Showing papers on "Shock (mechanics) published in 1988"

Electron heating and the potential jump across fast mode shocks

Abstract: Two different methods were applied to determine the cross-shock potential jump in the de Hoffmann-Teller reference frame, using a data set that represented 66 crossings of the terrestrial bow shock and 14 interplanetary shocks observed by various ISEE spacecraft, and one crossing each of the Jovian bow shock and the Uranian bow shock made by the Voyager spacecraft. Results for estimates of the electrostatic potential based on an estimate of the jump in electron enthalpy correlated well with estimates based on Liouville's theorem, although the Liouville-determined values were systematically the higher of the two, suggesting that significant irreversible processes contribute to the shape of the downstream distribution. The potential jump corresponds to approximately 12-15 percent of the incident ion ram kinetic energy, and was found not to be controlled by the Mach number, plasma beta, shock geometry, or electron to ion temperature ratios.

A mechanism for strong shock electron heating in supernova remnants

Abstract: It is shown that collisionless shock waves propagating away from a supernova may be directly responsible for the 10 keV X-ray emission seen in supernova remnants. A sequence of plasma instabilities (Buneman and ion acoustic) between the reflected and/or transmitted ions and the background electrons at the foot of the shock front can give rise to rapid anomalous heating of electrons. Hybrid simulations of a perpendicular collisionless shock are presented to demonstrate that this heating can arise within a self-consistently computed shock structure.

High‐speed photography of surface geometry effects in liquid/solid impact

Abstract: Recent theoretical studies of liquid/solid impact, in particular the geometric wave theory of Lesser and Field, have emphasized the importance of the detailed geometry in the contact area. In parallel with the theoretical work, we have developed a two‐dimensional technique using gels for impact and shock studies. A combination of high‐speed photography and schlieren optics allows the shocks in the liquid and solid, if it is transparent, to be visualized, as well as important processes, such as jetting, to be recorded. This paper describes the gel technique and gives results for a range of surface geometries for collision velocities of a few hundred meters per second. The relevance to damage initiation in liquid/solid impact problems, such as rain erosion, steam turbine blade erosion, and cavitation, are discussed.

Shock absorbing device

Abstract: A shock absorbing device comprising: an upper impermeable sheet; a lower impermeable sheet; an intermediate flexible impermeable sheet which is deformed to provide a grid of spaced peaks projecting on each surface thereof, the peaks being fixed to the upper and lower sheets to form two chambers of interconnected pockets, the upper one chamber being filled with a non-compressible liquid and the lower other chamber being filled with a compressible gas, the liquid and the gas being sealed therein.

Studies of aerothermal loads generated in regions of shock/shock interaction in hypersonic flow

Abstract: Experimental studies were conducted to examine the aerothermal characteristics of shock/shock/boundary layer interaction regions generated by single and multiple incident shocks. The presented experimental studies were conducted over a Mach number range from 6 to 19 for a range of Reynolds numbers to obtain both laminar and turbulent interaction regions. Detailed heat transfer and pressure measurements were made for a range of interaction types and incident shock strengths over a transverse cylinder, with emphasis on the 3 and 4 type interaction regions. The measurements were compared with the simple Edney, Keyes, and Hains models for a range of interaction configurations and freestream conditions. The complex flowfields and aerothermal loads generated by multiple-shock impingement, while not generating as large peak loads, provide important test cases for code prediction. The detailed heat transfer and pressure measurements proved a good basis for evaluating the accuracy of simple prediction methods and detailed numerical solutions for laminar and transitional regions or shock/shock interactions.

Shoe sole having compressible shock absorbers

Abstract: This is a sole with compressible shock absorbers which not only provides an improved shock absorbing function but also provides more comfort for the wearer. The shock absorbers consist of a polygonal replaceable air bellows placed in a polygonal recess on the forefoot section of the shoe and a cylindrical replaceable air cylinder placed in a circular recess on the heel section of the shoe. The air cylinder and air bellows are made of an integrally resilient air-tight material such that, during exercise, the shock absorbers absorb and then return the energy in a controlled upward direction, by way of the bellows-type body and the friction caused between the inside wall of the recess and the bellows itself. Moreover, since the shock absorbers can be placed easily by hand, the wearer can adjust the shoes in line with his weight and type of sport performed. The uses of the shoe are therefore maximized because the air cylinder and air bellows will continue to function even after the outer sole has experienced considerable wear and tear.

Acceleration of cosmic rays by shock waves

Abstract: Theoretical work on various processes by which shock waves accelerate cosmic rays is reviewed. The most efficient of these processes, Fermi acceleration, is singled out for special attention. A linear theory for this process is presented. The results found on the basis of nonlinear models of Fermi acceleration, which incorporate the modification of the structure caused by the accelerated particles, are reported. There is a discussion of various possibilities for explaining the generation of high-energy particles observed in interplanetary and interstellar space on the basis of a Fermi acceleration mechanism. The acceleration by shock waves from supernova explosions is discussed as a possible source of galactic cosmic rays. The most important unresolved questions in the theory of acceleration of charged particles by shock waves are pointed out.

Research in Applied Mathematics.

Abstract: : Most of the work has been on shock dynamics, a term we use for problems of the focusing of curved shocks, the diffraction of shocks by bodies or density layers, the propagation of shocks down curved tubes and channels, and the stability of converging shocks. Our earlier theoretical work is described in the book Linear and Nonlinear by G.B. Whitham, and references given there. This theoretical work on shock dynamics had been found by experimenters to be extremely useful in practical situations. However, the analytic results had been limited to fairly simple situations. The numerical scheme originally proposed was again limited and could not hope to handle some of the interesting practical situations. Keywords: Shock waves, Solitons, Roll waves; Nonlinear wave propagation; Numerical methods.

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.

Propagation of weak shocks through a random medium

Abstract: The propagation of weak shock waves (M_s = 1.007, 1.03 and 1.1) through a statistically uniform random medium has been investigated experimentally in a shock tube. The wave-from geometry, rise time and amplitude of initially plane shocks which have propagated through a random mixture of helium and refrigerant 12 are measured. The effect of shock propagation on the properties of the random medium is visualized with schlieren and shadow photography. The pressure histories of the distorted shock waves reflecting from a normal end wall are observed to be both peaked and rounded. In the rounded case the perturbed shock is found to be made up of a succession of weak, slightly curved fronts with a total effective rise time orders of magnitude greater than the classical Taylor thickness. The radius of curvature of the weakest shocks after propagating through the random medium is inferred from observations at two downstream stations to be about 7 times the integral scale of the gas inhomogeneities. It is concluded that the observed distortions of the wave fronts can best be explained in terms of random focusing and defocusing of the front by the inhomogeneities in the medium. A ray-tracing calculation has been used to interpret the experimental observations. It is found that geometrical considerations are sufficient to account for many of the effects observed on the shocks.

Shock waves in water

Abstract: The present paper gives a comprehensive examination of equations and charts for shock waves in water. The Tait equation of state is used in the analysis to describe the compressibility of water. The results for an oblique shock wave are presented in the hodograph plane where all possible velocity vectors downstream of the shock wave are seen to lay on a single curve (i.e., shock polar). The equation of the shock polar and the corresponding charts up to upstream Mach number of 3 are given, as well as a more convenient way of adapting the shock polar. The oblique shock‐wave detachment condition and the sonic flow condition downstream of a shock are also presented.

Colliding plasma structures: Current sheet and perpendicular shock

Abstract: They authors present results of a one-dimensional simulation of the interaction of a current sheet and a supercritical perpendicular shock in a collisionless plasma. The low-field region of the current sheet passes through the shock, allowing, for a short time, the complete reflection of the incident flow at the transition of the magnetic field to its downstream value. The counterstreaming ion beams relaunch the shock, while the low-field region remains downstream. The plasma in the low field region is unshocked but develops a complex multistream configuration, which in time develops to a more uniform distribution. They discuss the implications of the simulation results for observations within the magnetosheath and of active current sheets upstream of the bow shock.

The Hugoniot and shock sensitivity of a plastic‐bonded TATB explosive PBX 9502

Abstract: The plane shock response of PBX 9502 was measured from 0.5 to 25 GPa. The explosive is 95‐wt. % triamino‐trinitrobenzene powder coated with Kel‐F 800 plastic; porosity was 2.6%. Shock initiation sensitivity, the Hugoniot, and dynamic yield behavior were studied. Experiments done were explosively driven wedge tests and particle velocity history measurements using electromagnetic gauges at a light‐gas gun. A new analysis of the wedge test was implemented. Based on wedge test results, an approximation for the particle velocity at the shock front is constructed, which is used in functional composition with the Hugoniot to calculate the shock path of each experiment. Optimization on the parameters of the approximation and the Hugoniot simultaneously ensures agreement of the Hugoniot with the overall kinematics of the shock growth as well as the particle velocity/shock velocity data. The individual shock paths are used to examine the single‐curve buildup assumption; the assumption is not valid for PBX 9502. A ramping elastic precursor of 0.073 GPa strength was observed. The yield and weak shock behavior are interpreted in terms of porosity, crystalline anisotropy, and residual strain in the pressed, plastic‐bonded explosive.

Anisotropic phase transition of rutile under shock compression

Abstract: Shock recovery experiments for single crystal and powdered specimens of TiO2 with the rutile structure were performed in the pressure range up to 72 GPa. Single crystal specimens were shocked parallel to [100], [110] and [001] directions. X-ray powder diffraction analysis showed that the amount of α-PbO2 type TiO2 produced by shock-loading depended strongly on the shock propagation direction. The maximum yield (about 70%) was observed for shock loading to 36 GPa parallel to the [100] direction. In the [001] shock direction, the yield is much smaller than that of the [100] direction. This anisotropic yield was consistent with the observed anisotropy of the phase transition pressure in shock compression measurements. However, transformation to the α-PbO2 type cannot explain the large volume change observed above about 20 GPa. On the basis of the high pressure behavior of MnF2, we assumed that the high pressure phase was either fluorite or distorted fluorite type and that the phase conversion to the α-PbO2 type was induced spontaneously in the pressure reduction process.

Acceleration of cosmic rays in the solar wind termination shock. I. A steady state technique in a spherically symmetric model

Abstract: In this paper, it is shown that a steady state model for the combined acceleration and modulation of cosmic rays in the heliosphere is possible. In this first of a series of papers, a spherically symmetric heliosphere with a termination shock at 50 AU is adopted. It is shown that the accelerated spectrum develops naturally from the shock boundary condition and that the low-energy source of particles to be accelerated need not be specified explicitly. The demonstration solutions on the shock are the expected power laws, and it is shown where shock curvature and acceleration time scale effects set in. Some of the basic features of the anomalous cosmic-ray component are explained by the solutions, but, as expected, a detailed comparison with observations must await more extensive and realistic models. 25 references.

Critical energy criterion for the shock initiation of explosives by projectile impact

Abstract: The critical energy (Ec) criterion developed by Walker and Wasley for predicting the response of bare, heterogeneous explosives to flying-plate impact, is modified for application to flat-nosed rods and spherical projectiles. This modification redefines the shock duration term used in calculating Ec and implies that a minimum volume of explosive at a given shock energy is needed before initiation occurs. The modification also changes the L/D ratio of the projectile at which plate behaviour changes to rod behaviour from the currently accepted value of approximately 1/4 to 1/12. Comparisons between the modified criterion and published experimental data show that the same value of Ec can now be obtained from both flying-plate and more complex projectile impacts into the same explosive.

Seat sliding apparatus for vehicles

Abstract: A seat sliding apparatus for attachment to a vehicle comprises at least one lower rail fixedly secured to the vehicle; a corresponding upper rail slidably mounted on the lower rail, the upper rail being subject to a shock force in a direction separating the upper rail and the lower rail during operation of the vehicle; bearing device between the upper and lower rails for supporting the upper rail on the lower rail; and interlocking safety device on the upper and lower rails for maintaining substantial contact between the upper and lower rails and the bearing device when the shock force occurs.

Waves in liquids with vapour bubbles

Abstract: An investigation of wave processes in liquids with vapour bubbles with interphase heat and mass transfer is presented. A single-velocity two-pressure model is used which takes into account both the liquid radial inertia due to medium volume changes, and the temperature distribution around the bubbles. An analysis of the microscopic fields of physical parameters is aimed at closing the system of equations for averaged characteristics. The original system of differential equations of the model is modified to a form suitable for numerical integration. An elliptic equation is obtained to determine the field of the mixture average pressure at an arbitrary time through the known fields of the remaining quantities. The existence of the steady structure of shock waves, either monotonic or oscillatory, is proved. The effect of the initial conditions, shock strength, volume fraction, and dispersity of the vapour phase and of the thermophysical properties of the phases on shock-wave structure and relaxation time is studied. The influence of nonlinear, dispersion and dissipative effects on the wave evolution is also investigated. The shock adiabat for reflected waves is analysed. The results obtained have proved that the interphase heat and mass transfer determined by the thermal diffusivity of the liquid greatly influences the wave structure. The possible enhancement of disturbances in the region of their initiation is shown. The model has been tested for suitability and the results of calculations have been compared with experimental data.

The effectiveness of shock-absorbing insoles during normal walking

Abstract: This paper describes a study of the effectiveness of commercially available shock absorbing insoles when used in four different pairs of shoes during normal walking. The measurement method was based on the use of the Fourier Transform of the axial acceleration of the leg measured by an accelerometer mounted at the ankle. The magnitude of shock was measured by the "Shock Factor" which has been defined as the rms acceleration between 50 Hz and 150 Hz expressed as a proportion of that between 10 Hz and 150 Hz. Nine insoles were tested in each pair of shoes and the Shock factor for each combination was compared with the value obtained for the shoes alone. Statistically significant reductions of Shock Factor were noted in 58% of cases; the largest improvement (30% reduction in Shock Factor) was achieved by lightweight Sorbothane. The experimental technique has now been further developed to allow the measurement of Shock Factor by a portable Shock Meter.

Shock structure in classical magnetohydrodynamics

Abstract: How the structure of coplanar MHD fast and slow shocks depends upon the relative magnitudes of the dissipation coefficients contained in classical magnetohydrodynamics is examined. An asymptotic method is used in which the scale lengths for resistivity, thermal conduction, and viscosity are widely separated, and the qualitative dependence of the shock solutions upon the ordering of the scales is studied. Upper limit Mach numbers for both fast and slow shocks are defined at which resistivity and thermal conduction taken together can provide all the shock dissipation and at which viscosity is not needed.

Shock absorber for a bow mounted camera

Abstract: This invention relates to a shock absorption device for absorbing recoil shock when a camera is mounted to an archery bow. The present invention includes a first mechanism which operates to absorb shock in a generally horizontal plane when the shooting device is being used for shooting and a second mechanism which operates to absorb shock in a generally vertical direction when the shooting device is being used. The two mechanisms operate independently of one another. The mechanism operating in the horizontal plane uses first and second planar members which have polished, greased, flat surfaces abutting each other in a mating relation which members are held together with a wide elastic band tightly encasing the perimeter of the two members. The two members slide relative to one another when the shock force is applied, and after the shock force subsides, the elastic band restores the two members to their normal static resting position. The second absorption mechanism includes a member connected to the bow and another member connected to the camera. The two members are hinged together and are positioned one above the other with a foam-like material sandwiched between them. This mechanism absorbs shock forces which occur in a general vertical direction when the bow is being used.

Shock absorber for a car

Abstract: An apparatus for absorbing and relieving the shock inflicted from outside at the time of collision of a car which covers the majority of the covering portions that may be contacted from outside by tubular air sheets in which pressed air or helium gas is filled. Each of the front and rear bumpers (10, 20) comprises three bumper members (11, 12, 13), each for low, medium and high pressure shock absorption with different volumes and gas capacity. Each of the front and rear bum­ pers and the left and right side panels (41, 42, 43, 44, 60) are interconnected by longitudinal and transverse gas circulation pipes (30, 31) so that the gas of each air sheet can be dispersed when the shock is inflicted, thereby enhancing the shock ab­ sorption effect thereof. The gas circulation tubes are interconnected by a gas circulation controller (50) which controls the circulation of gas.

Shock absorber for mountain bicycles

Abstract: The headset of a conventional mountain 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 shock and vibration to the rider when riding over rough ground. A valve matching those in the mountain bicycle tires is located on 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 with the shock absorber unloaded is less than the tire pressure. The shock absorber design should permit hard use for long periods without maintenance.

Principles of measurement and instrumentation

Abstract: Instrument classification and characteristics measurement system errors instrument calibration signal processing, manipulation and transmission bridge circuits measurement of electrical signals data recording and presentation fibre optic sensors and transmission systems intelligent instruments instrumentation/computer networks temperature pressure flow level dimension measurement translational displacement transducers translational velocity acceleration, vibration and shock measurement rotational displacement transducers rotational velocity mass, force and torque measurement measurement of miscellaneous quantities.

Shock absorbing structure

Abstract: The present invention provides a shock absorbing structure made of a rubber-like elastic material having a hardness in the range of 5 to 60, according to a JIS-A type hardness meter and being composed of a base and a plurality of ridges or projections spaced apart in parallel on said base The shock absorbing structure according to the present invention is suitable as a shock absorbing material, especially, for shoes

Evolution of recurrent solar wind structures between 14 AU and the termination shock

Abstract: The solar wind conditions observed from Voyager 2 at approximately 14 AU are extrapolated to the region of the outer heliosphere bounded by the termination shock, using an MHD simulation model. Results from two simulation studies are presented for two sets of nearly recurrent solar wind interaction regions, with initial conditions generated from plasma and magnetic field data observed on March 1984 at 13.8 AU, and on November 1984 at 15.4, respectively. Each simulation describes an idealized recurrent solar wind structure in the supersonic region of the outer heliosphere out to the termination shock far beyond the present reaches of the Pioneer and Voyager spacecraft. It is shown that a collision between the forward shock and the reverse shock occurs approximately every 40 AU. When a forward shock interacts with the termination shock, the latter is weakened and moves outward; the termination shock is strengthened and moves inward when a reverse shock interacts with it.