Showing papers on "Shock wave published in 1983"

Magnetohydrodynamic Shock Waves in Molecular Clouds

Abstract: The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C*- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics

The dynamic and gravitational instabilities of spherical shocks

Abstract: In this paper we examine the stability of a thin spherical shock against dynamic and gravitational perturbations when the wavelength of the perturbation is large compared to the thickness of the dense shell. For dynamic perturbations, we find that a spherical isothermal shock, driven by a hot gas, is overstable against small perturbations. As a result, the fragmentation of the shell proceeds in an oscillatory manner. The smaller wavelength perturbations are most unstable, with the maximum growth rate being comparable to the inverse of the time it takes sound waves to propagate through the shell thickness. The nonradiating case is not subject to this overstability unless d ln P/d in rho is close to 1. The overstability can appear in cases where there is an ambient magnetic field. We also consider the growth of gravitationally driven perturbations. On cosmological time scales all such shocks are unstable, with the density perturbations growing as power laws in time. In the nonradiating case the maximum exponent is close to 1. On shorter time scales, only the isothermal shocks are unstable. Whenever the overstability mechanism is at work, gravitational instabilities can be neglected. We discuss the implications of these results for star formation, andmore » for hydrodynamic galaxy formation.« less

Electron velocity distributions near the Earth's bow shock

Abstract: New information is presented on the general characteristics of electron distribution functions upstream, within, and downstream of the earth's bow shock, thereby providing new insights into the instabilities in collisionless shocks. The results presented are from a survey of electron velocity distributions measured near the earth's bow shock between October 1977 and December 1978 using the Los Alamos/Garching plasma instrumentation aboard ISEE 2. A wide variety of distribution shapes is found within the different plasma regions in close proximity to the bow shock. It is found that these shapes can be classified into general types that are characteristic of three different plasma regions, namely the upstream region or electron foreshock, the shock proper where most of the heating occurs, and the downstream region or the magnetosheath. Evidence is provided that field-aligned, rather than cross-field, instabilities are the major source of electron dissipation in the earth's bow shock.

Evolution of ion distributions across the nearly perpendicular bow shock: Specularly and non‐specularly reflected‐gyrating ions

Abstract: Data from ISEE 1 and 2 spacecraft were used to study the evolution of the ion distributions in the perpendicular terrestrial bow shock. The plasma data were taken during passage of the spacecraft downstream of and through the shock. Solar wind ions had velocities ranging from Mach 2-12.4, and reflected ions featured a relative density of 1-3 percent of the solar wind density at Mach 2 to 15-25 percent at Mach 8-12. Computer simulations have indicated that the ions provide essential dissipation at the shock and gyrate about the magnetic field lines in the plasma rest frame at a speed twice that of the normal incident solar wind flow. The ion density decreases by up to two orders of magnitude at the forward end of the foot of the shock profile, suggesting that the ions are reflected by the shock specularly, and may enhance downstream ion thermalization.

Unsteadiness of the Separation Shock Wave Structure in a Supersonic Compression Ramp Flowfield

Abstract: Wall pressure fluctuations have been measured in a two-dimensional separated compression ramp-induced shock wave turbulent boundary-layer interaction The tests were made at a nominal freestream Mach number of 3 and at Reynolds numbers based on boundary-layer thickness of 78 X 10 and 14x 10 The wall temperature condition was approximately adiabatic Large-amplitude pressure fluctuations exist throughout the interaction, particularly near separation and reattachment In the upstream region of the flowfield, the unsteadiness of the separation shock wave structure generates an intermittent wall pressure signal Mean wall pressures in this region result from the superposition of the relatively low-frequency, large-amplitude, shock wave-induced fluctuations on the pressure signal of the undisturbed boundary layer This behavior is qualitatively similar to that observed in three-dimensional blunt fin-induced flows In these two flowfields, the length scale of the shock motion is a significant fraction of the distance from the interaction start to separation

Multiple spacecraft observations of interplanetary shocks Four spacecraft determination of shock normals

Abstract: ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best-fit normals for these five shocks. In addition to providing well-documented shocks for future investigations these data allow the evaluation of the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20 deg, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, the use of overdetermined shock normal solutions, utilizing plasma measurements, separation vectors, and time delays together with magnetic constraints, is recommended whenever possible.

Solar type II radio emission and the shock drift acceleration of electrons

Abstract: Many of the properties of energetic electrons and ions accelerated by interplanetary shock waves can be understood in terms of the shock drift acceleration mechanism. In this paper we show that the shock drift acceleration of electrons can be responsible for solar type II radio bursts as well. We review the shock drift acceleration mechanism and show that the streaming distribution of reflected electrons produced upstream of the shock front by this mechanism can be unstable to the generation of electrostatic plasma waves, which in turn interact to produce the observed radio emission. We derive constraints upon the density and energy of suprathermal electrons required to produce a typical type II burst.

Upstream Waves and Particles

Abstract: The region upstream from terrestrial, planetary, and interplanetary shocks in which the magnetic field lines are connected to the shock is filled with a variety of plasma waves, MHD waves, energetic electrons, and ions associated with the near presence of the shock. These upstream waves and particles present us with a natural plasma laboratory which is providing basic information on plasma instabilities and collisionless shock physics, as well as insight into how cosmic ray acceleration may occur in the interstellar medium. Much remains to be done, however, before our present empirical knowledge is woven into the stronger fabric of theoretical understanding.

Experimental investigation of turbulence properties in transonic shock/boundary-layer interactions

Abstract: Three flows resulting from shock wave/turbulent boundary-layer interactions occurring in a two-dimensional transonic channel have been investigated. The first flow corresponds to incipient shock induced separation, the second to a well separated case, the third to a situation where a large separated bubble forms. The flows were characterized by using two-color laser velocimetry. The results include, for the three flows, mean velocity and Reynolds stress distributions across the viscous layer. The turbulence measurements reveal that the first part of the interaction process entails a very large turbulence production with the development of a very strong anisotropy. In this zone, the neglect of normal stresses in the momentum and turbulence energy equation is not justified. The downstream relaxation toward a new equilibrium state is a very gradual process due to the long lifetime of the large structures which formed in the region of intense turbulence production.

Structure of perpendicular shocks in collisionless plasma

Abstract: The macrostructure of perpendicular shocks in the supercritical regime is investigated theoretically. The coupling between the shock precursor (foot region) associated with reflected ions and the magnetic ramp associated with electron Ohmic heating is analyzed using a model in which the ions are treated as a multifluid and the electrons as a massless fluid. The results are found to be in good agreement with a number of features exhibited by recent numerical simulations of perpendicular shocks, including the morphology of the reflected‐gyrating ion stream, the presence of potential and magnetic field overshoots, and the Mach number dependence of the shock structure.

Waves observed upstream of interplanetary shocks

Abstract: The properties of the waves that are present upstream of interplanetary, collisionless, quasi-parallel shocks are described. Two types of such waves have been detected, a higher frequency whistler mode wave and a lower frequency fast mode MHD wave. Both are typically circular or elliptically polarized right-hand waves which propagate along the ambient magnetic field with a 15 deg angle cone. The high frequency waves have sufficient group velocities to outrun the shock, and may be generated by cyclotron resonance with 100 eV to 1 keV shock electrons. The lower frequency waves must be generated locally by particles upstream of the shock, probably by 1-10 keV ions flowing away from the shock. Distinct changes in the spectra of upstream waves as a function of distance from the shock have been noted.

Subpicosecond electro-optic shock waves

Abstract: When an extremely short optical pulse is focused into an electro‐optic material, a moving polarization is produced which radiates in a Cerenkov‐like cone. At the boundary of this shock wave, the electric field consists of an extremely fast electrical transient with a correspondingly wide spectral distribution extending well into the far infrared. When appropriately coupled out of the material, this shock wave can be used as a fast electronic impulse generator or far‐infrared source without requiring subminiature transmission structures to guide the signal.

Interplanetary flow systems associated with cosmic ray modulation in 1977 - 1980

Abstract: The hydromagnetic flow configurations associated with cosmic ray modulation in 1977 to 1980 were determined using solar wind plasma and magnetic field data from Voyagers 1 and 2 and Helios 1. The modulation was related to two types of large scale systems of flows: one containing a number of transients such as shocks, post shock flows and magnetic clouds; the other consisting primarily of a series of quasi-stationary flows following interaction regions containing a stream interface and often bounded by a forward reverse shock pair. Each of the three major episodes of cosmic ray modulation was characterized by the passage of the system of transient flows. Plateaus in the cosmic ray intensity time profile were associated with the passage of systems of corotating streams.

Shock experiments on feldspar crystals

Abstract: Single crystals of sanidine, orthoclase, microcline, oligoclase, and labradorite as well as polycrystalline bytownite were shocked from 10.5 to 45 GPa and recovered. The direction of shock wave propagation in single crystals was always parallel to the crystallographic a-axis. Optical observations reveal a continuous sequence of shock effects: fracturing starts to develop at <10.5 GPa, planar elements develop at 10.5–14 GPa, mosaicism at 18–26 GPa, transformation of the crystal into diaplectic glass begins at 26–34 GPa, and formation of melt glass is observed at ∼42 GPa. Microcline, however, remains weakly birefringent even at 45 GPa. Refractive indices of diaplectic glass drop sharply at shock pressures between 30 and 40 GPa. Xray investigations reveal only a minor expansion of the unit cell of sanidine, whereas the other feldspars display no systematic change of lattice constants. The structural state (Al,Si-distribution) of feldspars is not affected by shock. EPR analyses reveal the disordering of the crystal structure and the formation of a short-range order phase in feldpars at pressures as low as 10.5–14 GPa. The beginning of the formation of diaplectic glass in the range from 18–22 GPa is evident from IR-spectra of shocked samples. It is assumed that diaplectic glass is an arrangement of grossly unchanged crystalline material and a glassy phase that is structurally indistinguishable from fusion-formed glass. The influence of the chemical composition and initial structural state of feldspars on the development of shock effects is believed to be less than the influence of exsolution lamellae and alteration products. Based on the results of these investigations, pressure ranges are given for the lower and upper boundary of the mixed-phase regime of the pV Hugoniot (p = shock pressure; V = specific volume) for all samples. Two independent methods for the determination of shock pressures from residual shock effects are proposed based on optical and IR-spectroscopic analyses.

Optically recording interferometer for velocity measurements with subnanosecond resolution

Abstract: An optically recording velocity interferometer system, called ORVIS, has been developed to measure particle velocity histories in shock wave experiments on condensed matter. The interferometer fringe motion is recorded with a high speed electronic streak camera, rather than with photomultiplier tubes and oscilloscopes as in previous interferometry systems. With this approach, the particle velocity of a witness foil in a detonation wave experiment was measured with 300‐ps time resolution. We believe that 20‐ps time resolution can be achieved with this technique which would represent an improvement of two orders of magnitude over previous measurement techniques.

The response of normal shocks in diffusers

Abstract: The frequency response of a normal shock in a diverging channel is calculated for application to problems of pressure oscillations in ramjet engines. Two limits of a linearized analysis arc discussed: one represents isentropic flow on both sides of a shock wave; the other may be a crude appr'l'I;imation to the influence of flow separation induced hy the wave. Numerical results arc given, and the influences of the shock wave on oscillations in the engine are discus,ed.

A source of the backstreaming ion beams in the foreshock region

Abstract: The proposed source mechanism for the 'reflected' ion beams observed in the bow shock region's foreshock places the origin of the beams in the magnetosheath, downstream of the quasi-perpendicular shock portion whose transition is characterized by the downstream ion populations. Particle simulations indicate that highly anisotropic downstream ion distributions can excite electromagnetic ion cyclotron waves which, in turn, pitch angle-scatter the gyrating ions in a few ion gyroperiods. As a result, some ions acquire large parallel velocities and move fast enough along the convecting downstream magnetic field to escape back across the bow shock into the upstream region. The results presented are in general agreement with ISEE reflected ion observations.

Turbulence: Space‐time statistical properties and behavior in supersonic flows

Abstract: Following a scientific cooperation beginning in 1954 with L. S. G. Kovasznay, and in his memory, in this paper is presented a survey of two of the main areas of research to which he contributed: (1) turbulence properties deduced through statistical conditional sampling and space‐time measurements of correlations and contingencies; these include celerities, optimum correlations, memories, and coherent structures; and (2) the study of turbulence in supersonic flows initiated by Kovasznay; this include the use of mass‐weighted averages, decrease of turbulence in expansion, increase in compression, strong Reynolds analogy extension, and temperature–velocity correlation conservation.

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.

Application Of Holographic Interferometry To Shock Wave Research

Abstract: Paper reports a successful application of holographic interferometry to the shock wave research. Four topics are discussed; i) transonic flow over an aerofoil, ii) shock wave propagation and diffraction past a circular cross-sectional 90° bend and two-dimensional straight or curved wedges, iii) stability of converging cylindrical shock waves and iv) propagation and focusing of underwater shock waves. Experiments were conducted on shock tubes equipped with a double exposure holographic interferometer. In each case isopycnics around shock waves were determined and three-dimensional shock wave interactions were also observed. Results are not only bringing forth new interesting findings to the shock wave research but also showing a further potentiality of holographic interferometry to the high speed gasdynamic study.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Heterogeneous shock-induced thermal radiation in minerals

Abstract: A 500 channel optical imaging intensifying and spectral digital recording system is used to record the shock-induced radiation emitted from 406 to 821 nm from transparent minerals during the time interval that a shock wave propagates through the sample. Initial results obtained for single crystals of gypsum, calcite and halite in the 30 to 40 GPa (300 to 400 kbar) pressure range demonstrate greybody emission spectra corresponding to temperatures in the 3,000 to 4,000 K range and emissivities ranging from 0.003 to 0.02. In the case of gypsum and calcite, distinctive line spectra, are superimposed on the thermal radiation. The observed color temperatures are a factor of 2 to 10 greater than the Hugoniot temperature, calculable on the basis of continuum thermodynamics and equation of state models for the shock states achieved in the three minerals. These observed high temperatures are believed to be real. We conclude that we are detecting a large number of closed spaced high temperature shear-band regions immediately behind the shock front. A shear instability model, such as proposed independently by Grady (1977, 1980), Ananin et al. (1974), and Horie (1980), in which small zones of highly deforming and melted material are produced which are the source of the detected thermal radiation and have a fractional effective area approximately numerically equal to the measured emissivity, can be used to predict an effective emissivity of 0.0065 directly behind the shock front. If shear band instability arises from viscous flow processes, Grady's model and mineral thermal properties yield for the shocked mineral viscosities values in the range of 10^9 to 10^(15) P immediately behind the shock front.

Stability of electron distributions within the Earth's bow shock

Abstract: The linear Vlasov stability of a class of electron velocity distributions which model those observed within the earth's bow shock is examined. The study is restricted to electrostatic waves which propagate parallel to the ambient magnetic field B. Two instabilities are identified which are driven by free energy in the direction parallel to B: an ion acoustic wave with real frequency below the ion plasma frequency and an electron acoustic wave with real frequency of several times the ion plasma frequency. The characteristics of the unstable waves are in accord with observations obtained within the bow shock, in particular, with the trend of the observed electrostatic waves to be polarized parallel to the magnetic field. The instabilities identified here may contribute to electron dissipation in collisionless shocks.

On the drift mechanism for energetic charged particles at shocks

Abstract: The energy changes of energetic charged particles at a plane shock due to the so-called drift mechanism are analyzed by using the ''adiabatic treatment.'' The analysis shows that for a fast MHD shock, particles lose energy owing to acceleration (curvature) drift in the magnetic field at the shock with the drift velocity being antiparallel to the electric field, and they gain energy owing to gradient drift parallel to the electric field. It is shown that particles with pitch angles aligned along the magnetic field which pass through the shock tend to lose energy owing to acceleration drift, whereas particles with pitch angles nonaligned to the magnetic field gain energy owing to gradient drift. Particles that are reflected by the shock always gain energy. Slow-mode shocks may be similarly analyzed, but in this case curvature drifts give rise to particle energy gains, and gradient drifts result in particle energy losses.

Electron velocity distributions near interplantary shocks

Abstract: Previous work on electron heating mechanisms operating at the supercritical earth's bow shock is extended to the generally lower Mach number interplanetary shocks. This is done by studying electron velocity distributions observed both upstream and downstream of interplanetary shocks observed by ISEE 3 between August 1978 and December 1979. It is found that perpendicular heating accompanied by little or no parallel heating occurs at the weaker shocks but that parallel heating is dominant for the stronger shocks. When perpendicular heating is dominant, downstream velocity distributions are for the most part Gaussian at low energies (core) and Lorentzian at high energies (halo). When parallel heating is dominant, however, these distributions are flat-topped at low energies, having power law extensions to higher energies. At the weaker shocks, the observations are consistent with electron heating resulting from conservation of the magnetic moment, whereas at the stronger shocks the heating is quite similar to that observed at the earth's bow shock.

Hydromagnetic wave dissipation in molecular clouds

Abstract: The damping of long wavelength, hydromagnetic waves in molecular cloud environments is studied with the aim of determining whether the supersonic motions observed in such clouds are likely to be due to the waves. It is found that Alfven waves propagating parallel to the average magnetic field are the longest lived wave modes. Such waves can typically survive for as long as one-million years if the wavelength is as long as a few tenths of a pc and the magnetic field is 0.1-1 milligauss. Nonlinear steepening of the waves followed by ion-neutral friction in the steepened wave profiles appears to be the most effective damping mechanism.

Transient Mixed-Flow Models for Storm Sewers

Abstract: A new dynamic mathematical model capable of simulating unsteady free surface-pressurized flows is presented. The model allows two options. The first option simplifies the mixed-flow problem to a free-surface flow problem by adding a hypothetical slot at the crown of the pipe. The second option treats the two flow regimes separately but joined together by an interface which is regarded as a shock wave. The speed, location, and intensity of the shock wave are computed at every time step. Interactions between the shock wave and various structures such as junctions, inflow drop structures, overflow relief structures, surge relief structures, and upstream and downstream ends are simplified using appropriate boundary conditions. The method of characteristics is used. The model is applied to a new combined sewer network and some numerical results are presented. It is shown that the pressurization phenomenon is a very dynamic phenomenon even without rapid change of any boundary condition. The pressurization wave tends to become very steep shortly after its formation and generates waterhammer as well as surges upon collision with hydraulic structures. This example serves as a warning that less than a fully dynamic model should not be used for a system which undergoes a pressurization process.

Blast waves with cosmic-ray pressure

Abstract: The effects of cosmic-ray pressure on the dynamics of self-similar, spherical blast waves and driven waves are investigated on the assumptions that the ratio of relativistic cosmic-ray pressure to total pressure at the shock front is a constant w and the the cosmic rays and thermal gas evolve as independent adiabatic fluids in the postshock flow. For blast waves from a point explosion in a uniform medium, the cosmic rays dominate the pressure near r = 0 if w>0. The solutions show that, if w is small, the ratio of cosmic-ray energy to total energy in the blast wave is several times w. The solutions are used to make specific predictions of the pion-decay ..gamma..-ray flux from a blast wave as a function of w. If w is large, the predicted fluxes from supernova remnants are close to the current observational limits. It is also noted that cosmic rays may limit the compression in the radiative shock waves of supernova remnants. The addition of cosmic pressure does not change the geneal nature of the driven wave self-similar solutions. The solutions are used to predict the pion-decay ..gamma..-ray flux from a young Type II supernova interacting with circumstellar material. Observations thesemore » ..gamma..-rays from extragalactic supernovae are not promising, but a galactic supernova could be very bright in ..gamma..-rays.« less

The average solar wind in the inner heliosphere: Structures and slow variations

Abstract: Measurements from the HELIOS solar probes indicated that apart from solar activity related disturbances there exist two states of the solar wind which might result from basic differences in the acceleration process: the fast solar wind (v 600 kms(-)1) emanating from magnetically open regions in the solar corona and the "slow" solar wind (v 400 kms(-)1) correlated with the more active regions and its mainly closed magnetic structures. In a comprehensive study using all HELIOS data taken between 1974 and 1982 the average behavior of the basic plasma parameters were analyzed as functions of the solar wind speed. The long term variations of the solar wind parameters along the solar cycle were also determined and numerical estimates given. These modulations appear to be distinct though only minor. In agreement with earlier studies it was concluded that the major modulations are in the number and size of high speed streams and in the number of interplanetary shock waves caused by coronal transients. The latter ones usually cause huge deviations from the averages of all parameters.

Turbulence Analysis of the Jovian Upstream 'Wave' Phenomenon

Abstract: As Voyager 2 approached Jupiter's bow shock, large-amplitude fluctuations were seen in both the magnetic field and plasma fluid velocity. These fluctuations generally coincided with the occurrence of long-lived energetic particle events similar to the upstream waves often observed near the earth's bow shock. In this paper an analysis of the magnetic field and plasma observations using spectral methods is presented. The characteristic spectral features related to the upstream waves are generally seen near 1 mHz. The measured correlation lengths of these fluctuations suggest that they are coherent over only a few wavelengths. The analysis is consistent with the hypothesis that these fluctuations are driven by streaming ions, possibly protons. No evidence for the existence of whistler waves is found. It is argued that some of the observed spectral features suggest that dynamical turbulent processes are occurring in the uptream wave region, including a possible observation of an inverse cascade of magnetic helicity to large spatial scales.