Shock Waves最新文献

On the battlefield, soldiers may be injured by the detonation of explosive charges, particularly to the head. Several injury mechanisms have been proposed in the literature to explain the observed brain lesions, and numerous experiments, such as post-mortem studies, have been carried out to investigate those hypotheses. In the current study, a new skull substitute has been developed and its mechanical behaviour has been evaluated under free-field blast conditions. Instrumentation included accelerometers, strain gauges, and pressure sensors mounted in multiple positions. The substitute was embedded on a rigid mount and subjected to the detonation of 21 explosive charges. The proposed experimental campaign included seven free-field scenarios with incident pressures ranging from 75 to 200 kPa and two blast durations of 1.2 ms and 2.0 ms. The time analysis of signals revealed an “ipsi-contralateral” effect for internal pressures, i.e., an overpressure at the ipsilateral site concomitant to a depressure at the contralateral site. A slight underestimation of the first peaks versus incident pressures was also observed when compared with the literature on post-mortem human subjects. For shell strains, an overestimation of the maximum values and the first peak values versus incident pressures was attested compared to the bibliographic data. According to the current findings, the newly designed skull substitute produces results in line with post-mortem human subjects in terms of first peak strain and peak internal pressure trends. The described methodology could be applied to develop new head substitutes and, in the future, investigate injury mechanisms.

This study is motivated by concerns for the safety of goods and people, as well as the need to provide individuals with the means to prevent and protect against accidental risks and terrorist threats. The research is one of the tasks in the ANR research project (mathrm{URB(EX)}^{{3}}), which aims at developing a fast-running, breakthrough model for blast consequences in urban configurations. The objective is to characterize the propagation of a shock wave along a straight street using experimental and numerical approaches. The shock wave results from the detonation of a gaseous explosive charge. The experiments are carried out at laboratory scale by applying the laws of similarity. Shock waves are studied using pressure profiles recorded by regularly distributed pressure sensors. Visualization is also used to illustrate various shock wave interactions between the two walls. The explosive charge is placed on the central axis of a street between two parallel walls. The shock wave propagation is analysed in terms of street width and height. It is demonstrated that the shock wave changes its propagation mode from 3D to 2D. It is also shown that several planar shock waves are correlated with the junction of two Mach stems. This study reveals secondary shock waves, as well as multiple shock waves, which can lead to a certain complexity in interpreting the measured pressure signals. The 3D to 2D mode transition zone is determined for each configuration, and an empirical law is established based on the different experimental results obtained. The law considers three parameters, namely the diameter of the explosive charge and the dimensions of the street (height and width).

It is important to understand the effects of explosions to ensure the safety of civilians and military personnel. Rapid City Planner (RCP) is a comprehensive software tool for predicting the effects of conventional and improvised explosive devices with geographic information system-based outcomes for munition safety, building/structure damage, protection of assets, and human vulnerability in real cities. Modelling cased explosives requires the consideration of casing fragmentation, which is modelled in RCP using three different methods having a different level of detail. The present study focuses on introducing and validating one of the methods, namely the fast primary fragmentation method. The fast fragmentation solver is used to simulate casing breakup of steel-cased cylindrical charges with various TNT- and RDX-based explosives. The predicted velocities were within less than one percent of theoretical Gurney velocities, and the fragment size distributions compared well with experimental data for each explosive. A TNT-filled artillery shell trial was used to validate the fast fragmentation solver in terms of polar distributions of initial fragment speed and number, as well as the spatial spread of fragment throw in a free-field environment. The RCP hydrocode solver is used to assess the equivalent bare charge model used by the fast fragmentation approach. The detailed and fast-modelling approaches produced similar peak overpressures, but underpredicted impulses, at standoff distances between 5 and 10 m (mid-to-far field range). The failure strain and fragment size distribution are shown to have little effect on the blast wave, whereas both would have a significant impact on subsequent fragment effects. Finally, a full-scale scenario was modelled in RCP to show blast and primary fragmentation effects in a coastal urban environment, including a demonstration of urban blast effects from blast pressure and primary fragment trajectories outcomes.

To prevent flow separation under overexpanded conditions in traditional large-area-ratio nozzles of rocket engines at sea level, the method of characteristics for wall pressure control is adopted. This method, which is based on thrust-optimized contours, can be implemented to redesign the latter half of a divergent contour to ensure that the wall pressure of the new contour is not less than the critical separation pressure of 0.03 MPa. The newly generated nozzle is named the full-flow nozzle. Then, the design method is verified by simulations, and the performance of full-flow nozzles is evaluated. The results show that the method of wall pressure control can achieve the intended purpose, and the newly generated contour ensures that the nozzle is not only in the full-flow state at sea level but also able to withstand combustion chamber or ambient pressure fluctuations. The combustion chamber pressure is 8.5 MPa, and the specific heat ratio of hot gas is 1.144. Compared with the thrust-optimized contour with an area ratio of 40, in which the flow tends to separate at sea level, the full-flow nozzle can increase the area ratio to 60. Thus, the vacuum specific impulse can be increased by approximately 5.24 s. Compared with the thrust-optimized contour nozzle with an area ratio of 60, the vacuum specific impulse of the full-flow nozzle with an equal area ratio is decreased by 1.57 s.

Detonation diffraction leads to either successful transmission of the detonation or quenching wherein the propagation mechanism is attenuated. The transmission behavior is governed by competing effects of energy release, curvature, and unsteadiness. There is a potentially unique critical diameter that will determine the diffraction outcome for every combustible mixture composition at each set of initial conditions. The critical diffraction diameter has been correlated to several detonation parameters to date; however, these correlations all have limitations. Analytical or quasi-analytical solutions to the diffraction problem, specifically those able to predict the critical diameter, are scarce. The present work develops several critical diameter models by uniting previous work on diffraction phenomena and the critical initiation energy problem. Curvature, decay rate, and energy-based models are established, and their critical diameter predictions are compared against a wide range of experimental critical diameter data. While detonation diffraction is a complex multifaceted phenomenon, a curvature-based one-dimensional model in this work is shown to accurately reproduce empirical critical diameter behavior at relatively low computational cost.

In recent years, terrorist attacks and accidental explosions in urban environments have occurred frequently, causing severe damage, even collapse, of building structures, and have become a major concern of modern society. The need to design and evaluate the blast resistance of building structures is rising markedly. The utmost requirement is the determination of blast loads acting on building structures, i.e., the reflected overpressure of blast waves. To better keep the balance between computational efficiency and prediction accuracy of complex blast wave propagation and its interactions with buildings, a practical numerical simulation approach integrating multiple existing techniques including the multi-stage method, graded mesh, mapping, and un-refinement technique is proposed based on ANSYS/AUTODYN. Firstly, the propagation of blast waves is simplified into three stages, i.e., propagation in the free air from the explosion center to ground zero, propagation after the ground reflection, and interaction with building structures. These three stages are modeled by 1D uniform meshes and 2D/3D graded meshes with increasing mesh sizes. Then, the mapping technique, including mesh un-refinement, is adopted to transfer the predicted results at the previous stage into the next stage. The corresponding meshing strategy against the scaled distances Z ((Z = R / root 3 of {W}), where R is the distance between the detonation point and the target surface, W is the equivalent charge weight of TNT) for each stage is recommended through mesh sensitivity analyses. Finally, the proposed approach and mesh sizes are validated against four series of explosive tests for a single house, an intersection, and two city blocks by comparing with both the overpressures and impulses of blast waves. Additionally, two solvers, i.e., Euler FCT and Euler multi-material, are compared. The former solver is recommended due to its greater efficiency and accuracy. The present work could provide a helpful reference for the blast-resistant design and evaluation of urban building structures.

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.