate distribution and flow transition dynamics.

Molecular-dynamics-based simulations have been carried out for crystalline Al-(text {Al}_{90}text {Sm}_{10}) metallic glass (MG) nanolaminates with different grain structures corresponding to varying values of shock intensities to analyze the structural evolution during shock-wave loading and spallation behavior of the nanolaminates. A transition from elastic–plastic behavior occurs in nanocrystalline NC-MG nanolaminates with increasing values of shock intensities when the shock traverses from the crystalline end to the MG end. On the other hand, an overdriven elastic front is observed for all values of shock intensities in columnar-grained CG-MG nanolaminates. When the shock-wave direction is reversed, a plastic wave dominates the shock profiles irrespective of the grain structures and shock intensity values. Adaptive common neighbor analysis (a-CNA) and dislocation analysis reveal that grain boundary-mediated plasticity is dominant in NC-MG nanolaminate specimens, while dislocation-mediated plasticity predominately governs the shock deformation behavior in CG-MG nanolaminates. The reflection of the rarefaction wave generated at the crystalline–amorphous interface aids in stacking fault generation in NC-MG nanolaminates but does not cause any structural changes in CG-MG nanolaminates. The spallation behavior of the nanolaminate specimens is significantly influenced by the grain structures and the presence of the free surfaces. The population of perfect icosahedral clusters (langle 0,0,12,0rangle ) decreases during the passage of shock as determined using Voronoi cluster analysis.

In this study, we investigate the role of a diffuse interface on the Richtmyer–Meshkov (RM) instability by performing two-dimensional simulations of a single-mode perturbation (wavelength (lambda )) imposed on a diffuse interface (thickness (delta )) between air and (hbox {SF}_6). By varying the ratio (0.1le delta /lambda le 0.5), we examine the effect of the interfacial diffusion thickness on the baroclinic vorticity, perturbation growth, and fluid entrainment. The initial circulation is conserved with respect to (delta ), causing a reduction of the initial vorticity magnitude, thus resulting in a reduction of perturbation growth. In the linear regime, the diffusion layer delays perturbation growth, but in the nonlinear regime, the growth becomes insensitive to the initial diffusion thickness, as shown by our power-law scaling accounting for the redistribution of vorticity along the interface. The initial diffusion thickness increases the overall volume of the roll-up, but decreases its surface area. Introducing a new metric (the inter-fluid distance, d) reveals that initially thicker interfaces increase material separation and reduce strain rates within the roll-up structures, resulting in longer diffusion length scales. These structures undergo a gradual thinning over time, causing the inter-fluid distance to decrease to scales comparable to the strain-dominated diffusion length. Therefore, while the strain rate dominates the vortex-core evolution early on, the effect of diffusion may become important at later times, with this transition delayed for thicker initial interfaces.

The interface between explosives and steel plates can vary in the clearance of a gap or the presence of a cushion, and the dimension of the interface region can also differ. These variations in the types of interface may affect the dynamic loading and energy absorption of steel plates driven by detonation. To investigate this issue, we conducted a numerical study on the influence of different interface types and thicknesses. Initially, we designed a simulation model of a detonation driving a steel plate, with one half featuring clearance between the explosive and the steel, and the other half filled with a cushion. We then carried out simulations to analyze the influence of varying clearance and cushion thickness on the dynamic loading and energy absorption of the steel plate. The results indicate that a small-thickness clearance between explosive and steel can increase the kinetic energy of the steel plate, and there may be an optimal clearance thickness to maximize the energy absorption of the steel plate. As the clearance thickness is increased, the first loading pressure in the steel decreases, and the spallation and recompression processes in the steel gradually transform into an approximately uniform loading process without fracture. On the other hand, filling the clearance with a cushion has a negative effect on the energy absorption of the steel plate, and the kinetic energy of the steel plate decreases nearly linearly with an increase of the cushion thickness. As the cushion thickness is increased, the first loading pressure in the steel decreases less, and the dynamic behaviors of spallation and recompression may occur. Lastly, we briefly discuss interfaces with uneven thickness, which should be strictly controlled to prevent the occurrence of unexpected phenomena.

High-fidelity microphones can be used to characterize laser-generated microblast waves (“microshocks”) in tabletop experiments. This study probes both spherical and hemispherical microshocks, analogous to height-of-burst or surface-burst geometries, at distances of 1–15 cm and laser energies in the range of ~  300–630 mJ under face-on ((0^{circ })) or side-on ((90^{circ })) microphone orientations. We take a Kingery–Bulmash-style analysis approach and calculate the characteristic fitting parameters for time of arrival of the microshock. Blast waves from these laser energies cover scaled distances of ~  2–50 m/(hbox {kg}^{mathrm {1/3}}), roughly equivalent to the detonation of a few grams of TNT probed from several meters away. We compare the experimental results to BlastX simulations and tabulated data from a variety of sources. Under this experimental configuration, a 302-mJ laser pulse is equivalent to a TNT charge in the mass range 1–18 (upmu )g and the 628-mJ pulse is within the range 10–45 (upmu )g. This corresponds to a laser energy to shock coupling ratio when compared to 100% TNT equivalence of 1–24% and 7–29%, respectively. This work informs microblast scaling expectations for experiments using laser-induced shock waves as a microscale energetic characterization technique and provides connections between laboratory and free-field detonation testing.

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.