Showing papers on "Shock (mechanics) published in 1984"

Propagation of nonlinear compression pulses in granular media

Abstract: The study of mechanics of a granular medium is of substantial interest, both scientifically and for the solution of applied problems. Such materials are, for example, good buffers for shock loads. Their, study is important for the development of processes of the pulse deformation of several porous materials. A review of studies of small deformations and elastic wave propagation in these media was carried out in [i] on the basis of discrete models. The structure of a stationary shock wave was analyzed in [2] as a function of its amplitude. i. Statement of the Problem. The problem of nonstationary, nonlinear perturbations in one-dimensional granular media is stated in the present paper on the basis of the wellknown interaction between neighboring granules. As an interaction law we choose the Hertz law [3]

The adiabatic energy change of plasma electrons and the frame dependence of the cross‐shock potential at collisionless magnetosonic shock waves

Abstract: The adiabatic energy gain of electrons in the stationary electric and magnetic field structure of collisionless shock waves was examined analytically in reference to conditions of the earth's bow shock. The study was performed to characterize the behavior of electrons interacting with the cross-shock potential. A normal incidence frame (NIF) was adopted in order to calculate the reversible energy change across a time stationary shock, and comparisons were made with predictions made by the de Hoffman-Teller (HT) model (1950). The electron energy gain, about 20-50 eV, is demonstrated to be consistent with a 200-500 eV potential jump in the bow shock quasi-perpendicular geometry. The electrons lose energy working against the solar wind motional electric field. The reversible energy process is close to that modeled by HT, which predicts that the motional electric field vanishes and the electron energy gain from the electric potential is equated to the ion energy loss to the potential.

Monte Carlo shock-like solutions to the Boltzmann equation with collective scattering

Abstract: The results of Monte Carlo simulations of steady state shocks generated by a collision operator that isotropizes the particles by means of elastic scattering in some locally defined frame of reference are presented. The simulations include both the back reaction of accelerated particles on the inflowing plasma and the free escape of high-energy particles from finite shocks. Energetic particles are found to be naturally extracted out of the background plasma by the shock process with an efficiency in good quantitative agreement with an earlier analytic approximation (Eichler, 1983 and 1984) and observations (Gosling et al., 1981) of the entire particle spectrum at a quasi-parallel interplanetary shock. The analytic approximation, which allows a self-consistent determination of the effective adiabatic index of the shocked gas, is used to calculate the overall acceleration efficiency and particle spectrum for cases where ultrarelativistic energies are obtained. It is found that shocks of the strength necessary to produce galactic cosmic rays put approximately 15 percent of the shock energy into relativistic particles.

The energy spectrum of 35‐ to 1600‐keV protons associated with interplanetary shocks

Abstract: We present the results of a statistical study on the proton energy spectra in the range of 35–1600 keV during the one-hour interval centered on the time of arrival of the shock front at the spacecraft of 75 interplanetary shocks that cover the period from August 1978 until December 1980, using the low-energy proton experiment on ISEE 3. The strength of the shocks was determined by calculating the ratio of the downstream to upstream plasma density by using the ion data obtained by the Los Alamos solar wind instrument. The shock events were sub-divided into four different classes based on the behavior of their low-energy (35–238 keV) spectral index. The signatures of the shock events and their spectral index-time profiles in the different classes are (1) smooth profiles associated with oblique, strong, and fast shocks, which roughly corresponds with predictions following from diffusive shock acceleration theory; (2) irregular profiles mainly associated with quasi-perpendicular shocks and (3) spikelike profiles associated with quasi-perpendicular shock spike events, where the properties within both classes point at predominant shock drift acceleration; (4) flat profiles, mainly associated with weak shocks accompanied by little or no shock-accelerated particles. Strong and fast oblique shocks are found to be the most effective particle accelerators. The spectrum at the shock can generally be described by two power laws with a breakpoint energy near 250 keV. For only 15% of the events the spectrum followed a power law over the full energy range. We found that the low-energy spectral index, measured immediately downstream of shocks associated with clear flux enhancements, is related to the shock strength according to predictions from first-order Fermi acceleration, irrespective of the assigned diffusive or drift character of the event.

Collisionless dissipation in quasi‐perpendicular shocks

Abstract: Microscopic dissipation processes in quasi-perpendicular shocks are studied by two-dimensional plasma simulations in which electrons and ions are treated as particles moving in self-consistent electric and magnetic fields. Cross-field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.

Numerical Computations of Turbulence Amplification in Shock-Wave Interactions

Abstract: Numerical computations are presented which illustrate and test various effects pertinent to the amplification and generation of turbulence in shock wave-turbulent boundary layer interactions. Several fundamental physical mechanisms are identified. Idealizations of these processes are examined by nonlinear numerical calculations. The results enable some limits to be placed on the range of validity of existing linear theories. Additional results are given which are of a fundamentally nonlinear nature.

Shock hazard protection system

Abstract: New device for protecting human life and property against electrical shock and more specifically to a shock hazard prevention system for disconnecting an electrical load from an electrical source when a shock hazard condition exists within the load. According to the invention, the installation comprises a source operatively connected to a load by first (110) and second (120) conductors, a detector (200) associated with the load, a control circuit (300) connected to the detector by a sensing or third conductor (130), and an interruptor circuit (400) associated with the source and connected to the control circuit (300).

Plasma and energetic particle structure upstream of a quasi-parallel interplanetary shock

Abstract: ISEE 1, 2 and 3 data from 1978 on interplanetary magnetic fields, shock waves and particle energetics are examined to characterize a quasi-parallel shock. The intense shock studied exhibited a 640 km/sec velocity. The data covered 1-147 keV protons and electrons and ions with energies exceeding 30 keV in regions both upstream and downstream of the shock, and also the magnitudes of ion-acoustic and MHD waves. The energetic particles and MHD waves began being detected 5 hr before the shock. Intense halo electron fluxes appeared ahead of the shock. A closed magnetic field structure was produced with a front end 700 earth radii from the shock. The energetic protons were cut off from the interior of the magnetic bubble, which contained a markedly increased density of 2-6 keV protons as well as the shock itself.

Forced oscillation experiments in supercritical diffuser flows

Abstract: Low-frequency oscillations induced in ramjet inlets by combustion instabilities were simulated by mechanically modulating the exit area of a two-dimensional, supercritical diffuser at frequencies up to 330 Hz. Boundary layers were attached below a terminal shock Mach number of 1.27, and shock-induced separation occurred above this value up to the experimental limit of 1.35. Shock position histories were obtained and streamwise distributions of static/total dynamic pressures were determined both on the wall and within the flow for various shock strengths and frequencies. Excitation at the natural frequencies of the shock motion produced no obvious resonance effects. For weak shocks, the perturbations and their reflections from the shock are reasonably approximated by one-dimension al, acoustic considerations, but this description fails for strong shocks.

Structure of the November 12, 1978, quasi‐parallel interplanetary shock

Abstract: The jump in plasma parameters exhibited by the intense interplanetary shock event of Nov. 12, 1978 is analyzed using ISEE 1, 2 and 3 data. Magnetic and electric field measurements indicated that the shock magnetic field profile was similar to the earth bow shock profile. Data on the electron and proton densities, temperatures, bulk velocities and alpha particles showed a steady electron temperature increase across the shock on a 12 earth radii scale. The upstream and downstream flow parameters are found to be within 10 percent of Rankin-Hugoniot jump conditions. The shock moved at 614 km/sec and had three dissipative scales, one a few Larmor radii determined by the magnetic field jump, a second 10 earth radii correlated with the electron equilibrium and the other 30 earth radii connected to the energetic proton foreshock.

SHAPESET: A process to reduce sidewall curl springback in high-strength steel rails

Abstract: High-strength steel rails for structural members can reduce vehicle weight, but sidewall curl springback in these rails causes assembly difficulties with adjacent parts. A process to greatly reduce curl springback, known as SHAPESET, was investigated for a variety of high-strength steels, including two dual-phase steels. Gridded blanks were stamped by the two-step SHAPESET process and by conventional methods. The SHAPESET process nearly eliminated sidewall curl and was not sensitive to material strength. The first step of SHAPESET stamps a preform to eliminate the shock line, which restricts the spread of strain in the sidewall. The second step firmly clamps the preform flanges, in order to strain the part to the final shape. In comparison, the curl springback could only be reduced in a conventional stamping process by increasing the drawbead restraining force, which tended to produce failure by splitting.

Gradual hard X-ray events and second phase particle acceleration

Abstract: In the second phase acceleration process the close time coincidence between the gradual hard X-ray burst and the type II shock wave is presumed due to shock acceleration of the electrons producing the gradual phase burst. We point out that recent studies of gradual hard X-ray bursts place the source heights well below the heights of 2–10 × 105 km traversed by the shock. Gradual phase energetic electrons therefore cannot be accelerated in the shock but must be produced elsewhere. We propose the loop systems of long decay X-ray events (LDEs) as the sites of the gradual phase electron production.

High-pressure quasi-isentropic impact experiments

Abstract: A method has been developed for generating quasi-isentropic compression waves in flat plate impact experiments. Peak stresses can be in the multimegabar range, thus allowing investigations of material properties in a high-pressure, low-temperature regime which is inaccessible either quasi-statically or by conventional shock wave experiments. The key to the technique is a thin impactor with a smooth but strong gradient in shock impedance. Novel impactor fabrication and testing methods have been developed to attain the excellent uniformity and relatively close control of the shock impedance profile which are needed. In using the method, it is normally desirable to obtain time-resolved measurements of either pressure or particle velocity at two distances from the impact surface. The stress-volume loading path followed by the specimen can then be obtained from a centered-wave analysis, provided certain assumptions are valid. The data and results of several experiments are presented.

Acoustic shock generation by ultrasonic imaging equipment

Abstract: The pulses generated by ultrasonic imaging equipment have been observed to form acoustic shocks in water within a range of a few centimetres under normal operating conditions. The commonly held view of pulse propagation from ultrasonic imaging equipment is that the acoustic pulse has the form of a damped sine wave which will project largely unchanged in waveform. Any waveform changes which do occur result from diffraction effects and from the scattering and attenuation properties of tissue. The theory on which this understanding is based assumes that propagation laws are linear. This paper presents experimental evidence that this assumption is quite invalid at the pressures generated by commercial pulse-echo imaging equipment in common use. Measurements in water of the pulse waveforms using a calibrated broad-band polymer hydrophone have demonstrated that pulse distortion and shock formation commonly occur due to the inherent nonlinearity of the propagation medium. This fact must be considered du...

Distribution properties of the system failure time in a general shock model

Abstract: In this paper we study some distribution properties of the system failure time in general shock models associated with correlated renewal sequences ( X n , Y n ) . Two models, depending on whether the magnitude of the nth shock X n is correlated to the length Y n of the interval since the last shock, or to the length of the subsequent interval to the next shock, are considered. Sufficient conditions under which the system failure time is completely monotone, new better than used, new better than used in expectation, and harmonic new better than used in expectation are given for these two models.

The forward‐reverse shock pair at large heliocentric distances

Abstract: An unsteady one-dimensional numerical magnetohydrodynamic (MHD) model is developed in order to study the essential physical processes involved in the development of the forward-reverse shock pair in the heliosphere. In the model, MHD shocks are treated as boundary surfaces which divide the domain of interest in the r-t plane into several flow regions. The positions of the shock boundary surfaces between two neighboring flow regions are determined by shock speed. On the basis of integrations of the model, it is found that the strong MHD disturbances generated in a corotating interaction region (CIR) propagate at a fast speed relative to the moving material, and that the wave propagation speed is greater in CIR than in its surroundings. This causes disturbances in CIR to pile up and form a shock pair. The newly formed shock pair will in turn propagate outward from the leading edge to interact with ambient rarefaction regions. This interaction accounts for the double sawtooth configuration observed in velocity profiles of shock pairs. It is also demonstrated that the merging of two shocks produces a stronger shock and constant surface on its backside. Computer generated velocity profiles based on the model are presented.

Scale lengths in quasi-parallel shocks

Abstract: Examples of an interplanetary and the bow shock illustrate the small relative size of the electrostatic layer relative to the scale of the magnetic fluctuations in quasi-parallel shocks. While both examples are supercritical, the interplanetary example is marginally so, showing a thickness in absolute and convected ion larmor radii units that is thicker (approximately 13 U/omega sub ci) than at the bow shock (approximately omega sub ci). The fluid speed changes abruptly in the quasi-parallel shock on this shorter scale. The increase in electron and ion random energies also is clearly seen on this shorter scale. In the interplanetary example the scale of the electric layer is certainly less than 1/60th that of the up or downstreams magnetic fluctuations. The thickness of the earth's bow shock deceleration layer is dramatically narrower than any domain of upstream waves as controlled by reflected, intermediate, or diffuse ions.

Active and Passive Shock/Boundary Layer Interaction Control on Supercritical Airfoils,

Abstract: : This paper presents experimental investigations which aimed at improving the off-design performance of supercritical airfoils by active or passive control of the shock/boundary layer interaction (SBLI) through boundary layer suction in the shock region or ventilation respectively. The experiments were carried out in the DFVLR 1m x 1m transonic windtunnel Gottingen, using the advanced supercritical airfoil VFW VA-2 designed to have a largely fixed shock position at off-design conditions. The basic model was equipped with an exchangeable control device within the shock region to allow measurements with either surface clean, suction through a single slot, double slot or perforated strip or ventilation through a double slot or perforated strip. The effectiveness of the different SBLI control methods is evaluated from surface pressure distribution, wake and boundary layer measurements as well as Schlieren observations. It is shown that local boundary suction in the shock region mainly delays the shock-induced separation to greater shock strength and stabilizes the shock in its rearward position up to higher incidence, resulting in substantial improvements in the airfoil characteristics at off-design conditions. Moreover, without any suction, favorable passive effect is observed by boundary layer ventilation on the double slot and perforated strip configurations with a plenum underneath. This leads to a weakening of the shock wave, offering a large potential for off-design drag reduction.

Shoe incorporating shock absorbing partially liquid-filled cushions.

Abstract: A pair of thin-walled hollow partially liquid-filled cushions (51, 52) are enclosed in cavities (71, 72) formed in the sole (66) of a shoe. The first cushion (51) is positioned to coincide with the plantar pads on the lower sides of the wearer's metatarsals (23); the second (52) to coincide with the tuberosity of the wearer's calcaneum (24).

The dynamics and upstream distributions of ions reflected at the Earth's bow shock

Abstract: Using test particle trajectories in a simplified model of a supercritical, oblique, collisionless shock, we investigate the interaction of solar wind thermal ions with the earth's bow shock. We present results for shocks with shock angle (θBn) between 35° and 60° and show that their velocity space signatures are consistent with observations of many upstream ion events, particularly gyrating ions and field-aligned ion beams. The shock is modeled as a finite, planar discontinuity in the magnetic field and in the electrostatic potential, which is also given an overshoot at the shock. We find that the shock can produce backstreaming ions by reflecting a small fraction of the incident distribution. The reflected particle trajectories are of three types: specularly reflected, multiple traversal, and multiple bounce. All backstreaming reflected ions are found to suffer specular reflection on their first encounter with the shock surface; their subsequent behavior is determined by the angle θBn and their initial conditions.

Shock wave properties of anorthosite and gabbro

Abstract: Shock wave experiments have been conducted on San Gabriel anorthosite and San Marcos gabbro to peak stresses between 5 and 11 GPa using a 40-mm-bore propellant gun. Particle velocity wave profiles were measured directly at several points in each target by means of electromagnetic gauges, and Hugoniot states were calculated by determining shock transit times from the gauge records. The particle velocity profiles yielded sound velocities along the release adiabats which indicate a retention of shear strength upon shock compression for anorthosite, with a loss of strength upon release to nearly zero stress. Sound velocities of anorthosite shocked to peak stresses between 6 and 10 GPa were measured to be between 5.1 and 5.3 km/s upon release to nearly zero stress as compared to ∼6.9 and 5.4 km/s for the expected longitudinal and bulk wave speeds. Stress density release paths in the anorthosite indicate possible transformation of albite to Jadeite + (quartz or coesite), with the amount of albite transformed ranging from as low as 0.05 to as much as 0.19 mass fraction in the 6–10 GPa shock stress range. Electrical interference effects precluded the determination of accurate release paths for San Marcos gabbro. Because of the apparent loss of shear strength during unloading from the shocked state, the fluidlike rheology of anorthosite which is indicated implies that calculations of energy partitioning due to impact onto planetary surfaces based on elastic-plastic models will underestimate the amount of internal energy deposited in the impacted surface material.

Rapid-tuning frequency-doubled ring dye laser for high resolution absorption spectroscopy in shock-heated gases

Abstract: This letter describes a ring dye laser system and its initial application to fully resolved absorption line shape measurements of the OH molecule during the brief test time available following shock waves generated in a shock tube. The operating wavelength has been extended into the UV through intercavity second harmonic generation.(AIP)

Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

Abstract: Teleseismic long-period body waves from the 24 February 1981 Gulf of Corinth earthquake and its two principal aftershocks of 25 February (02h35m) and 4 March (21h58m) 1981 are studied to determine source characteristics Focal mechanisms, along with observed surface fault breaks, suggest that the Corinth earthquake sequence represents normal faulting due to the N-S trending extension Depths of the three shocks, estimated by matching synthetic seismograms to observations, are found to lie between 4 and 12 km The azimuthal variation of observed body-wave duration indicates that the main shock is a multiple event and that the main rupture occurred about 3 to 4 sec after a relatively small foreshock and propagated toward the W-NW Seismic moments deduced from the body-wave synthetics are 81 × 10 25 , 27 × 10 25 , and 22 × 10 25 dyne-cm for the main, 25 February and 4 March shocks, respectively Average final displacements and stress drops are estimated to be 37 cm and 10 bars for the main shock (for a circular fault of radius 15 km); 22 cm and 8 bars for the 25 February shock, and 18 cm and 7 bars for the 4 March shock (for circular faults of radius 11 km) The striking features of the earthquake sequence are the low stress drops of the main shock and its two principal aftershocks, and the clear eastward migration of aftershock activities The unusually long source-time function rise times (4 sec for the main shock, 25 sec for both aftershocks) and low stress drops suggest an overall slow energy release during the earthquake sequence