Showing papers on "Shock (mechanics) published in 1989"

Shock absorbing system for footwear application

Abstract: This invention relates to a new shock absorber which may be used as an insole or as a midsole for an article of footwear. The shock absorber comprises a multi-cell membrane which may be embedded in a flexible envelope or which may be used itself as a one-piece multi-cell membrane insole or midsole. The shock absorber exhibits improved shock absorbing characteristics which increases the comfort of the wearer of the shoes and reduces damage to the foot during athletic exercises.

The effects of target compliance on liquid drop impact

Abstract: When a liquid drop impacts a solid surface, the contact periphery at first expands more quickly than the compression wavefronts in either liquid or solid. The liquid behind the shock envelope is compressed and high pressures of order ρCV result, where ρ is the density of the liquid at ambient pressure, C the shock velocity in the liquid, and V the impact velocity. At a later stage, the shock envelope overtakes the contact periphery and a jetting motion, which releases the high pressures, commences. The magnitude and duration of the high pressures are critical in explaining the damage mechanisms and erosion processes caused by liquid impact. The experiments described in this paper use the two‐dimensional gel and photographic techniques developed for visualizing the shocks, recording the onset of jetting, and measuring jet velocities. This particular study is primarily concerned with the effect of target compliance on the early stages of impact. It is shown that the greater the target compliance, the longer...

Observation of the formation of an optical intensity shock and wave breaking in the nonlinear propagation of pulses in optical fibers.

Abstract: We have observed the formation of an optical intensity shock and the subsequent wave breaking in the nonlinear propagation of 1-psec pulses in an optical fiber. The wave breaking manifests itself as the appearance of oscillations trailing the shock, which are due to the beating of widely separated frequency components which bridge the shock. The experimental results are in good agreement with numerical solutions of the nonlinear Schr\"odinger equation.

On the refraction of shock waves

Abstract: This paper discusses the refraction of plane shock waves in media with arbitrary equations of state. Previous work is reviewed briefly, then a rigorous definition of wave impedance is formulated. Earlier definitions are shown to be unsatisfactory. The impedance is combined with the boundary conditions at the media interface to study both head-on and oblique shock incidence. The impedance determines the nature of the reflected and transmitted waves, their intensities, and the fractions of energy and power that are reflected and transmitted. The refractive index is also defined and determines whether or not a wave will be refracted, and also helps determine whether the wave system will be regular or irregular. The fundamental law of refraction is derived and shown to be a consequence of the fact that an arbitrary point on a shock or an expansion wave follows a ray path of minimum time between any two points on the path. This is a generalization of Fermat's Principle to media that are deformed and convected by the waves propagating through them.

Nonequilibrium molecular motion in a hypersonic shock wave

Abstract: Molecular velocities have been measured inside a hypersonic, normal shock wave, where the gas experiences rapid changes in its macroscopic properties. As first hypothesized by Mott-Smith, but never directly observed, the molecular velocity distribution exhibits a qualitatively bimodal character that is derived from the distribution functions on either side of the shock. Quantitatively correct forms of the molecular velocity distribution function in highly nonequilibrium flows can be calculated, by means of the Direct Simulation Monte Carlo technique.

On the effect of a tangential discontinuity on ions specularly reflected at an oblique shock

Abstract: A study is conducted for the case of a tangential discontinuity (TD) that convects into a shock at some arbitrary angle, in order to clarify events observed in proximity to the earth's bow shock. Very different behavior is noted to result depending on the sense of ion gyration relative to the TD. The fact that particles can be injected into the TD's plane, so that they travel upstream close to the TD, implies that active current sheet (ACS) events thus far assumed to be generated by the solar wind's interaction with a large-density reflected component are actually detached from the bow shock. In other geometries, ions stay close to the shock after their TD interaction, implying that ACS events are modifications of the shock.

Modeling two‐dimensional detonations with detonation shock dynamics

Abstract: One of the principal shortcomings of the computer models that are presently used for two‐dimensional explosive engineering design is their inadequate treatment of the explosive’s detonation reaction zone. Current methods lack the resolution to both calculate the broad gas expansion region and model the thin reaction zone with reasonable detail. Recently an alternative method for modeling the reaction zone has been developed. This method applies when the radius of curvature of the shock is large compared to the reaction‐zone length. In this limit, the dynamics of the interaction between the chemical heat release and the two‐dimensional flow in the reaction zone is quasisteady. It is summarized by a relation Dn(κ), between the local normal shock velocity Dn and shock curvature κ. When this relation is combined with the kinematic surface condition (an equation that describes how disturbances move along the shock), the two‐dimensional reaction‐zone calculation is reduced to a one‐dimensional calculation.

"Electron acceleration at nearly perpendicular collisionless shocks, 1, One-dimensional simulations without electron scale fluctuations""

Abstract: Under certain conditions electrons can be reflected and effectively energized at quasi-perpendicular shocks. This process is most prominent close to the point where the upstream magnetic field is tangent to the curved shock. A theoretical explanation of the underlying physical mechanism has been proposed which assumes conservation of magnetic moment and a static, simplified shock profile are performed. Test particle calculations of the electron reflection process in order to examine the results of the theoretical analysis without imposing these restrictive conditions. A one-dimensional hybrid simulation code generates the characteristic field variations across the shock. Special emphasis is placed on the spatial and temporal length scales involved in the mirroring process. The simulation results agree generally well with the predictions from adiabatic theory. The effects of the cross-shock potential and unsteadiness are quantified, and the influence of field fluctuations on the reflection process is discussed.

Fast Fermi and gradient drift acceleration of electrons at nearly perpendicular collisionless shocks

Abstract: Electrons can be reflected and effectively energized at quasi-perpendicular shocks. At a curved shock this process is most prominent where the upstream magnetic field and the shock surface are tangent. A theoretical explanation of the underlying physical mechanism in terms of fast Fermi acceleration has been proposed in the literature. The theory uses properties of the de Hoffmann-Teller frame and assumes conservation of magnetic moment in a static, simplified shock profile. Here, the discussion is extended in order to clarify certain aspects of the reflection process and to pinpoint the physical mechanisms that are operative and dominant from the viewpoint of the normal incidence frame. By reducing the analysis to the essential physical content and solving the pertinent energy equation, the equivalence of fast Fermi and gradient drift acceleration is shown.

Comparative study of high-resolution shock-capturing schemes for a real gas

Abstract: Recently developed second-order explicit shock-capturing methods, in conjunction with generalized flux-vector splittings, and a generalized approximate Riemann solver for a real gas are studied. The comparisons are made on different one-dimensional Riemann (shock-tube) problems for equilibrium air with various ranges of Mach numbers, densities and pressures. Six different Riemann problems are considered. These tests provide a check on the validity of the generalized formulas, since theoretical prediction of their properties appears to be difficult because of the non-analytical form of the state equation. The numerical results in the supersonic and low-hypersonic regimes indicate that these produce good shock-capturing capability and that the shock resolution is only slightly affected by the state equation of equilibrium air. The difference in shock resolution between the various methods varies slightly from one Riemann problem to the other, but the overall accuracy is very similar. For the one-dimensional case, the relative efficiency in terms of operation count for the different methods is within 30%. The main difference between the methods lies in their versatility in being extended to multidimensional problems with efficient implicit solution procedures.

On the computation of shock shapes in nonequilibrium hypersonic flows

Abstract: Several models using different descriptions of high temperature air are used to compute the bow shock shapes on experimental model configurations The test conditions result in nonequilibrium chemical reaction and thermal excitation of the gas which has a first-order effect on the shock shapes The computed results are compared to experiment and demonstrate that the model using seven chemical species and six temperatures predicts the experimental shock shapes very well The other gas models, including perfect gas, equilibrium gas and one-temperature chemical nonequilibrium models are less accurate The results illustrate the necessity of including both thermal and chemical nonequilibrium in the description of the gas The use of these experimental data makes it possible to verify aspects of current and future chemically reacting flow algorithms

Observation of mirror waves downstream of a quasi-perpendicular shock

Abstract: We have performed auto and cross spectral analysis of the electron density and vector magnetic field fluctuations recorded aboard ISEE 1 and 2 in the magnetosheath close the Earth's bow shock when in quasiperpendicular configuration. The analysis shows the characteristic signatures of mirror type modes downstream of the undershoot of the shock. copyright American Geophysical Union 1989

Herbig-Haro objects as the heads of radiative jets

Abstract: The interpretation of certain HH objects as the heads of nonadiabatic supersonic jets is examined using two-dimensional numerical simulations. It is found that radiative jets develop a dense shell between the jet shock and the leading bow shock when the cooling distance behind either one of these shocks is smaller than the jet radius. It is proposed that the radiatively cooling shell may account for the variable emission pattern from objects like HH 1. Also, it is suggested that HH objects with measured space velocities that exceed the spectroscopically inferred shock velocities could correspond to heavy jets in which the bow shock is effectively adiabatic. Low-excitation objects in which these velocities are comparable may represent light jets where the jet shock is nonradiative.

Shock compression and release in high-strength ceramics

Abstract: Shock compression and release particle velocity data have been obtained for silicon carbide, titanium diboride, boron carbide, and zirconium dioxide with a laser velocity interferometer (VISAR). Peak impact stresses in these experiments range between 20 and 50 GPa. Iterative numerical methods were used to obtain dynamic compression and release stress-strain behavior of the ceramics. 16 refs., 4 figs., 3 tabs.

Microscopic structure at the shock in the asymmetric simple exclusion

Abstract: We study for a semi-infinite one dimensional initial distribution the asymptotic behaviour in the hydrodynamical limit at the shock. In this case the location of the shock is naturally identified by the position of the leftmost particle of the system for which we prove a central limit theorem. From this we deduce that at the shock local equilibrium does not hold

ATB shock absorber

Abstract: The headset of a conventional All Terrain Bicycle is modified to permit the load supported by the front wheel to be transferred to the bicycle frame through a column of compressed air to minimize the shock and vibration to the rider when riding over rough ground. A valve matching those in the bicycle tires is located on the centerline of the steering axis above the handle bar stem and permits the compressed air pressure to be adjusted for different rider weights through use of a conventional bicycle pump. The typical air pressure required is less than the tire pressure. This IMPROVED ATB SHOCK ABSORBER includes provisions for hydraulically damping the air spring motion. The shock absorber design should permit hard use for long periods without maintenance.

Shock load detection device

Abstract: A shock load detection device (30) mounted in a housing (12) of a computer disk drive unit (10) for measuring a mechanical shock imposed thereon and issuing a command signal to stop writing of a read/write head (26) should a predetermined force value be exceeded, includes a sensor (32) for detecting a mechanical shock load imposed thereon and generating an electrical charge proportional to the mechanical shock load. An amplifier (58, 60) is connected to the sensor (32) for producing a voltage signal proportional to the electrical charge received from the sensor. A comparator (64) is connected to the amplifier (58, 60) for measuring the voltage signal and comparing the voltage signal with a predetermined range of force values. A write-fault signal (56) is received from the comparator when the voltage signal received by the comparator falls within the predetermined range of force values for stopping the writing of the read/write head (26).

Shock absorbing pad structure for athletic equipment

Abstract: A shock absorbing pad structure for athletic equipment, such as shoulder pads, has a foam member, having an undulated configuration formed by a plurality of elevations and dperessions, arranged in a staggered relationship with respect to one another, disposed within a flexible, substantially air impermeable enclosure. The flexible enclosure has at least one air permeable portion disposed therein.

Shock absorbing unit

Abstract: The present invention relates to a new and novel device constructed so as to prevent damage to a manually operated remote control device capable of controlling the operation of a television or other electronic equipment, the shock absorbing unit being of unitary construction having at both of its ends identical cavity structures capable of encapsulating the ends of a manually operated remote control device, the cavity structures being affixed to a singular band member resulting in there being provided shock absorbing means about all surfaces of the manually operated remote control device thereby protecting same from damage.

Safety line shock absorber

Abstract: A safety line shock absorber that is designed to gradually break the fall and stop the user without the shock that occurs when the standard lanyard reaches full extension. The shock absorber incorporates the structure of a bungy cord in combination with an elongated strip of flat webbing that is folded longitudinally upon itself a plurality of times in an accordian type manner to form a plurality of lapped portions. A plurality of transverse rows of stitching across the respective lapped portions will tear apart and allow the webbing to be stretched out to its unfolded length when excessive force is applied to the longitudinal axis of the safety line shock absorber when a workman hooked up to the shock absorber unexpectedly takes a fall. The shock absorber has a cover strip that will tear anytime the stitched lapped portions of the webbing and the bungy cord has been used to gradually break a fall and reduce the shock of the last workman reaching the end of the line during a fall and ripping out the stitching of the webbing and the cover strip thus alerting the workman that it should be discarded and that it is no longer safe to use.

A validation study of four Navier-Stokes codes for high-speed flows

Abstract: A code validation study has been conducted for four different codes for solving the compressible Navier-Stokes equations. Computations for a series of nominally two-dimensional high-speed laminar separated flows were compared with detailed experimental shock-tunnel results. The shock wave-boundary layer interactions considered were induced by a compression ramp in one case and by an externally-generated incident shock in the second case. In general, good agreement was reached between the grid-refined calculations and experiment for the incipient- and small-separation conditions. For the most highly separated flow, three-dimensional calculations which included the finite-span effects of the experiment were required in order to obtain agreement with the data. The finite-span effects were important in determining the extent of separation as well as the time required to establish the steady-flow interaction. The results presented provide a resolution of discrepancies with the experimental data encountered in several recent computational studies.

Inception length to a fully-developed fin-generated shock wave boundary-layer interaction

Abstract: An experimental study of fin-generated shock wave turbulent boundary-layer interactions confirmed previous observations that, sufficiently far from the fin apex, such interactions become conical. The inception length to conical symmetry was found to increase weakly with Mach number for Mach numbers from 2.5 to 4 and fin angles from 4 to 22 deg. For the range of interactions examined, the inception length was found to depend primarily upon the inviscid shock angle, this angle ranging from 21 to 40 deg. The behavior of the inception length with shock angle can be broadly divided into two categories. For 'weak' interactions with shock angles less than about 35 deg, the inception length decreased as the shock angle increased. For 'strong' interactions with shock angles greater than about 35 deg, the inception region was small and was approximately constant at three boundary-layer thicknesses in length. In the latter, strong interaction case, the inception length was an order of magnitude smaller than that found in the weakest interactions examined, to the extent that strong interactions were practically fully-developed from the apex.

Femtosecond optical shocks and wave breaking in fiber propagation

Abstract: Detailed analysis is presented of the recent observation [ Phys. Rev. Lett.62, 531 ( 1989)] and new measurements of the formation of an optical intensity shock and the subsequent wave breaking in the nonlinear propagation of visible 1-psec pulses in an optical fiber. The observed shock has a duration of 150 fsec, and the wave breaking is manifested by 400-fsec oscillations in the wake of the shock. The observations are compared with numerical integrations of the nonlinear Schrodinger equation (NLSE). The calculations generally agree with the measurements; however, two measured features are predicted neither by the NLSE nor by the NLSE when it is modified to include higher-order effects. The measured features are explained by the presence of small satellites and asymmetry in the input pulse.

Spring element for a shock isolating mount

Abstract: A spring element for a shock isolating mount is adapted for multiple assembling and disassembling. The spring element comprises an annular segment having holding tabs extending radially inwardly from one end thereof; and aligning tabs which are shorter and wider than the holding tabs, and which extend radially inwardly from the same end of the annular segment.

Two-temperature radiative shocks with electron thermal conduction

Abstract: The influence of electron thermal conduction on radiative shock structure is studied for both one- and two-temperature plasmas. The dimensionless ratio of the conductive length to the cooling length determines whether or not conduction is important, and shock jump conditions with conduction are established for a collisionless shock front. Approximate solutions are obtained, with the assumptions that the ionization state of the gas is constant and the cooling rate is a function of temperature alone. In the absence of magnetic fields, these solutions indicate that conduction noticeably influences normal-abundance interstellar shocks with velocities 50-100 km/s and dramatically affects metal-dominated shocks over a wide range of shock velocities.