Shock Waves最新文献

In this paper, we present results from spontaneous ignition of aluminium particle clouds in a series of shock tube experiments. For all experiments, the shock propagates along a narrow pile of 40-(upmu )m aluminium particles. The study includes shock Mach numbers in the range from 1.51 to 2.38. The results are visualised using photographic techniques and pressure gauges. The combination of two Phantom high-speed video cameras and a beamsplitter allows a compact schlieren setup mounted together with a dark-film high-speed camera. While the schlieren technique allows the shock features to be identified, the dark-film camera is used to capture the ignition and burning of the aluminium particle clouds. Based on extensive image processing and shock tube relations for reflected shocks, spontaneous ignition of the aluminium particle cloud is found to take place for reflected shock gas temperatures above 635 K. For increasing Mach numbers, we find a decreasing trend for the ignition delay. Additionally, the burning time is observed to decrease with increasing Mach number, indicating that the burning process is more efficient with increasing gas temperature.

We describe the implementation of several thermo-chemical analyses in Cantera and the Shock and Detonation Toolbox (SDT), that can be employed to investigate the chemical dynamics of planar steady detonation. A MATLAB graphical user interface has also been developed to post-process the data provided by the detonation codes. These utilities will be made available on request and in the future releases of the SDT.

In this paper, we provide the experimental evidence of a free-standing conical shock and the compressive confocal characteristics region in the Busemann intake flow. The experiments are carried out in the DRDC Trisonic Wind Tunnel at freestream Mach number 3.0 with a Busemann ring model. The Taylor-McColl equations are integrated to obtain the Busemann streamline and hence the inner surface of the Busemann ring. The CFD analysis of the flow using a locally adaptive unstructured Euler finite-volume code is in agreement with the experiments.

An experimental investigation of the shock wave structure, Hugoniot states, and spall strength of a shock-compressed porous carbon fiber–epoxy composite was conducted. To generate high dynamic pressures in the material, the impact of flat-plate aluminum projectiles accelerated by explosive planar shock wave generators to velocities ranging from 0.65 to 5.05 km/s was used. Particle velocity profiles were recorded on the composite surface–water window interface with a multichannel VISAR laser interferometer. On the velocity profiles for the composite with a transverse fiber orientation, a single shock wave was recorded, while for the parallel orientation, a two-wave structure was observed. It was found that the shock wave compressibility of the porous composite did not depend on the fiber orientation relative to the direction of shock wave propagation. A kink on the Hugoniot curve was observed at the pressure of 19 GPa. The results obtained for the porous composite were compared with data for a non-porous carbon–epoxy composite and epoxy resin used as a matrix in the composites. When analyzing dynamic fracture of the porous composite under shock compression, it was found that the spall strength of the material was significantly lower than that of epoxy resin.

A launcher’s engine and in particular its nozzle are subjected to several loads during the rocket launch. Most of these loads are dynamic, such as the external pressure pulse caused by the blast wave bouncing back from the floor and engulfing the nozzle. Nevertheless, they usually are considered as quasi-static in buckling computations as conservative design methods. Few studies have been investigated on dynamic buckling of thin shells subjected to external pressure pulse. Thus, a large program including experimental tests and numerical simulations have been conducted by the CNES, the French Space Agency. The main objectives are a better understanding of dynamic buckling and establishing a robust design methodology. In this context, two experimental means used for producing dynamic pulses are here considered and investigated, to explore the dynamic buckling of such structures. In one case, the shock wave is produced using a solid explosive, in the shape of a stick in which a nitrate ammonium/sodium nitrate mix is encapsulated. In another setup, the shock wave is produced using a commercial apparatus named DaisyBell. A hydrogen/oxygen mixture is detonated within a conical shock tube, producing a directional free-air-like blast. Both apparatuses are designed to be hanged above snowpack for avalanche preventive release, thus can be held at the desired height using a crane. The pulse intensity measured at the tested sample level can be tuned by moving the explosive up or down. A simplified model of the nozzle, in the form of a cylindrical shell, is proposed for the analysis. This study aims at showing how both apparatuses can be used to simulate free-air-like blasts and can cause the dynamical buckling of a steel cylindrical shell structure.

The differences of flow characterization at the different stages of flame acceleration and transition to detonation in tubes with smooth walls, solid obstacles, and fluid jets are studied, especially the effects of flow instabilities on the process. The two-dimensional viscous unsteady reactive Navier–Stokes equations with a detailed chemistry model are solved numerically based on the structured adaptive mesh refinement technique in Adaptive Mesh Refinement Object-oriented C(++). During the ignition to a low-speed flame stage, it is found that initial pressure wave interactions with the wall and Rayleigh–Taylor instabilities, induced by the density and pressure gradient misalignment between the ignition region and unburned gas, accelerate the wrinkling and deformation of the flame surface. Consequentially, the flame wrinkles trigger Darrieus–Landau instabilities and as a result the flame accelerates. At the main acceleration stage, the Kelvin–Helmholtz instability formed in the wake of solid obstacles and the strong Kelvin–Helmholtz instability caused by the jets lead to the formation of strong turbulent structures in the flowfield and accelerate the flame propagation. Richtmyer–Meshkov instabilities caused by the interactions of reflected shock waves and the flame surface lead to flame acceleration in the case with solid obstacles. Compared to the tube with fluid jets, although the solid obstacles induce stronger Richtmyer–Meshkov instabilities, the effect of Kelvin–Helmholtz instabilities is not obvious. In general, Darrieus–Landau instabilities and Rayleigh–Taylor instabilities dominate at the initial flame-developing stage, and Kelvin–Helmholtz instabilities and Richtmyer–Meshkov instabilities play a more critical role in the flame acceleration due to interactions of the flame, the shock, solid obstacles, and vortices during the deflagration propagation stage.

A theoretical analysis is presented to elucidate the relationship between the skin friction topology of the secondary separation bubble and surface pressure structure in the fin-generated swept shock-wave/boundary-layer interaction. This theoretical method is based on the intrinsic relation between skin friction and surface pressure, and the variational method is applied to extract skin friction fields when the boundary enstrophy flux is modeled. The skin friction topology extracted from a surface pressure field in swept shock-wave/boundary-layer interaction is studied as the relevant parameters to surface pressure vary. It is found that the formation of the secondary separation bubble characterized as a topological change of skin friction is directly related to the geometrical features of the surface pressure plateau. The extracted skin friction topology of the secondary separation bubble is compared with computational fluid dynamics results and surface oil visualizations in two examples. The developed method provides a useful tool for understanding of complex flow structures in shock-wave/boundary-layer interactions particularly in pressure-sensitive paint measurements.

PTFE/Al/W granular composite is a kind of impact-initiated energetic material and may well enhance damage to the impacted targets. To gain insight into response behavior of PTFE/Al/W granular composite under different loadings, the combined approach of experiments and theoretical analyses is used in this paper. More specifically, the combinations of quasi-static compression, dynamic tests, and ballistic impact experiments are conducted. Cylindrical PTFE/Al/W granular composite specimens, with a density of 7.7 (hbox {g/cm}^{3}) and a diameter of 10 mm, are fabricated by cold press molding, sintering, and cooling. Moreover, a high-speed imaging technique is used to record response process of the specimens in ballistic impact experiments. The experimental and analytical results show that the response behavior of PTFE/Al/W granular composite is significantly influenced by the loading strain rate. When the strain rate is less than (3.6times 10^{3},hbox {s}^{-1}), only mechanical response is observed in the quasi-static compression and dynamic tests. However, when the strain rate is higher than (4times 10^{4},hbox {s}^{-1}), the chemical reaction is found in the ballistic impact experiments. Furthermore, chemical response shows an enhanced trend with increasing of the loading strain rate.

Crystals of energetic materials, such as 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX), embedded in plastic binders are the building blocks of plastic-bonded explosives (PBX). Such heterogeneous energetic materials contain microstructural features such as sharp corners, interfaces between crystal and binder, intra- and extra-granular voids, and other defects. Energy localization or “hotspots” arise during shock interaction with the microstructural heterogeneities, leading to initiation of PBXs. In this paper, high-resolution numerical simulations are performed to elucidate the mechanistic details of shock-induced initiation in a PBX; we examine four different mechanisms: (1) shock-focusing at sharp corners or edges and its dependency on the shape of the crystal and the strength of the applied shock; (2) debonding between crystal and binder interfaces; (3) collapse of voids in the binder located near an HMX crystal; and (4) the collapse of voids within HMX crystals. Insights are obtained into the relative contributions of these mechanisms to the ignition and growth of hotspots. Understanding these mechanisms of energy localization and their relative importance for hotspot formation and initiation sensitivity of PBXs will aid in the design of energetic material-driven systems with controlled sensitivity, to prevent accidental initiation and ensure reliable performance.

Launching large (> 1 g) well-characterized projectiles to velocities beyond ({10},mathrm{km,s}^{-1}) is of interest for a number of scientific fields, but is beyond the reach of current hypervelocity launcher technology. This paper reports the development of an explosively driven light-gas gun that has demonstrated the ability to launch 8-mm-diameter 0.36-g magnesium projectiles to ({10.4},,mathrm{km,s}^{-1}). The implosion-driven launcher (IDL) uses the linear implosion of a pressurized tube to shock-compress helium gas to a pressure of 5 GPa, which then expands to propel a projectile to hypervelocity. The launch cycle of the IDL is explored with the use of down-bore velocimetry experiments and a quasi-one-dimensional internal ballistics solver. A detailed overview of the design of the 8-mm launcher is presented, with an emphasis on the unique considerations which arise from the explosively driven propellant compression and the resulting extreme pressures and temperatures. The high average driving pressure results in a launcher that is compact, with a total length typically less than a meter. The possibility to scale the design to larger projectile sizes (25 mm diameter) is demonstrated. Finally, concepts for a modified launch cycle which may allow the IDL to reach significantly greater projectile velocities are explored conceptually and with preliminary experiments.