Showing papers on "Shock (mechanics) published in 1992"

Relativistic Magnetosonic Shock Waves in Synchrotron Sources: Shock Structure and Nonthermal Acceleration of Positrons

Abstract: The theoretical properties of relativistic, transverse, magnetosonic collisionless shock waves in electron-positron-heavy ion plasmas of relevance to astrophysical sources of synchrotron radiation are investigated. Both 1D electromagnetic particle-in-cell simulations and quasi-linear theory are used to examine the spatial and kinetic structure of these nonlinear flows. A new process of shock acceleration of nonthermal positrons, in which the gyrating reflected heavy ions dissipate their energy in the form of collectively emitted, left-handed magnetosonic waves which are resonantly absorbed by the positrons immediately behind the ion reflection region, is described. Applications of the results to the termination shocks of pulsar winds and to the termination shocks of jets emanating from the AGN are outlined.

Insoluble surfactant spreading on a thin viscous film: Shock evolution and film rupture

Abstract: Lubrication theory and similarity methods are used to determine the spreading rate of a localized monolayer of insoluble surfactant on the surface of a thin viscous film, in the limit of weak capillarity and weak surface diffusion. If the total mass of surfactant increases as t(alpha), then at early times, when spreading is driven predominantly by Marangoni forces, a planar (axisymmetric) region of surfactant is shown to spread as t(1 + alpha)/3 (t(1 + alpha)/4) . A shock exists at the leading edge of the monolayer; asymptotic methods are used to show that a wavetrain due to capillary forces exists ahead of the shock at small times, but that after a finite time it is swamped by diffusive effects. For alpha < 1/2 (alpha < 1), the diffusive lengthscale at the shock grows faster than the length of the monolayer, ultimately destroying the shock; subsequently, spreading is driven by diffusion, and proceeds as t1/2. The asymptotic results are shown to be good approximations of numerical solutions of the governing partial differential equations in the appropriate limits. Additional forces are also considered: weak vertical gravity can also destroy the shock in finite time, while effects usually neglected from lubrication theory are important only early in spreading. Experiments have shown that the severe thinning of the film behind the shock can cause it to rupture: the dryout process is modelled by introducing van der Waals forces.

Dynamics of cluster-surface collisions.

Abstract: The structure, energetics, and dynamics of shock conditions generated in a nano-cluster upon impact on a crystalline surface are investigated with molecular-dynamics simulations for a 561-atom argon cluster incident with a velocity of 3 kilometers per second onto a sodium chloride surface. The "piling-up" shock phenomenon occurring upon impact, coupled with cascades of energy and momentum transfer processes and inertial confinement of material in the interior of the cluster, creates a transient medium lasting for about a picosecond and characterized by extreme local density, pressure, and kinetic temperature. The nano-shock conditions and impulsive nature of interactions in the newly formed compressed nonequilibrium environment open avenues for studying chemical reactivity and dynamics catalysed via cluster impact.

Relativistic, perpendicular shocks in electron-positron plasmas

Abstract: One-dimensional particle-in-cell plasma simulations are used to examine the mechanical structure and thermalization properties of collisionless relativistic shock waves in electron-positron plasmas. Shocks propagating perpendicularly to the magnetic field direction are considered. It is shown that these shock waves exist, and that they are completely parameterized by the ratio of the upstream Poynting flux to the upstream kinetic energy flux. The way in which the Rankine-Hugoniot shock jump conditions are modified by the presence of wave fluctuations is shown, and they are used to provide a macroscopic description of these collisionless shock flows. The results of a 2D simulation that demonstrates the generality of these results beyond the assumption of the 1D case are discussed. It is suggested that the thermalization mechanism is the formation of a synchrotron maser by the coherently reflected particles in the shock front. Because the downstream medium is thermalized, it is argued that perpendicular shocks in pure electron-positron plasmas are not candidates as nonthermal particle accelerators.

Spectral Analysis of Impact Shock during Running

Abstract: The purpose of this study was to determine the effects of increasing impact shock levels on the spectral characteristics of impact shock and impact shock wave attenuation in the body during treadmill running. Twelve male subjects ran at 2.0, 3.0, 4.0, and 5.0 m s−1 on a treadmill. Axial accelerations of the shank and head were measured using low-mass accelerometers. The typical shank acceleration power spectrum contained two major components which corresponded to the active (5–8 Hz) and impact (12–20 Hz) phases of the time-domain ground reaction force. Both the amplitude and frequency of leg shock transients increased with increasing running speed. Greatest attenuation of the shock transmitted to the head occurred in the 15–50 Hz range. Attenuation increased with increasing running speed. Thus transmission of the impact shock wave to the head was limited, despite large increases in impact shock at the lower extremity.

Shock-induced collapse of single cavities in liquids

Abstract: A two-dimensional method was used to observe the interactions of plane shock waves with single cavities. This allowed study of processes occurring within the cavity during collapse. Results were obtained from high-speed framing photography. A variety of collapse shock pressures were launched into thin liquid sheets either by firing a rectangular projectile or by using an explosive plane-wave generator. The range of these shock pressures was from 0.3 to 3.5 GPa. Cavities were found to collapse asymmetrically to produce a high-speed liquid jet which was of approximately constant velocity at low shock pressures. At high pressures, the jet was found to accelerate and crossed the cavity faster than the collapse-shock traversed the same distance in the liquid. In the final moments of collapse, high temperatures were concentrated in two lobes of trapped gas and light emission was observed from these regions. Other cavity shapes were studied and in the case of cavities with flat rear walls, multiple jets were observed to form during the collapse.

Nonstationarity of a two-dimensional quasiperpendicular supercritical collisionless shock by self-reformation

Abstract: Two‐dimensional electromagnetic particle simulations evidence a self‐reformation of the shock front for a collisionless supercritical magnetosonic shock propagating at angle θ0 around 90°, where θ0 is the angle between the normal to the shock front and the upstream magnetostatic field. This self‐reformation is due to reflected ions which accumulate in front of the shock and is observed (i) in both electric and magnetic components, (ii) for both resistive and nonresistive two‐dimensional shocks, and (iii) over a cyclic time period equal to the mean ion gyroperiod measured downstream in the overshoot; resistive effects may be self‐consistently included or excluded for θ0≂90° according to a judicious choice of the upstream magnetostatic field orientation. The self‐reformation leads to a nonstationary behavior of the shock; however, present results show evidence that the shock becomes stationary for θ less than a critical value θr, below which the self‐reformation disappears. Present results are compared to p...

Physical Model of the Swept Shock Wave/Boundary-Layer Interaction Flowfield

Abstract: New data are presented on the structure of fin-induced, swept shock/boundary-layer interactions. These data are obtained using a nonintrusive planar laser scattering (PLS) imaging technique. A range of interaction strengths, from barely separated to very strongly separated, is covered for freestream Mach numbers of 3 and 4. These new data, when combined with previous results on the flowfield and interaction "footprint," are sufficient to allow the construction of a physical model for the swept interaction flowfield structure and behavior. This physical model is presented and discussed in terms of six detailed flowfield maps which take advantage of the inherent quasiconical behavior of the class of interactions considered.

Shock metamorphism of quartz with initial temperatures −170 to + 1000° C

Abstract: Shock experiments on quartz single crystals with initial temperatures −170 to +1000°C showed that ambient temperature does not affect the type of defects formed but can lower the pressure of complete amorphization. The amount of glass recovered increases with both pressure and temperature, and the shock-induced phase transformation of quartz is temperature-activated with an apparent activation energy of <60 kJ/ mol. The phase transformation is localized along three types of transformation lamellae (narrow, s-shaped, and wide) which contain fractured and/or high-pressure phases. Transformation lamellae are inferred to form by motion of linear collapse zones propagating near the shock front. Equilibrium phases, such as stishovite, were not recovered and are probably not formed at high shock pressures: the dominant transformation mechanism is inferred to be solid-state collapse to a dense, disordered phase. Melting occurs separately by friction along microfaults, but no high-pressure crystal phases are quenched in these zones. Shock of quartz thus produces two types of disordered material, quenched melt (along microfaults) and diaplectic glass (in transformation lamellae); the quenched melt expands during P-T release, leaving it with a density lower than quartz, while recovered diaplectic glass has a density closer to that of quartz. At low pressures (< 15 GPa), quartz transforms mostly by shear melting, while at higher pressures it converts mostly along transformation lamellae. We find that shock paleopiezometers using microstructures are nominally temperature-invariant, so that features observed at impact craters and the K/T boundary require in excess of 10 GPa to form, regardless of the target temperature. Shock comminution will be much more extensive for impacts on cold surfaces due to lack of cementation of fragments by melt glass; shock on hot surfaces could produce much more glass than estimated from room-temperature experiments. Because of the shock-impedance mismatch between quartz specimen and steel capsule, the incident shock wave reverberates up to a final pressure. The dynamic compression process is quasi-isentropic with high strain rates. Preheating and precooling achieves final shock pressures and temperatures representative of single-shock states of room temperature quartz and of quartz on known planetary surfaces. Stress histories were calculated by detailed 1- and 2-dimensional computer simulations. The stress history throughout the sample is relatively uniform, with minor variations during unloading. Significant differences between impact pressures calculated by the shock-impedance-match method and specimen pressures calculated by computer simulations indicate the importance of modeling shock recovery experiments computationally.

Acoustic emission and sonoluminescence due to cavitation at the beam focus of an electrohydraulic shock wave lithotripter.

Abstract: The acoustic emission from cavitation in the field of an extracorporeal shock wave lithotripter has been studied using a lead zirconate titanate piezoceramic (PC4) hydrophone in the form of a 100-mm diameter focused bowl of 120-mm focal length. With this hydrophone directed at the beam focus of an electrohydraulic lithotripter radiating into water, it is possible to identify signals well above the noise level, at the 1-MHz resonance of the hydrophone, which originate at the beam focus. Light emission, attributed to sonoluminescence, is also shown to originate at the focal region of the lithotripter, and the signal obtained from a fast photomultiplier tube directed at the focus has similarities in structure and timing to the detected acoustic signals. The multiple shock emission resulting from a single discharge of an electrohydraulic source is shown to result in two separate bursts of cavitational activity separated by a period of 3–4 ms. The signal burst corresponding to the primary shock has a duration of about 600 μs with little noticeable structure. The signal burst associated with the secondary shock has a reproducible structure with two distinct peaks separated by about 200 μs depending on the shock amplitude. The timing and structure of each burst is shown to be in reasonable agreement with the theoretical predictions made by Church (1989) based on the Gilmore model of bubble dynamics. In particular, it is shown that it is possible to obtain precise measurements of the time delay between the separate peaks within the signal burst detected following the secondary shock and this may, as predicted, provide a method of determining the size of bubbles remaining after the primary shock.

Method and apparatus for isolating electronic boards from shock and thermal environments

Abstract: A method and apparatus for isolating electronic devices from mechanical shock and thermal environments utilizes a metal outer protective shell surrounding the electronic device to be protected. Interposed between the protective shell and the electronic device are a thermal insulating layer, a layer of phase change material and a layer of mechanical shock absorbent material.

The failure waves and spallations in homogeneous brittle materials

Abstract: The formation and propagation of failure waves in shock compressed materials was observed in a series of plane shock wave experiments with glass and fused quartz. Results of stress, free surface velocity wave profile measurements, and computer simulations show that the failure wave nucleates at the surface of the compressed sample and then propagates into the sample with a decreasing subsonic velocity. The dynamic tensile strength is significantly higher than those of ceramics and minerals.

An Analysis of Combustion Studies in Shock Expansion Tunnels and Reflected Shock Tunnels

Abstract: The effect of initial nonequilibrium dissociated air constituents on the combustion of hydrogen in high-speed flows for a simulated Mach 17 flight condition was investigated by analyzing the results of comparative combustion experiments performed in a reflected shock tunnel test gas and in a shock expansion tunnel test gas. The results were analyzed and interpreted with a one-dimensional quasi-three-stream combustor code that includes finite rate combustion chemistry. The results of this study indicate that the combustion process is kinetically controlled in the experiments in both tunnels and that the presence of the nonequilibrium partially dissociated oxygen in the reflected shock tunnel enhances the combustion. Methods of compensating for the effect of dissociated oxygen are discussed.

Apparatus and method for preventing data corruption in disk drives from mechanical shock during write operations

Abstract: Apparatus for preventing data corruption on a disk due to mechanical shock occurring during the write process to the disk includes a mechanical shock sensor to sense mechanical shocks having a magnitude exceeding a predetermined threshold Write disable circuitry responsive to the mechanical shock sensor interrupts the write current to the disk drive write head Repositioning circuitry then repositions the data head over the original data track and the incomplete data that was interrupted by the mechanical shock is rewritten A method for preventing data corruption on a disk due to mechanical shock experienced by a disk drive during the write process to the disk includes the steps of sensing a mechanical shock having a magnitude exceeding a predetermined threshold; storing information identifying the data being written at the onset of the sensed shock; interrupting the write current to the write head; repositioning the data head to the original track; and rewriting the data which was interrupted because of the sensed shock

Apparatus for sensing operating shock on a disk drive

Abstract: This invention provides a shock load detection device (24) mounted on a disk drive unit (10). The relatively planar shock sensor (24) is mounted on a relatively rectangular disk drive (10) such that the plane of the sensor (24) intersects the planes aligned with the sides of the disk drive (10) at oblique angles. The detection device (24) is preferably comprised of a piezoelectric polymer film (42) encapsulated and electrically shielded by a metallic or metallic coated package (41). Electrical contacts are preferably provided for on the piezoelectric polymer film (42) and are preferably connected to amplification and comparator circuitry (52, 54) which issues a write fault signal upon detection of external force in excess of a predetermined threshold. The shock sensor (24) may be mounted on the computer disk drive unit (10) in such a manner as to be at a 45° angle to each of the x, y and z axis of the orthogonal co-ordinate system so that linear and torsional forces may be monitored.

Solids Under High-Pressure Shock Compression: Mechanics, Physics, and Chemistry

Abstract: Since the 1950s shock compression research contributed greatly to scientific knowledge and industrial technology. As a result, for example, our understanding of meteorite impacts has substantially improved, and shock processes have become standard industrial methods in materials synthesis and processing. Investigations of shock-compressed matter involve physics,electrical engineering, solid mechanics, metallurgy, geophysics and materials science. The description of shock-compressed matter presented here, which is derived from physical and chemical observations, differs significantly from the classical descriptions derived from strictly mechanical characteristics. This volume, with over 900 references, provides an introduction for scientists and engineers interested in the present state of shock compression science.

Constraints on the Acceleration of Anomalous Cosmic Rays

Abstract: The recent determination of an upper limit of 0.2 pc on the distance to the source of the anomalous cosmic rays, and the implied upper limit on their age imposes a significant observational constraint on possible acceleration mechanisms. Adiabatic cooling in the expanding wind makes the constraints significantly more stringent. In the light of these constraints, the rate of second-order Fermi acceleration is much too low to play a significant role. Diffusive shock acceleration, even at the strong termination shock of the solar wind, is sufficient only if one discards the standard expression for the maximum acceleration rate based on the Bohm limit. Only diffusive acceleration at the quasi-perpendicular solar wind termination shock has a high enough acceleration rate to be the source of the anomalous component.

Spectroscopy of a Balmer-dominated filament in the Cygnus Loop with the Hopkins Ultraviolet Telescope

Abstract: A fair UV spectrum of a nonradiative filament in the Cygnus Loop covering the wavelength range 830-1860 A at 3.5-A resolution was obtained using the Hopkins Ultraviolet Telescope (HUT) on the Astro-1 space shuttle mission. Nonradiative shock models which include a more sophisticated treatment of Lyman line transfer are calculated. It is found that the HUT spectrum can be explained in terms of a shock with velocity 175-185 km/s propagating into a low-density medium. This shock velocity can be reconciled with the 135-km/s width of the broad component of H-alpha in this filament if equilibration of the postshock electron and ion temperatures proceeds much more rapidly than Coulomb equilibration time scales. The time required for a 180-km/s shock to develop a partial recombination zone is short, about 200 yr if n is approximately equal to 2/cu cm. This suggests that the shock is decelerating as it encounters denser material. The current analysis patently favors rapid equilibration of electrons and ions behind the shock.

The influence of the large-scale interplanetary shock structure on a low-energy particle event

Abstract: We have developed a numerical model to study the influence of the large-scale shock topology on the associated low-energy (less than 2 MeV) particle event upstream of the shock. It includes particle injection at the solar corona, two-dimensional MHD simulation of the propagation of the shock, modeling of particle propagation through the interplanetary medium, and particle injection at the shock. The model does not attempt to simulate the physical processes of shock particle acceleration, therefore the injection at the shock is represented by a numerical source function in the equation that describes particle propagation

Bicycle shock-absorbing apparatus

Abstract: A bicycle shock-absorbing apparatus includes an upper tube telescopically received in a lower tube. A plurality of compressive members are positioned inside the upper tube. A socket bolt member having a socket portion and a rod portion is mounted on a mount member of the lower tube by its rod portion while allowing its socket portion to be slidably received in a lower portion of the upper tube and to contact against a bottom one of the compressive members. When the bicycle receives a shock from an uneven terrain, the upper tube slides downward inside the lower tube while the compressive members therein will be compressed, thereby absorbing the shock.

Thermal equilibration in a shock wave.

Abstract: Absolute-intensity measurements of luminous emissions from shocked silicon show significant disagreement with predictions that assume thermal equilibrium between electrons and ions is reached immediately behind the shock front. Taking into account both electron-ion equilibration and electron thermal conduction, we have estimated an electron-ion coupling coefficient of 10 16 W/m 3 K. This finding will also have important ramifications in pyrometric determinations of shock temperatures

Shock absorber and a hermetically sealed scroll gas expander for a vehicular gas compression and expansion power system

Abstract: Energy of shock force normally absorbed by shock absorber in an automotor vehicle is recovered and converted to electric power.

Quasielastic release in shock‐compressed solids

Abstract: Shock‐ and release‐wave measurements are reported for 6061‐T6 aluminum [J. R. Asay and L. C. Chhabildas, in Shock Waves and High‐Strain‐Rate Phenomena in Metals, edited by M. A. Meyers and L. E. Murr (Plenum, New York, 1981), pp. 417–431], oxygen‐free‐electronic copper, and a Si‐bronze alloy. Significant departure from ideal elastic‐plastic response is observed in all three materials. Experimentally determined release‐wave profiles show evidence for the onset of reverse plastic flow immediately upon release from the shocked state. This phenomenon is analyzed in terms of internal stresses acting on straight dislocation pileups and pinned dislocation loops created by the shock‐compression process. Following shock compression and prior to release, the internal stresses are opposed by the applied shear stress; that is, they exactly balance each other and no plastic flow occurs. As the applied stress is reduced in the unloading wave, reverse plastic flow occurs immediately due to internal reverse stresses acting on these pileups and pinned loops. This effect reduces the longitudinal modulus, and hence, the release‐wave speed in what we normally think of as the ‘‘elastic‐wave’’ regime. Interpretations of quasielastic release‐wave data and calculations are expressed in terms of micromechanical concepts.

Apparatus and method for shock attenuation in a disk recording and/or reproduction system using variable gain acceleration sensor

Abstract: A disk recording and/or reproduction device subject to acceleration forces in which the transducer track following function is augmented by an acceleration responsive transducer positioning control. The acceleration response is calibrated as a function of the position error of the transducer with respect to the center of the track being followed.

Shock absorbing glove

Abstract: A shock absorbing glove for sports and industrial activities having air or gas filled compartments on the palm portion to function as cushions to absorb shocks and impacts transmitted to the palm by an object gripped by the glove.

Shock mount assembly

Abstract: Vibration and shock mounting of an apparatus such as a disk drive is dampened through the use of one or more O-rings held in place by a bolt with a pre-determined load force applied to the mounting bracket of the device. One or more notches or slits may be selectively employed as required to equalize the force applied to multiple mounting brackets by the device.