Propagation Of Shock Front Research Articles

Pulsed Laser Induced Decomposition of Energetic Polymers: Comparison of ultraviolet (355 nm) and infrared (9.3 μm) initiation

AbstractIrradiation of the energetic polymer GAP (glycidyl azido polymer) by a high power pulsed UV laser leads to its rapid decomposition. A large amount of solid and gaseous material is released, and in the presence of an inert gas, a shock wave develops. Comparison with an inert polymer indicates that the energy released by the exothermicity of the decomposition reaction contributes significantly to the shock formative energy. The energy released in the micro‐explosion can be estimated from the analysis of the shock front propagation velocity. It is found that irradiation of polymers in which GAP is diluted by an inert polymer, may lead to a higher shock intensity than irradiation of neat GAP. Possible causes for this apparent inconsistency (which is not observed upon initiation by a pulsed infrared laser) are discussed.

The 1990 May 24 solar cosmic-ray event

This paper presents an integrated analysis of GOES 6, 7 and neutron monitor observations of solar cosmic-ray event following the 1990 May 24 solar flare. We have used a model which includes particle injection at the Sun and at the interplanetary shock front and particle propagation through the interplanetary medium. The model does not attempt to simulate the physical processes of coronal transport and shock acceleration, therefore the injections at the Sun and at the shock are represented by source functions in the particle transport equation. By fitting anisotropy and angle-average intensity profiles of high-energy (>30 MeV) protons as derived from the model to the ones observed by neutron monitors and at GOES 6 and 7, we have determined the parameters of particle transport, the injection rate and spectrum at the source. We have made a direct fit of uncorrected GOES data with both primary and secondary proton channels taken into account.

Investigation of material removal in pulsed-laser deposition of ceramics

The material removal of the ceramics alumina and zirconia by CO 2 and excimer laser radiation is investigated both experimentally and theoretically. The experimental investigations include microbalance measurements of the mass loss of the target as a measure of mass removal as well as high speed photography and emission spectroscopy as a measure of geometry and dynamics of the vapor-plasma state generated. The theoretical investigations combine the laser-induced phase transition from solid state to vaporized gas of the ceramic targets and the dynamics of the emerging vapor-plasma state. The plasma is described in a two-fluid approximation by use of non-dissipative gas-dynamical equations incorporating absorption of laser radiation in the plasma and the dynamics of its ionization state. Experimental and calculated results show two plasma regions above the target surface in the pressure regime 10 -2−1 × 10 0 mbar used for thin film deposition: one attached to the irradiated target surface and another concerning the developing and propagating shock front. The calculated and the experimental mass loss vs. the laser fluence are in qualitative agreement but with the maximum shifted for the calculations to smaller fluences.

Molecular dynamics simulation of cluster-ion fragmentation

Recent experiments have shown that it is possible to “bounce” low energy (few eV/atom) molecular ions from surfaces and observe the inelasticity of the rebound and the fragmentation patterns. In order to understand these data and to improve our intuition of such phenomena, we have performed molecular dynamics simulations of the impacts of Au clusters with barriers. These calculations not only allow us to deal with laboratory experiments but also give insight into the collisions of interstellar grains that result from the propagation of supernova-driven shock fronts through molecular clouds. In this case we want to determine to what extent the grains are heated and fragmented as a function of the shock velocity. The relevance of these results to the application of cluster-ion bombardment in thin-film deposition will also be discussed.

Shock-induced crystal-glass transition in solid silica

Abstract Solid silica SiO2 occurs under a variety of forms, α-quartz being the stable one at ambient conditions. Shock wave compression induces in this mineral typical defects which appear at the optical microscope as very straight and narrow (≤1μm) lamellae parallel to {10 n} planes. Transmission electron microscopy on both naturally (meteorite impacts) and experimentally (gun shots) shocked quartz shows that they are thin lamellae of amorphous silica. Theoretical computations show that the shock-induced crystal-amorphous transition results from instabilities of the shear modulus in {10 n} planes for P>10 GPa. The elastic misfit energy in the shock front is relaxed by the nucleation of amorphous and more compressible zones the growth of which is then driven by the propagating shock front. The boundaries delineating these defects are sharp and move at very high speed.

An implicit difference scheme for a moving boundary hyperbolic problem

In this paper an implicit difference scheme is defined for a moving boundary hyperbolic problem, which describes a shock front propagation in a constant state. We have reformulated the problem to a fixed boundary domain where an implicit difference scheme is proposed. As is well known, the equivalent condition for the convergence of a consistent scheme is its stability. However, the only reliable methods of stability analysis are based on linear theory. Moreover, the pertinent literature provides simple examples where the linearization of a nonlinear scheme leads to incorrect stability results. On an experimental basis a discrete perturbation stability analysis was then considered. In order to investigate the convergence of the scheme we considered a particular example where an approximate similarity solution is known. In this case, we point out the numerical convergence of the scheme. Even more important is that a possible way to assess the numerical accuracy when the similarity solution does not exist is suggested.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. II: Application in extracorporeal shock wave lithotripsy

To better understand the mechanism of stone fragmentation during extracorporeal shock wave lithotripsy (ESWL), the model developed in Part I [P. Zhong and C.J. Chuong, J. Acoust. Soc. Am. 94, 19-28 (1993)] is applied to study cavitation microjet impingement and its resultant shock wave propagation in renal calculi. Impact pressure at the stone boundary and stress, strain at the propagating shock fronts in the stone were calculated for typical ESWL loading conditions. At the anterior surface of the stone, the jet induced compressive stress can vary from 0.82 approximately 4 times that of the water hammer pressure depending on the contact angles; whereas the jet-induced shear stress can achieve its maximum, with a magnitude of 30% approximately 54% of the water hammer pressure, near the detachment of the longitudinal (or P) wave in the solid. Comparison of model predictions with material failure strengths of renal calculi suggests that jet impact can lead to stone surface erosion by combined compressive and shear loadings at the jet impacting surface, and spalling failure by tensile forces at the distal surface of the stone. Comparing responses from four different stone types suggests that cystine is the most difficult stone to fragment in ESWL, as observed from clinical experience.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. I: Theoretical formulation

To understand the physical process of the impingement of cavitation microjet and the resultant shock wave propagation in an elastic solid, a theoretical model using geometrical acoustics was developed. Shock waves induced in both the jet head (water) and the solid were analyzed during a tri-supersonic impact configuration when the contact edge between the jet head and the elastic boundary expands faster than the longitudinal wave speed in the solid. Impact pressure at the boundary was solved using continuity conditions along the boundary normal. Reflection and refraction of shock waves from a solid-water interface were also included in the model. With this model, the impact pressure at the solid boundary and the stress, strain as well as velocity discontinuities at the propagating shock fronts were calculated. A comparison with results from previous studies shows that this model provides a more complete and general solution for the jet impact problem.

Properties of Mira Photospheres

The first crude pictures of the structure of Mira photospheres were derived from velocity analyses of emission and absorption lines appearing shifted, doubled and doppler-broadened in Mira spectra. Positions and shapes of profiles change with phase and often do not repeat in successive cycles. Non-repeating cycles are also found in colors (e.g., TiO bands in Fig. 9 of Spinrad & Wing 1969) and in light curves and present, of course, a problem in matching specific observed and modeled cycles. The large geometric extension of Mira photospheres was demonstrated by monochromatic radius observations of o Ceti and R Leo by Labeyrie et al. (1977) and Bonneau et al. (1982) which, however, did not systematically follow the stars through different phases and cycles.The structure of a Mira photosphere is essentially determined by the propagation of shock fronts. A shock front enters the bottom of the photosphere once each cycle and travels outwards.

A moving boundary hyperbolic problem for a stress impact in a bar of rate-type material

In this paper we present some results obtained by studying the mathematical model describing a moving boundary hyperbolic problem related to a time dependent stress impact in a bar of Maxwell-like material. Due to the impact a shock front propagates with a finite speed. Here our interest is to underline the influence of the dissipative term on the propagation of the shock front. In the framework of the similarity analysis we are able to reduce the moving boundary hyperbolic problem to a free boundary value problem for an ordinary differential system. It is then possible, by applying two numerical transformation methods, to solve the free boundary value problem numerically. The influence of the dissipative term is evident: the free boundary (that defines the shock front propagation) is an increasing function of the dissipative coefficient.

Study of laser-driven shock wave propagation in Plexiglas targets

An experimental study of laser-driven shock wave propagation in a transparent material such as Plexiglas using a high-speed optical shadowgraphy technique is presented in this paper. A Nd:glass laser was used to produce laser intensity in the range 1012-1014 W/cm2 on the target. Optical shadowgrams of the propagating shock front were recorded with a second-harmonic (0.53-μm) optical probe beam. Shock pressures were measured at various laser intensities, and the scaling was found to agree with the theoretically predicted value. Shock pressure values have also been obtained from a one-dimensional Lagrangian hydrodynamic simulation, and they match well with experimental results. Shadowgrams of shock fronts produced by nonuniform spatial laser beam irradiation profiles have shown complete smoothing when targets with a thin coating of a material of high atomic number such as gold were used. Shock pressures in such coated targets are also found to be considerably higher compared with those in uncoated targets.

Blast pressure prediction between buildings

Consideration is given to the prediction of airborne shock-front propagation between buildings. An experiment is described wherein peak overpressures were measured during the propagation of such shock fronts among buildings where adjacency effects of screening and diffraction influenced the measurements. The results demonstrate that the usual laws of spherical spreading are little influenced by the presence of the buildings. This fortuitous, but somewhat surprising, result is discussed in terms of the propagation properties peculiar to shock fronts.

Molecular dynamic simulation of energy fluxes in pulsed molecular systems

In a condensed material the underlying mechanisms and dynamics of energy transfer and exchange between a propagating shock front and molecular units or defects within the material and from these to the local environment, are central for a theory of “hot spot” formation and explosives initiation. The shock fronts of interest are associated with a moderate-to-strong shock propagating through a condensed fluid or a well-ordered solid. Simulations using molecular dynamics have enabled us to obtain a general microscopic description of shocked systems on temporal and spatial scales as yet simultaneously unobtainable from experiment. We will discuss a series of studies carried out for a model nitromethyl radical incorporated into a matrix of atoms several times heavier than the heaviest of the radical atoms. Answers to important questions were obtained from the studies. These include whether or not significant conversion of shock energy from the heavy weakly-bound host lattice into the high-frequency internal modes of the molecular unit can occur despite what appears to be a severe impedance mismatch between host and molecule, the effect of the initial temperature on the energy flux to and within the unit, the “rise time” associated with the excitation of the molecule and the rate at which the high-frequency internal modes of strongly-bonded molecular structures can be excited.

Wave fronts in elastic media with temperature dependent properties

This paper deals with the propagation of shock and acceleration wave fronts in elastic media with temperature dependent properties. The partial differential equations governing the evolution of such waves are derived and solved using the method of Charpit. Solutions for wave front propagation in a thermoelastic layer with exponentially temperature dependent properties are obtained.

On transient electric fields observed in chemical release experiments by rockets

As a follow‐up to the successful chemical release experiment Trigger in 1977, the Trigger Optimized Repetition rocket was launched from Esrange on October 24, 1984. As in the Trigger experiment, a large‐amplitude electric field pulse of 200 mV/m was detected shortly after the explosion. The central part of the pulse was found to be clearly correlated with an intense layer of swept up ambient particles behind a propagating shock front. The field was directed toward the center of the expanding ionized cloud, which is indicative of a polarization electric field source. Expressions for this radial polarization field and the much weaker azimuthal‐induced electric field are derived from a simple cylindrical model for the field and the expanding neutral cloud. Time profiles of the radial electric field are shown to be in good agreement with observations.

On the propagation of a weak shock front: Theory and application

In this paper the kinematics of a weak shock front governed by a hyperbolic system of conservation laws is studied. This is used to develop a method for solving problems, involving the propagation of nonlinear unimodal waves. It consists of first solving the nonlinear wave problem by moving along the bicharacteristics of the system and then fitting the shock into this solution field, so that it satisfies the necessary jump conditions. The kinematics of the shock leads in a natural way to the definition of ldquoshock-raysrdquo, which play the same role as the ldquoraysrdquo in a continuous flow. A special case of a circular cylinder introduced suddenly in a constant streaming flow is studied in detail. The shock fitted in the upstream region propagates with a velocity which is the mean of the velocities of the linear and the nonlinear wave fronts. In the downstream the solution is given by an expansion wave.

Resistance to shock-front propagation in solids

The structure of a strong shock front is modeled using a wall of dislocations. Motion of these dislocations changes the density of the solid medium and simultaneously relaxes the shear strains that would otherwise exist behind the front. The dislocation motion is highly dissipative because it requires atoms in intimate contact to slide over one another. This transports momentum down the velocity gradient. It is shown that the energy dissipation rate associated with dislocation motion is larger than for any other general mechanism in the system, including electron-electron, electron-phonon, and phonon-phonon interactions. The reason for this is that electrons are not effective in transporting momentum, and atomic thermal velocities are substantially smaller than the velocities of the dislocations which move with the shock front. The viscosity coefficient associated with the dislocations is propertional to the shock velocity, so the viscous drag stress is proportional to the square of the shock velocity. This can lead to large drag pressures.

A mechanism for dislocation generation in shock-wave deformation

Numerous articles describing the changes in structure and mechanical response of metals and alloys induced by shock waves have been published in the past twenty years (I-3). Increases in the concentration of point (4) and line defects (5), formation of twins (6), martensite (7) and precipitates (8) have been observed in the residual structures. Smith (9) proposed the f i rs t model accounting for dislocation generation in shock loading. According to this model, a two-dimensional array of dislocations separated the high-density shockedmaterial from the virgin material; the translation of this two-dimensional interface coincided with the shock-front propagation. Aware of the fact that the passage of such an interface would result in undeformed material, Smith (9) hypothesized the presence of sources and sinks of dislocations, moving with the velocity of the shock front. Hornbogen (lO), based on the fact that shock-loaded iron (between 7 and II GPa) presents a substructure characterized by straight screw dislocations, proposed a modification to Smith's (9) model. Shock loading would induce the formation of loops; the edge components would propagate with the front, basically forming a Smith interface. However, the screw components would stay behind and would therefore be the salient residual feature of the substructure. Cowan ( l l ) suggested that at shock pressures generating shear stresses above the theoretical shear stress of the material a difference in dislocation generation mechanism might be expected. Above this termed by him supercritical stress, the limitations imposed on dislocation velocity would no longer be valid, and a Smith (9) interface might become possible. No specific mechanism is however proposed by him.

Sequential formation of subgroups in OB associations

view Abstract Citations (1208) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Sequential formation of subgroups in OB associations. Elmegreen, B. G. ; Lada, C. J. Abstract We reconsider the structure and formation of OB associations in view of recent radio and infrared observations of the adjacent molecular clouds. As a result of this reexamination, we propose that OB subgroups are formed in a step-by-step process which involves the propagation of ionization (I) and shock (S) fronts through a molecular cloud complex. OB stars formed at the edge of a molecular cloud drive these I-S fronts into the cloud. A layer of dense neutral material accumulates between the I and S fronts and eventually becomes gravitationally unstable. This process is analyzed in detail. Several arguments concerning the temperature and mass of this layer suggest that a new OB subgroup will form. After approximately one-half million years, these stars will emerge from and disrupt the star-forming layer. A new shock will be driven into the remaining molecular cloud and will initiate another cycle of star formation Several observed properties of the OB associations are shown to follow from such a sequential star-forming mechanism. These include the spatial separation and systematic differences in age of OB subgroups in a given association, the regularity of the subgroup masses, the alignment of subgroups along the galactic plane, and their physical expansion. Detailed observations of ionization fronts, masers. IR sources, and molecular clouds are also in agreement with this model. Finally, this mechanism (e.g., T. Tauri stars) which may have formed ahead of the shock as part of the original cloud collapsed and fragmented. Publication: The Astrophysical Journal Pub Date: June 1977 DOI: 10.1086/155302 Bibcode: 1977ApJ...214..725E Keywords: B Stars; Lyman Spectra; Nebulae; O Stars; Star Clusters; Stellar Evolution; Interstellar Gas; Protostars; Shock Fronts; Stellar Models; Stellar Structure; Astrophysics full text sources ADS | data products SIMBAD (10)

Magnetic measurements in shock waves

In [5] a method was proposed in which the measurement winding was located directly in the specimen to be studied. The change in induction was determined by the decrease in area of the specimen cross section and, consequently, the decrease in winding area, as well as by the change in magnetization. Since in this method the cross-sectional area always decreases to some degree, a signal always exists, even when a change in specimen magnetization does not occur. In such a configuration, where the magnetic field is directed perpendicular to the direction of shock-front propagation, it is experimentally impossible to separate the effect of shock-induced magnetic anisotropy, produced by stress anisotropy in the shock-wave front, from the demagnetization connected with shock heating and the dependence of the Curie point on pressure. In the method proposed presently the magnetic field is parallel to the direction of shock-wave front propagation and thus it is possible in experiment to distinguish the effect of demagnetization produced by change of the Curie point with increase in pressure and shock heating from the induced magnetic anisotropy, while in the configuration used there is no signal connected with deformation of the induction sensor cross section. 2. We will analyze the situation in which a shock-wave front propagates parallel to a magnetic field. Let the specimen to be studied, surrounded by turns of the measurement winding, be located in a constant homogeneous magnetic field H e . The initial magnetic flux passing through the measurement winding will be