Showing papers on "Shock (mechanics) published in 1983"

An introduction to the theory of diffusive shock acceleration of energetic particles in tenuous plasmas

Abstract: The central idea of diffusive shock acceleration is presented from microscopic and macroscopic viewpoints; applied to reactionless test particles in a steady plane shock the mechanism is shown to produce a power law spectrum in momentum with a slope which, to lowest order in the ratio of plasma to particle speed, depends only on the compression in the shock. The associated time scale is found (also by a macroscopic and a microscopic method) and the problems of spherical shocks, as exemplified by a point explosion and a stellar-wind terminator, are treated by singular perturbation theory. The effect of including the particle reaction is then studied. It is shown that if the scattering is due to resonant waves these can rapidly grow with unknown consequences. The possible steady modified shock structures are classified and generalised Rankine-Hugoniot conditions found. Modifications of the spectrum are discussed on the basis of an exact, if rather artificial, solution, a high-energy asymptotic expansion and a perturbation expansion due to Blandford. It is pointed out that no steady solution can exist for very strong shocks; the possible time dependence is briefly discussed.

Multiple spacecraft observations of interplanetary shocks Four spacecraft determination of shock normals

Abstract: ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best-fit normals for these five shocks. In addition to providing well-documented shocks for future investigations these data allow the evaluation of the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20 deg, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, the use of overdetermined shock normal solutions, utilizing plasma measurements, separation vectors, and time delays together with magnetic constraints, is recommended whenever possible.

Solar type II radio emission and the shock drift acceleration of electrons

Abstract: Many of the properties of energetic electrons and ions accelerated by interplanetary shock waves can be understood in terms of the shock drift acceleration mechanism. In this paper we show that the shock drift acceleration of electrons can be responsible for solar type II radio bursts as well. We review the shock drift acceleration mechanism and show that the streaming distribution of reflected electrons produced upstream of the shock front by this mechanism can be unstable to the generation of electrostatic plasma waves, which in turn interact to produce the observed radio emission. We derive constraints upon the density and energy of suprathermal electrons required to produce a typical type II burst.

General shock models associated with correlated renewal sequences

Abstract: In this paper we define and analyze a general shock model associated with a correlated pair (Xn, Yn ) of renewal sequences, where the system fails when the magnitude of a shock exceeds (or falls below) a prespecified threshold level. Two models, depending on whether the nth shock Xn is correlated to the length Yn of the interval since the last shock, or to the length Yn of the subsequent interval until the next shock, are considered. The transform results, an exponential limit theorem, and properties of the associated renewal process of the failure times are obtained. An application in a stochastic clearing system with numerical results is also given.

The response of normal shocks in diffusers

Abstract: The frequency response of a normal shock in a diverging channel is calculated for application to problems of pressure oscillations in ramjet engines. Two limits of a linearized analysis arc discussed: one represents isentropic flow on both sides of a shock wave; the other may be a crude appr'l'I;imation to the influence of flow separation induced hy the wave. Numerical results arc given, and the influences of the shock wave on oscillations in the engine are discus,ed.

Formation of shock‐spike events at quasi‐perpendicular shocks

Abstract: This paper is an analysis of how shock-spike particle events are formed by single-encounter “shock-drift” acceleration at quasi-perpendicular fast-mode interplanetary shocks. A set of time-reversed equations, valid in the upstream or downstream solar wind frames, express a particle's initial or pre-shock-interaction kinetic energy and pitch angle as functions of its final or post-shock-interaction kinetic energy and pitch angle. These equations and particular forms of the initial or ambient energy spectrum and angular distribution yield model-predicted intensity enhancements, energy spectra, and pitch angle distributions for comparison with spacecraft observations. It is shown that the final energy spectrum can be harder than, softer than, or equal to the ambient spectrum, depending upon, for example, the point along the final spectrum one chooses to examine, whether one looks at the upstream of downstream spectrum, and how rapidly the ambient spectrum decreases with increasing energy. It is shown that for a given form of the ambient energy spectrum, an upstream ambient angular distribution peaked toward (away from) the shock along the magnetic field yields a final energy spectrum that is harder (softer) in both the upstream and downstream regions than the spectrum produced for an isotropic ambient distribution. The single-encounter scheme is shown to account for many features observed in the short-lived, impulsive shock-spike events. In particular, the existence of the high-intensity spike in close temporal association (of the order of minutes) with the shock passage is consistent with a kinematical accumulation of particles accelerated and transmitted downstream of the shock. The single-encounter acceleration process is also consistent with some features of energetic storm particle events observed at radial distances of 14–22 AU from the sun.

Application Of Holographic Interferometry To Shock Wave Research

Abstract: Paper reports a successful application of holographic interferometry to the shock wave research. Four topics are discussed; i) transonic flow over an aerofoil, ii) shock wave propagation and diffraction past a circular cross-sectional 90° bend and two-dimensional straight or curved wedges, iii) stability of converging cylindrical shock waves and iv) propagation and focusing of underwater shock waves. Experiments were conducted on shock tubes equipped with a double exposure holographic interferometer. In each case isopycnics around shock waves were determined and three-dimensional shock wave interactions were also observed. Results are not only bringing forth new interesting findings to the shock wave research but also showing a further potentiality of holographic interferometry to the high speed gasdynamic study.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Inflatable packaging material

Abstract: A packaging material is formed from a pair of juxtaposed sheets. The material includes a plurality of continuous passages formed between the sheets and extending between two of its edges. Each of said passages is delimited by a pair of spaced partitions, at least one of which follows a sinuous path between the edges. The passages are inflatable by admission of air to provide a shock absorbing facility.

Shock attenuation system.

Abstract: A shock attenuation system (S) comprising a plurality of shock attenuating columns (9) of a substantially resilient elastomeric material. The columns are so dimensioned and configured that, when subjected to an axial impact force of predetermined magnitude, they resiliently deform for attenuating the shock resulting from the impact force. The columns then spring back substantially to their undeformed shape.

Backstreaming ions from oblique earth bow shocks

Abstract: One-dimensional, hybrid (particle ions, fluid electrons) numerical simulations of oblique shock structure are used to analyze the mechanism by which solar wind ions can be reflected off the earth's bow shock front and escape upstream. It is shown that no backstreaming ions are produced in the simulation over 50-90 degrees of theta(nB), the angle between the interplanetary magnetic field and the shock normal. Backstreaming ions are emitted for theta(nB) of 45 degrees or less, with characteristics in reasonable agreement with those of observed reflected ion streams. The simulated shock structure becomes rather turbulent as theta(nB) decreases from 45 to 30 degrees. Analysis of the backstreaming ion trajectories shows that all backstreaming ions are originally specularly reflected. The oblique shock structure and effects such as whistler formation and ion heating are discussed.

Shock compression of diamond crystal

Abstract: Two shock wave experiments employing inclined mirrors have been carried out to determine the Hugoniot elastic limit (HEL), final shock state at 191 and 217 GPa, and the post-shock state of diamond crystal, which is shock-compressed along the intermediate direction between the and crystallographic axes. The HEL wave has a velocity of 19.9 ± 0.3 mm/µsec and an amplitude of 63 ± 28 GPa. An alternate interpretation of the inclined wedge mirror streak record suggests a ramp precursor wave and then another HEL value. The maximum post-shock density achieved upon release from the ∼200 GPa shock state is ∼3.95 Mg/m³, which compares to the initial density 3.52 Mg/m³. This result suggests an elastic unloading effect or shock-induced transition to a denser (possibly metallic) phase.

Optimum Replacement of a System Subject to Shocks

Abstract: A system is subject to shocks that arrive according to a nonhomogeneous or homogeneous Poisson process. Since each shock weakens the system and makes it more expensive to run, it is desirable to determine a replacement policy for the system. We consider the possibility of periodic replacement of the system, and exhibit a necessary and sufficient condition for the existence of an optimal finite period. The problem is investigated for both finite and infinite time horizons. Finally, we study a particular model for a system that fails on the second shock.

Collisionless dissipation processes in quasi‐parallel shocks

Abstract: The evolution of collisionless, quasi-parallel shocks (the angle between the shock normal and the upstream magnetic field being less than 45 deg) is examined using two dimensional particle simulations. Reflected ions upstream from the shock are observed with average guiding center velocity and gyrational energy which agree well with the prediction of simple specular reflection. Strong ion heating through the shock ramp is apparently caused by large amplitude whistler turbulence. A flux of suprathermal electrons is also the magnetic field direction. Much stronger ion heating occurs in the shock than electron heating. The relevance of this work to the earth's bow shock is discussed.

The structure and emission spectrum of a nonradiative shock wave in the Cygnus Loop

Abstract: The ultraviolet spectrum of a Balmer-line filament located just outside the main body of optical filaments in the Cygnus Loop confirms the nonradiative shock wave theory for its origin. This theory is extended to include the hydrogen two-photon continuum, and the possibility of slow electron-ion equilibration in the postshock gas is considered. The shock velocity is inferred from the H-alpha profile. Comparison of model calculations with optical and ultraviolet spectra favors Coulomb equilibration behind a 170 km/s shock over models with rapid equilibration due to plasma turbulence. Elemental abundances in the preshock gas are found to be typical of diffuse interstellar clouds. The ram pressure behind the shock is higher than the pressures inferred for bright optical filaments. It is suggested that the bright optical filaments are regions of thermally unstable cooling behind shocks faster than the shock velocities inferred from their optical and ultraviolet spectra.

On the structure of the magnetic slow switch‐off shock

Abstract: A particle code is used to simulate the evolution of a magnetic slow shock. The initial state is two uniform plasmas related by the Rankine-Hugoniot jump conditions and separated by a transition layer a few ion gyroradii thick. The code follows the evolution of the system in time. Two principal features of the results are the upstream escape of the hot shocked plasma and a damped, left-handed circularly polarized wave on the trailing edge of the shock. Analysis of the trailing wave train indicates that ion cyclotron interaction is important in heating the plasma. The upstream escape of particles results in a temporal broadening of the shock profile. The implications of the simulation results in magnetic merging processes in the neighborhood of an x type neutral point are described.

Impact of a liquid mass on a perfectly plastic solid

Abstract: The use of steady, normal and oblique shock configurations is explored in calculating the pressure and deformation produced by the impact of a liquid mass on a plane solid surface. Since pressures generated are very large, the change in bulk modulus of the liquid (water) is accounted for by using an equation of state following Field, Lesser & Davies (1979). The impact of a plane-ended liquid mass is analysed using a normal shock for the cases of a rigid surface and a perfectly plastic surface. For the former, it is found that pressures somewhat in excess of the ‘water-hammer’ pressure of linear acoustic theory are predicted, and for the latter there is a critical impact velocity below which no deformation occurs. Above this velocity the surface deforms at a constant rate, producing a pit with maximum depth at the centre. If the liquid mass is wedge-shaped then an oblique shock is formed, which is attached to the contact point provided that the impact Mach number is large enough, as originally shown by Heymann (1969). Pressure and deformation velocity can again be calculated for the cases of rigid and perfectly plastic surfaces respectively. For a rigid surface it is confirmed that pressures considerably in excess of the plane-ended case are produced at shock detachment. For the plastic surface, it is found that there is no critical impact velocity and deformation can occur at any velocity as shock detachment is approached. For a cylindrical liquid mass with a conical tip, the pit produced again has maximum depth at the centre, but with a considerably increased value. The possible use of these models for pitting caused by microjets associated with cavitation bubbles and by impact of liquid drops is discussed.

Shock-associated noise in supersonic jets

Abstract: This paper examines the fundamental mechanism of broadband shock noise in an improperly expanded supersonic jet. The study includes circular convergent and convergent-divergent nozzles. The main source of shock noise is determined to be the transient interaction between the shock front and the convected vorticity within the jet plume. The discussion of the noise generation mechanism is based on detailed numerical analysis, theoretical modeling, refined measurements of the jet mean flow, shock-cell structure, turbulence, and noise. Results in this study provide a broad-based generalization for the Harper-Bourne and Fisher analysis and prediction method.

Shock absorber for fall protection system

Abstract: A shock absorber as designed for use in fall protection systems and may be incorporated integrally with either a belt or lanyard or may be used as a separate component in conjunction with conventional belts and lanyards. A shock absorbing mechanism of the invention utilizes a piercing member having smooth edges which pierce a piece of webbing material. When force is applied to opposite ends of the shock absorber, the piercing member stretches and breaks latitudinal fibers of the webbing material. This allows near complete retention of the strength of the longitudinal fibers as those fibers are not damaged during the absorbing operation.

Factors controlling the location of the Venus bow shock

Abstract: The location of the Venus bow shock determined from magnetic field measurements during the first and third years of Pioneer Venus orbiter operation is examined and compared with nearly simultaneously obtained interplanetary solar wind data to determine those factors that control the size of the Venus bow shock The location of the intersection of the bow shock with the terminator that is best determined by the data does not vary significantly between years 1979 and 1981 and is only 16 percent more distant than the Venera 9 and 10 shock when account is taken of solar wind aberration Alfvenic Mach number and magnetosonic Mach number affect the size of the bow shock significantly Solar wind dynamic pressure has a lesser effect No significant asymmetries in the shock shape were found either as a result of the orientation of the clock angle of the IMF in the terminator plane or the angle of the IMF relative to the shock normal

Direct observation of shock splitting in a vapor–liquid system

Abstract: The splitting of a single liquefaction shock wave into a vapor‐phase forerunner shock and a following condensation shock is observed photographically in shock‐tube experiments. The behavior of the two‐shock system is qualitatively described by equilibrium shock conditions, but the state downstream of the forerunner shock is found to be far from equilibrium. It is proposed that this state is limited by the critical supersaturation for homogeneous nucleation. The overall pressure jump for the two‐shock system is much greater than would be predicted by an equilibrium calculation.

Lower hybrid drift instability with temperature gradient in a perpendicular shock wave

Abstract: Finite beta effects and an electron temperature gradient are included in the present study of the perpendicular bow shock geometry's lower hybrid instability, where the flute mode that is stable at the shock for constant electron temperature is destabilized in the case of a sufficiently great temperature gradient. Numerical solutions are given for cases in which the ion distribution is either drifting Maxwellian or consists of two Maxwellians, to represent the effect of reflected ions at the shock. A discussion is presented of the implications of results obtained for ion and electron heating and electron acceleration at the bow shock.

A new method to measure homogeneous nucleation rates in shock tubes

Abstract: The centered expansion wave of a shock tube is utilized to expand and supersaturate a condensable vapor in small concentration in an inert carrier gas. The supersaturated state, located at the rear of the expansion wave, is preserved for a controlled period and then terminated by a recompressing shock wave. During the period of supersaturation, condensation nuclei are formed homogeneously. The nucleation rate is measured as a function of supersaturation by a Mie-light scattering technique. The method is tested using water and the results are compared with classical nucleation theory.

Shock absorbing mechanism for rearview mirror assembly of motor vehicle

Abstract: A shock absorbing mechanism for a rearview mirror assembly of a motor vehicle includes a mirror body movable in a horizontal direction with respect to a mounting stay mounted to the motor vehicle in such a manner that the mirror body is initially located at the neutral standing position and is movably supported to be inclined when an external force is applied thereto. The mirror body is automatically returned to the standing position upon removal of the external force or is inclined toward one side of the motor vehicle upon necessity.

Reinforced explosive shock tube

Abstract: Low energy explosive shock tubing is provided which consists of a two-ply, inner and outer layer plastic tube having a plurality of lengthwise textile filaments bonded into the interface between the plastic layers. The textile filaments are chosen for their low elongation properties and the resultant shock tube resists stretching especially in warm borehole environments.