Showing papers on "Shock wave published in 1978"

Particle Acceleration by Astrophysical Shocks

Abstract: A new mechanism is proposed for acceleration of a power-law distribution of cosmic rays with approximately the observed slope. High-energy particles in the vicinity of a shock are scattered by Alfven waves carried by the converging fluid flow leading to a first-order acceleration process in which the escape time is automatically comparable to the acceleration time. Shocks from supernova explosions propagating through the interstellar medium can account for the acceleration of galactic cosmic rays. Similar processes occurring in extragalactic radio sources can lead to efficient in situ acceleration of relativistic electrons.

Observations of two distinct populations of bow shock ions in the upstream solar wind

Abstract: Observations upstream of the earth's bow shock with the LASL/MPI fast plasma experiments on ISEE 1 and 2 reveal the presence of two distinct and mutually exclusive populations of low energy (no more than 40 keV) ions apparently accelerated at the bow shock. The first of these, the 'reflected' population, is characterized by (1) sharply peaked spectra seldom extending much above about 10 keV/ion and (2) relatively collimated flow coming from the direction of the shock. On the other hand, the 'diffuse' ions are distinguished by relatively flat energy spectra above about 10 keV and broad angular distributions. They are by far the most commonly observed upstream ion event. A close causal association is suggested between the diffuse ion population in the upstream solar wind and energetic plasma ions observed within the magnetosheath.

Solar wind stream interfaces

Abstract: Results are presented for a superposed epoch analysis of discontinuous solar wind interfaces. The average time-space profiles of stream interfaces are discussed with reference to fluid properties (flow speed, pressure ridge, density, electron and proton temperatures) and kinetic properties (electron core and halo, flow speed fluctuations, electron heat flux, alpha particles). Other aspects of stream interfaces are described, such as the persistence of individual interfaces, shock associations, the sector boundaries of the interplanetary magnetic field, and sudden impulses in the geomagnetic field. Interface position is considered in terms of the observed temperature jump. A conceptual model of high-speed stream evolution is proposed.

Optical emission from a fast shock wave - The remnants of Tycho's supernova and SN 1006

Abstract: The faint optical filaments in Tycho's supernova remnant appear to be emission from a shock front moving at 5600 km/s. The intensity of the hydrogen lines, the absence of forbidden lines of heavy elements in the spectrum, and the width of the filaments are explained by a model in which a collisionless shock wave is moving into partially neutral gas. The presence of the neutral gas can be used to set an upper limit of approximately 5 x 10 to the 47th power ergs to the energy in ionizing radiation emitted by a Type I supernova. The patchy neutral gas is probably part of the warm neutral component of the interstellar medium. The existing information on the remnant of SN 1006 indicates that its emission is similar in nature to that from Tycho's remnant.

Existence of rarefaction shocks in a laser‐plasma corona

Abstract: General conditions under which rarefaction shocks can exist in the expanding corona of a plasma heated by a laser are derived. In particular, for the case of a two‐electron temperature isothermal plasma with temperatures Th and Tc, such a shock is shown to occur if Th/Tc≳5+√24. The case of rarefaction shocks induced by the ponderomotive force is also briefly discussed.

Numerical Analysis of Two-Dimensional Nonsteady Detonations

Abstract: In the present work, a system of two-dimension al nonsteady hydrodynamic and chemical kinetic equations was numerically integrated for an exothermic system. An assumed two-step reaction model simulates an oxyhydrogen mixture. The calculation starts from a plane Chapman-Jouguet detonation as an initial condition. Two-dimensional disturbances are generated by artificially placing nonuniformities ahead of the detonation front. Regardless of the difference in the given initial disturbances, a fixed number of triple shock waves were produced for a fixed combination of mixture model and geometry when the transition period was over. This shows that for a given detonation tube geometry, any exothermic system has its own characteristic multidimensional structure. The obtained number of triple shock waves contained in the detonation front was in agreement with existing experimental observations under the same condition.

The formation of high-density z-pinches

Abstract: Three regimes in the formation of a simple pinch are delineated. For a rapidly rising current, the formation of the pinch by a strong shock is first considered. A slug model is developed which, unlike the snowplough model, describes the radial structure between piston and shock. The final equilibrium pinch is described and radial profiles of density and temperature are obtained. Under the conditions of a strong shock, a weakly compressed pinch is formed in which the pinch radius is related to the wall radius and is independent of the current applied. By programming the current in time, regimes of singular compression and adiabatic compression can be obtained. A much more highly compressed plasma is produced. Such a current profile can readily be produced by a line storage energy source.

Shock steepening and prompt thermal emission in supernovae

Abstract: A calculation of supernova shock propagation down the exponential density gradient at the outside of the presupernova giant envelope predicts a strong quasi-thermal burst of ultraviolet and soft X-ray luminosity accompanying the emergence of the radiative shock at the stellar photosphere Burst duration is of order several thousand seconds, with characteristic luminosities of 10/sup 45/ ergs s/sup -1/ in the thermal continuum, peaked at about 500 A Total UV energy radiated is of order several times 10/sup 48/ ergs Expansion velocities in the outer 5 x 10/sup -3/ M/sub sun/ of material reach nearly 10/sup 4/ km s/sup -1/, in excellent agreement with peak values inferred from Doppler-shifted lines in the spectra of these objects No underlying source of energy (pulsar, etc) is required; the simple shock wave model for supernova light curves can account for all of the major features of observed Type II events A hard X-ray burst (kTapprox10--100 keV) results from the transition to an ion-viscous shock in the outermost surface layers

Plasma near Venus from the Venera 9 and 10 wide-angle analyzer data

Abstract: Preliminary results of ion and electron plasma measurements near Venus are presented and discussed. The data were obtained with wide-angle plasma analyzers carried on the Venera 9 and 10 spacecraft. On the basis of 33 bow shock crossings the position of the shock is quite stable and agrees well with theoretical predictions of Spreiter et al. with H/r/sub 0/=0.01 and a stagnation point altitude of approx.500 km. This observation lends strong support to the assumption that the solar wind interacts with the upper ionosphere of Venus and not with a planetary magnetic field. These spacecraft are the first to explore the optical umbra of Venus. Close to the planet a stable population of electrons and an ill-defined population of positive ions were found; this region is called the corpuscular umbra. The corpuscular umbra and the transition region are separated by a zone which contains both positive ions and electrons and is characterized by a flow velocity reduced in comparison with that of the transition region. This zone is called the corpuscular penumbra. The distribution of plasma density behind the bow shock (including the optical umbra of the planet) is given, and the existence of a Venusian plasma magnetic til ismore » revealed.« less

Studies of Laser-Driven Shock Waves in Aluminum

Abstract: We have observed the structure and velocity of laser-driven shock waves in aluminum foils. We have measured shock velocities as high as 13 km/s and shock luminosity rise-times less than 50 ps, and we have inferred pressures of 200 GPa and shock-front thicknesses 0.7 \ensuremath{\mu}m. These results suggest that such techniques may be used for measuring equation-of-state parameters and studying the detailed structure of shock fronts.

Shock waves at a fast-slow gas interface

Abstract: This paper presents the results of experiments on plane shock waves refracting at air/SF6 and He/CO2 interfaces. These are called fast-slow gas combinations because the speed of sound in the incident shock gas is greater than that in the transmitting shock gas. Our work was based on a generalization of the von Neumann (1943) classification of shocks into two classes called weak and strong. We introduced two subclasses of each of these, giving in all four groups of phenomena for study. This is possibly an exhaustive list, at least for conditions where the gases are approximately perfect. We present data on all four groups and study various transition conditions both within and across the groups. Our results appear to conflict with a previously reported irregular refraction; in fact we could apparently completely suppress this wave system by attention to our gas purity and boundary conditions. In its place we found a different system which appears to be a new phenomenon. We found another new system which has the appearance of a Mach-reflexion type of refraction but with its shock dispersed into a band of wavelets. It is interesting that the wavelets remain intense enough to induce identifiable vortex sheets in the flow. Finally we found yet another refraction of the Mach-reflexion type which had no detectable vortex sheet emanating from the triple point: such a system was foreshadowed by von Neumann.

Propagation of perturbations in a gas-liquid mixture

Abstract: The present investigation has been performed over a wide range of the dimensionless parameters characterizing the process of propagation of pressure perturbations in a gas-liquid mixture; these are the Reynolds number, and a dispersion parameter responsible for the relation between the values of dispersion and signal intensity. The values of the above parameters were changed mainly by varying the initial perturbation. The results obtained show a complete agreement between the Burgers-Korteweg-de Vries model and the real process of propagation of long-wave perturbations in a liquid with gas bubbles. In addition to signal propagation with the formation of monotonic and oscillatory shock waves, the propagation of signals in the form of solitary waves (solitons) and wave packets was observed experimentally. Data have been obtained on signal damping, energy dissipation and the influence of mixture viscosity on the signal evolution.

Magnetic reconnection: Acceleration, heating, and shock formation

Abstract: Numerical analysis of two-dimensional compressible MHD magnetic reconnection has shown that (1) the reconnection process is strongly dependent on boundary conditions but weakly dependent on internal conditions, (2) a slow mode shock layer is formed on the downstream side of the magnetic separatrix, and (3) plasma is heated up to a considerable degree near the X type neutral line and is accelerated up to the Alfven speed by the slow mode shock.

Shock waves at a slow-fast gas interface

Abstract: This paper describes the results of our experiments with shock waves refracting at a CO2/CH4 interface. The refraction is slow-fast because the speed of sound in the incident gas (CO2) is less than that in the transmitting gas (CH4). We found three phenomena which apparently have not been reported before and which all have free precursor shocks in their wave systems; schlieren photographs of them are presented. As a result of the present and earlier work, we can assert that there exist at least four different free precursor refractions. Theoretical studies suggest that the slow-fast phenomena can be conveniently classified into three groups characterized by different ranges of values of the inverse strength ζi of the incident shock i. The classification may be an exhaustive list of the phenomena, at least when the gases are nearly perfect, but we cannot be sure. We present experimental data on all the phenomena in each group, including data on the transition conditions from one wave system to another both within and across the groups.

Molecular dynamics of shock waves in one-dimensional chains

Abstract: The behavior of shock waves in one-dimensional chains has been explored in a series of molecular-dynamics computer experiments. Three "realistic" nearest-neighbor pair potentials were considered---Lennard-Jones 6-12, Toda, and Morse---as well as three truncated forms---harmonic, cubic, and quartic. Over a wide range of shock strengths the particle velocity profiles and shock speeds for a given form of potential can be characterized in strength by $\ensuremath{\alpha}\ensuremath{ u}$, where $\ensuremath{\alpha}$ is the cubic anharmonicity coefficient and $\ensuremath{ u}$ is the particle velocity in units of the long-wavelength sound speed. For strong shocks ($\ensuremath{\alpha}\ensuremath{ u}g1$), steady hard-rod-like velocity profiles are observed for the "realistic" potentials and the quartic truncated form, but not for the harmonic or cubic forms. The shock thickness in the harmonic chain grows as the cube root of time, while the shock thickness in the anharmonic chain grows linearly with time, in proportion to shock strength. This evolution of the shock thickness is unaffected by initial equilibration of the chain at finite temperature. If either a heavy- or light-mass defect is included, the shock wave is reflected and the relaxation process is slowed behind the defect.

A shocked cloudlet model for Herbig-Haro objects.

Abstract: The results of moderately high dispersion spectra of Herbig-Haro objects Nos. 1 and 2 are reported. Relative emission line intensities, radial velocities, and emission line widths have been measured for seven of the knots composing HH 2. A theoretical model is proposed in which a strong stellar wind from an embedded pre--main-sequence star produces shock waves upon interaction with small ambient cloudlets. Radiating gas from bow shocks formed aroung dense clouds can produce the salient features of most HH objects including their low excitation spectra, radial velocities, relative emission line widths between lower and higher excitation lines, luminosities, and variational time scales. For the knots composing HH 1 and HH 2, a characteristic stellar wind velocity of about 100 km s/sup -1/ and a mass loss rate of approx.10/sup -5/ to 10/sup -6/Msunyr/sup -1/ from the embedded source are required. The preshock stellar wind density of about 200 cm/sup -3/ at a distance of approx.10/sup 17/ cm from the star is expected to yield post--bow-shock conditions typical of HH 1 and HH 2. The results of detailed plane shock calculations performed independently by Raymond and Dopita for HH objects are employed to discuss the differences expected between bow shock andmore » plane shock spectra. The required mass flow rates and acceleration time scale of the cloudlets indicate that HH objects are short-lived phenomena (approx.10/sup 3/yr). Polarization is associated with those objects which exhibit significant continuum, and it is suggested that these objects represent a later phase of evolution in which a dust envelope has been partially cleared from the star, permitting reflection of continuum from dust associated with the cloudlets.« less

Magnetohydrodynamics of atmospheric transients. II. Two-dimensional numerical results for a model solar corona.

Abstract: A systematic study of dynamic response of the inner solar corona is made within the context of two-dimensional, time-dependent plane hydromagnetics. The governing equations are written in r-phi coordinates (i.e., in the solar equatorial plane), and numerical solutions are obtained by introducing an impulsive temperature enhancement within a rectangular region (i.e., 'box') in an initially isothermal corona in magnetohydrostatic equilibrium. Effects of the magnetic field configuration are illustrated for initially open (radial) and closed (azimuthal) magnetic fields by comparison with the nonmagnetic response. The channeling or blocking effects by the magnetic fields on the mass motion of solar plasma as the consequence of the evolution of fast and slow mode MHD shock waves are demonstrated. Some physically significant applications of the results, useful for the interpretation of observations, are discussed. It is noted, for example, that coronal transients observed in white light probably occur within essentially radial field topologies.

Numerical Solution of Three-Dimensional Shock Wave and Turbulent Boundary-Layer Interaction

Abstract: A rapid numerical scheme is used to solve the complete mass-averaged Navier-Stokes equations for supersonic turbulent flow over a three-dimensional compression corner. A simple eddy viscosity model is developed, and the interaction of a swept shock wave and a three-dimensional turbulent boundary layer is studied. Good agreement is obtained between the present results and experimental measurements for the case of a wedge with an angle of 6 deg on a flat-plate sidewall. For the case of a 12-deg wedge angle, the computed results do not show the existence of a peak pressure found experimentally. However, the range of interaction, the plateau pressure, and the peak heat transfer are closely predicted for all cases. The high heat transfer near the axial corner is due to the thinning of the boundary layer and inflow of fresh high-momentum fluid. The heat transfer is relieved through pressure reduction and boundary-layer thickening.

Current-driven instabilities in a laminar perpendicular shock

Abstract: The linear theory of fully electromagnetic plasma instabilities driven by currents flowing across a magnetic field is investigated, with applications to the laminar perpendicular bow shock. The theory utilizes the local approximation, assumes unmagnetized ions and magnetized electrons, and includes the effects of gradients in the magnetic field magnitude and density. Oblique propagation is considered, and wave numbers may be of the order of or larger than the reciprocal electron Larmor radius. An increase in the electron beta does not significantly affect the ion acoustic instability but does reduce the growth rates of the Buneman and the modified two-stream instabilities. If Te » Ti within the shock, results indicate that the ion acoustic instability is the most important growing mode, for any reasonable beta. If Te < Tι throughout the shock, further work is necessary to determine which instability is most important.

H2 cooling, dissociation, and infrared emission in shocked molecular clouds

Abstract: Models are presented of interstellar shocks in molecular clouds over ranges of ambient molecular density from 1000 to 10 million per cu cm and shock velocity from 6 to 14 km/s. Estimates of H2-H2 collisional-excitation rates are used to derive the H2 radiative cooling rates from vibrational-rotational quadrupole transitions as a function of n(H2) and temperature. The emissivities integrated through the shock of the strongest infrared lines in the v = 1-0, 2-0, and 2-1 bands of H2. The effectiveness of H2 dissociative cooling is considered for the highest-velocity shocks. The H2 line intensities from such shocks are compared with those produced by the 'competitive' mechanism of UV pumping for two likely driving mechanisms of shocks - wind-driven shells and expanding H II regions.

Shock Wave Propagation in Layered Composites

Abstract: Stress histories in multilayered composite targets were recorded using manganin gages in plane impact powder gun experiments.The results exhibit an interesting resonance behavior due to the multiple wave reflections in the layers. This phenomenon is well known in com posites with periodic structure near the linear elastic region, but was not expected to be important at higher stresses, contrary to our results.Elasto-plastic calculations carried out with a finite difference pseudo- viscosity hydro-code did reproduce the experimental result and helped us to better understand the mechanism of shock wave propagation in the composites.

Experimental Investigations of Cavitation Bubble Collapse by a Water Shock Tube

Abstract: This paper deals experimentally with the mechanism of an impulsive pressure generated by a collapsing bubble. In a water filled shock tube, an expansion wave and a subsequent compression wave are applied to single, twin and triadic bubbles. The growth, collapse and rebound of bubbles situated at various distances from a solid boundary are observed by means of high-speed photography and in-line Fraunhofer holography using a pulsed dye laser. The results indicate that the impulsive pressure is caused by a shock wave radiated at the instant of the rebound of a collapsing bubble, and that the subsequent jet impingement does not produce any detectable effects. The pressure pulse is found to be of the order 104 ∼ 105 atm, and its duration 2 ∼ 3 μsec.

Acceleration of cosmic rays by shock waves.

Abstract: The problem of the origin of galactic cosmic rays is a particularly difficult one despite the fact that rather detailed measurements of the properties of cosmic rays can be made, at least in the vicinity of the Sun. The current situation has been well reviewed by Lingenfelter (1) who points out that there are several linked problems to be solved, namely the question of sources and acceleration mechanisms, propagation within the galaxy, escape from the galaxy and of course solar modulation which affects the interpretation of the observations, especially below ~ 1 GeV/nuc. It is usually supposed that these problems can be treated separately, so that the sources (perhaps supernovae, pulsars, black holes, flare stars, etc.) simply provide cosmic rays with given elemental and isotopic abundances and given spectra, which then propagate independently by diffusion through the interstellar medium, producing secondaries and perhaps losing energy as they do so until they eventually leave the galaxy by some means, which is usually described in terms of a “free escape” boundary condition to the diffusion equations.

Perturbation of a Discontinuous Transonic Flow

Abstract: The main difficulty in perturbing a discontinuous transonic How is in the representation of the shift in the location of the discontinuity (shock wave). Herein presented is a method of overcoming this difficulty by using a distorted airfoil as the initial case rather than the real physical airfoil; the distortion is chosen such that the shock location is unchanged by the perturbation. The distorted airfoil is obtained by the use of a strained coordinate system. A direct consequence of the theory is the derivation of an algebraic similarity relation between related airfoils with shock waves at different locations. Results for simple examples are shown. N important problem in aerodynamics is the accurate prediction of the pressures on an airfoil that is oscillating with small amplitude in a transonic flow; this prediction is necessary for the satisfactory estimation of flutter parameters. An important physical feature of such flows can be the presence of an oscillating shock wave, which should be accurately represented in any solution procedure because of the relatively large pressure fluctuations in the region bounded by the extremities of the shock motion. It is therefore desirable to represent the shock wave by the correct discontinuity rather than by the rapid compression exhibited by the commonly used "shock capture" finite-difference methods.' Such a discontinuous representation of the shock can be obtained in steady flow by using either a finite- difference method with shock fitting,2 or by the integral equation method.3 The shock fitting technique has recently been applied4 to unsteady flows. The present work is ultimately directed toward the development of a method of treating oscillating shock waves mainly through the integral equation approach. In the limit of zero frequency, the problem of computing the flow around an oscillating airfoil reduces to a steady perturbation problem with the airfoil geometry perturbed by an amount proportional to the amplitude of the oscillation. The feature of a shock increment is retained in this problem since it is unlikely that the perturbed airfoil will have a shock wave in the same location as the initial airfoil. The steady perturbation case is therefore a good starting point for deriving a fundamental approach for computing oscillatory flows, and it is this problem that is considered in this paper. In addition, a perturbation solution is useful in other ways; for example, when the flow over a given airfoil for a range of freestream Mach numbers is required, since once one nonlinear result is obtained, the other required results can be . obtained from the linear perturbation solution. As suggested earlier, the main difficulty in perturbing a discontinuous transonic flow is in the representation of the shift in the position of the discontinuity (shock wave), because for most small perturbations the physical aspects of the flow require that there be the same number of shock waves in both the initial and perturbed states but at different locations. An examination of the usual form for a perturbation solution indicates that this physical feature cannot be correctly represented except in the trivial case where no shock movement occurs. In the method herein presented, the

On the Rayleigh-Taylor instability in stellar explosions

Abstract: After a shock wave has accelerated the outer layers of a star during a stellar explosion, a rarefaction wave moves back into the stellar material, resulting in the conversion of internal energy into kinetic energy This additional acceleration can be Rayleigh-Taylor unstable Two-dimensional hydrodynamic calculations are presented for the case of Type II supernovae, showing that the instability results in the ejection of a clumpy shell It is unlikely that radiation transport can dump the instability for normal Type II supernove We further conjecture that the structure of nova shells is a consequence of t We fur ther conjecture that the structure of nova shells is a consequence of this instability; in this case, rings are formed by the smearing action of high rotational velocities