Showing papers on "Shock wave published in 1979"
TL;DR: In this article, the relativistic motion of a quasi-steady jet is modeled as a superluminal expansion in which the moving component and the stationary component would have comparable Doppler-boosted fluxes, and specific models for the dynamical and radiative properties of the jet and individual shocks are presented.
Abstract: Variable extragalactic radio sources, associated with the nuclei of galaxies and quasars, are interpreted in terms of a supersonic relativistic jet. It is proposed that radio emission originates both from the quasi-steady jet itself and from behind strong shock waves which either propagate in the jet, or which are formed behind dense condensations (clouds) that are accelerated to relativistic speeds by the flow. In this way the source could display apparent superluminal expansion in which the moving component (associated with a shock) and the stationary component (associated with the optically-thick core of the jet) would have comparable, Doppler-boosted fluxes. Specific models for the dynamical and radiative properties of the jet and of individual shocks are presented. Kinematical consequences of the relativistic motion are described for flux and polarization measurements, as well as for VLBI observations of superluminal sources. It is argued that the majority of bright compact sources are observed along lines of sight making small (< or approx. =10/sup 0/) angles to the jet velocity. This hypothesis has important consequences for the interpretation of low-frequency variable sources, optically-violent variable quasars, Lacertids, and extended double sources. These are briefly outlined, and some specific observational tests are proposed.
1,908 citations
TL;DR: In this article, the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas.
Abstract: The paper analyzes the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas. It is noted that such fast shocks initially dissociate almost all preexisting molecules. Discussion covers the physical processes which determine the post shock structure between 10 to the 4 and 10 to the 2 K. It is shown that the chemistry of important molecular coolants H2, CO, OH, and H2O, as well as HD and CH, is reduced to a relatively small set of gas phase and grain surface reactions. Also, the chemistry follows the slow conversion of atomic hydrogen into H2, which primarily occurs on grain surfaces. The dependence of this H2 formation rate on grain and gas temperatures is examined and the survival of grains behind fast shocks is discussed. Post shock heating and cooling rates are calculated and an appropriate, analytic, universal cooling function is developed for molecules other than hydrogen which includes opacities from both the dust and the lines.
777 citations
TL;DR: In this article, the authors derived an adiabatic index of roughly 4 3 coming from the degenerate leptons, but lowered slightly by electrons changing into neutrinos and by the nuclei dissolving into α-particles, right up to nuclear matter densities.
438 citations
TL;DR: Theoretical models of radiative shocks are constructed, with special attention to the transfer of ionizing radiation as mentioned in this paper, and the strongest emission lines in the UV, optical, and infrared are tabulated, as well as post-shock column densities of metal ions potentially observable by UV absorption spectroscopy.
Abstract: Theoretical models of interstellar radiative shocks are constructed, with special attention to the transfer of ionizing radiation. These models are 'self-consistent' in the sense that the emergent ionizing radiation (the UV precursor) is coupled with the ionization state of H, He, and the metals in the preshock gas. For shock velocities of at least 110 km/s the shocks generate sufficient UV radiation for complete preionization of H and He, the latter to He(+). At lower velocities the preionization can be much smaller, with important consequences for the cooling function, the shock structure, and the emission. For models with shock velocities of 40 to 130 km/s the intensities of the strongest emission lines in the UV, optical, and infrared are tabulated, as well as postshock column densities of metal ions potentially observable by UV absorption spectroscopy. Possible applications to supernova remnants and high-velocity interstellar gas are assessed.
383 citations
TL;DR: In this article, it was shown that the double-humped distribution would be expected only in a region too small to be resolved by the plasma measurements made so far, by comparing electrostatic noise at or near the ambient solar wind plasma frequency with times when the interplanetary magnetic field probably connects to the earth's bow shock.
Abstract: Scarf et al. (1971) and Dunckel (1974) have shown that there are, very frequently, intense electrostatic waves whose frequency is near the plasma frequency upstream of the earth's bow shock and that these waves are correlated with the presence of energetic electrons from the bow shock. Also, Fredericks et al. (1971) have postulated a two-stream instability. The paper investigates these phenomena further, by comparing electrostatic noise at or near the ambient solar wind plasma frequency with times when the interplanetary magnetic field probably connects to the shock. Evidence is presented that Scarf et al. and Fredericks et al. were correct in their explanation of the phenomenon, and that the double-humped distribution would be expected only in a region too small to be resolved by the plasma measurements made so far.
317 citations
TL;DR: In this paper, the optical and UV emission-line intensities and column densities of ions observable in the visible and UV wavelengths were derived for plane-parallel steady-state shock waves in the interstellar medium.
Abstract: Calculations of the optical and UV emission-line intensities and column densities of ions observable in the optical and UV are presented for plane-parallel steady-state shock waves in the interstellar medium. The range of shock velocities considered is from 50 to 200 km/s, and preshock densities are between 1 and 300 per cu cm. The ionization state of the preshock gas is found to be an important parameter, and several sets of elemental abundances corresponding to various amounts of depletion onto grains are used. The results are compared with observations of the Cygnus Loop, the Vela supernova remnant, and Herbig-Haro objects.
302 citations
TL;DR: In this paper, the authors measured the transition conditions in pseudosteady and steady flow in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon.
Abstract: Experiments were conducted in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon to measure the transition condition in pseudosteady and steady flow. The transition condition in the steady flow, in which the wall was eliminated by symmetry, agrees with the calculated von Neumann condition. In the real gases this calculation assumed thermo-dynamic equilibrium after the reflected shock. In the pseudosteady flow of reflexion from a wedge the measured transition angle lies on the Mach-reflexion side of the calculated detachment condition by an amount which may be explained in terms of the displacement effect of the boundary layer on the wedge surface. A single criterion based on the availability of a length scale at the reflexion point explains the difference between the pseudosteady and steady flow transition condition and predicts a hysteresis effect in the transition angle when the shock angle is varied during steady flow. No significant effects on the transition condition due to finite relaxation length could be detected. However, new experiments in which interesting relaxation effects should be evident are suggested.
245 citations
TL;DR: In this article, a detailed study of the three-dimensional shape and location of the bow shock has been carried out, where the influence of the different solar wind conditions has been reduced by normalising the observed crossings to an average solar wind dynamical pressure (N0 = 9.4 cm−3, V0 = 450 km s−1).
155 citations
148 citations
TL;DR: In this article, the authors show that energetic electrons are injected into the region upstream from the earth's bow shock in a thin sheet which lies just behind the sheet of interplanetary magnetic field lines that are tangent to the shock surface.
Abstract: ISEE spacecraft observations show that energetic (not less than 16 keV) electrons are injected into the region upstream from the earth's bow shock in a thin sheet which lies just behind the sheet of interplanetary magnetic field lines that are tangent to the shock surface. Lower energy electrons leave the shock over a much broader region. Although the energetic electron intensity varies, the sheet is nearly always present and may be a quasi-steady state feature of the bow shock. The electron velocity distribution in the thin sheet is strongly peaked and is responsible for excitation of electron plasma waves.
137 citations
TL;DR: In this paper, the authors used velocity-interferometer measurements of aluminum, copper, and steel to measure the rise time of the shock wave in all three materials and found that the peak time for the wave transition occurs within 3 ns in all materials.
Abstract: Time‐resolved measurements of shock‐wave rise times have been accomplished for aluminum, copper, and steel to stress levels of 41, 96, and 139 GPa, respectively, using velocity‐interferometer techniques. To within the time resolution of the technique, the shock transition is found to occur within 3 ns in all materials. Based on this upper limit for the transition time, limiting viscosity coefficients of 1000, 3000, and 4000 P are obtained for 6061‐T6 aluminum, OFHC copper, and 4340 steel, respectively, at strain rates above 108 s−1. It is found that the effective viscosity can be expressed as parameters in a Maxwellian relation for an elastic‐plastic solid, in which the viscosity is related to an effective relaxation time. It is also shown that viscosity is inversely proportional to mobile‐dislocation density, which implies that the density of mobile dislocations obtained during shock compression in these materials is well over 109/cm2.
TL;DR: In this paper, an externally driven magnetic reconnection is simulated for two two-dimensional MHD models with anomalous resistivity: a closed boundary model where an O-type neutral point is formed between two X-Type neutral points and an open-ended model where only an X type neutral point was formed.
Abstract: An externally driven magnetic reconnection is simulated for two two-dimensional MHD models with anomalous resistivity: a closed boundary model where an O-type neutral point is formed between two X-type neutral points and an open-ended model where only an X-type neutral point is formed. Computer runs have shown the following: (1) X-shaped (Petschek type) slow shocks are formed. Upon examination of their fine structure it is found that the Rankine-Hugoniot shock conditions are extremely well satisfied. (2) In the closed boundary case the plasmas entering into the magnetic island through the X-type neutral points are moderately accelerated along the magnetic island boundaries. Two pairs of vortices are formed internally in such a way that the plasmas are confined. Consequently, the region of the island expands as the pressure increases. As reconnection proceeds, another set of vortices is generated in the expanding island causing the plasma to become turbulent. (3) In the open-ended case the plasma acceleration is intensified, and strong jet streams are generated on the downstream side of the slow shocks, with the speed approaching the Alfven speed of the upstream region. Interestingly, the jet streams tend to concentrate preferentially along the plasma sheet boundaries (shock fronts). This is attributed to sharp pressure gradients along the magnetic field lines as a result of slow shocks whose fronts obliquely intersect the field lines. These features of strong jetting of plasmas along the plasma sheet boundaries can explain the recent observations of high-velocity proton flows in the earth's magnetotail during substorm expansions. (4) Examination of the energy conversion rate has shown that the externally driven magnetic reconnection acts as a powerful magnetic energy converter.
01 Jul 1979
TL;DR: In this paper, the authors examined the aero-acoustics associated with model nozzles operating supersonically and compared the shock structure and radiated shock noise of Mach 1.5 and 2.0 noizles with those of a convergent nozzle over a wide nozzle pressure ratio range corresponding to a fully expanded Mach number between unity and 237.
Abstract: The paper examines the aeroacoustics associated with model nozzles operating supersonically. In particular, the shock structure and radiated shock noise of Mach 1.5 and 2.0 nozzles are compared with those of a convergent nozzle over a wide nozzle pressure ratio range corresponding to a fully expanded Mach number between unity and 2.37. The nozzles were operated unheated both with and without a tab for screech tone suppression. The measurements show differences between the shock cell spacing of convergent and convergent-divergent nozzles, and the scaling relation appears to be a function of the exit-to-throat velocity ratio of each nozzle type. The acoustic measurements indicate the extent of the pressure ratio range where a C-D nozzle achieves a noise reduction benefit. At the design point of the Mach 1.5 nozzle, the total integrated sound power from this nozzle is 6 dB less than a convergent nozzle operating at the same pressure ratio and thrust
TL;DR: Solutions of the Navier-Stokes equations for strong shock waves in a dense fluid agree well with recent atomistic simulations using nonequilibrium molecular dynamics as discussed by the authors, which is the state-of-the-art.
Abstract: Solutions of the Navier-Stokes equations for strong shock waves in a dense fluid agree well with recent atomistic simulations using nonequilibrium molecular dynamics.
01 Jan 1979
TL;DR: In this paper, an experimental study to establish the basic mechanisms of transition from deflagration to spherical detonation downstream of flow obstructions has been performed, and it was found that large scale eddies are essential for detonation.
Abstract: An experimental study to establish the basic mechanisms of transition from deflagration to spherical detonation downstream of flow obstructions has been performed. The flow obstructions consist of circular and rectangular orfice plates, multiple rectangular orifice plates and circular hole perforated plates. In order to keep the apparatus reasonably small and also to facilitate photographic observations at low initial pressures, acetylene-oxygen mixtures at equimolar composition were used throughout. The principal diagnotics were stereoscopic streak and framing schlieren photography. The apparatus consists of a spherical flame chamber connected to a cylindrical detonation chamber via an orifice plate mounted in the port-hole connecting the two chambers. The mechanism of detonation initiation is found to be due to the intense mixing of the hot combustion products with the unburned gases. Large scale eddies are found to be essential for detonation. These eddies provide the main mechanism of entrainment, and are also partially responsible for providing energy to maintain the intensity of the finer scale turbulence. When wire screens are used in conjunction with the orifice plates, the formation of detonation is facilitated, indicating that the fine scale turbulence which is produced plays a role in rapid mixing of the entrained gases inside the large scale eddies. Under critical conditions, when the initiation results from one single eddy, it appears that shock wave amplification by coherent energy release (SWACER) inside an eddy, where a gradient field of induction time due to entrainment is present, is most probable. This suggests that the minimum size of the required eddies must be at least of the order of the detonation kernel (i.e., transverse wave spacing) of the mixture. The present experimental findings that the obstructions which produce eddies of size of the order of transverse wave spacing lead readily to detonation initiation support this view.
TL;DR: In this article, the authors examined magnetic field profiles across 6 terrestrial bow shocks and one interplanetary shock and examined the behavior of a low Mach number shock with an apparent wavelength of 180 km.
Abstract: ISEE-1 and -2 magnetic field profiles across 6 terrestrial bow shocks and one interplanetary shock are examined. The interplanetary shock illustrates the behavior of a low Mach number shock. It had an upstream whistler wave precursor with an apparent wavelength of 180 km. The shock thickness was about 90 km for the thickness of the final field jump or 270 km for the exponential growth of the precursor wave packet. The ion inertial length was 50 km, upstream of the shock.
TL;DR: In this article, a systematic numerical study of the structure of blast waves produced by constant-velocity and accelerating flames propagating away from the center of a spherical source region was conducted, showing that the maximum overpressure generated by such flames is no higher than that generated by a constant velocity flame that travels through the entire source region at the maximum velocity reached when the acceleration process ceases.
TL;DR: In this paper, low-frequency self-excited oscillations involving a normal shock wave and the subsonic flow behind it were investigated experimentally in a two-dimensional diffuser.
Abstract: Low-frequency, self-excited oscillations involving a normal shock wave and the subsonic flow behind it were investigated experimentally in a two-dimensional diffuser. The preshock supersonic flow was uniform and steady, and the exit pressure was constant. Static wall pressure fluctuations at numerous streamwise locations were recorded and analyzed statistically for flow conditions ranging from subsonic to a shock Mach number of 1.38. Root-mean-square fluctuation intensities, power spectra, and space-time correlation maps were computed. The pressure fluctuations correlated well with the shock motion over streamwise distances comparable to the diffuser length. Both downstream-convected and upstream-propagating pressure disturbances were instrumental in sustaining the oscillations. The amplitudes of upstream-moving perturbations diminished with increasing shock strength, while the opposite was true of the downstream-moving disturbances.
TL;DR: In this article, an analytical description of strictly periodic shock waves passing through a stellar atmosphere is developed which allows predictions to be made of the onset of instability of the system against mass loss by hydrodynamic ejection.
Abstract: An analytical description of strictly periodic shock waves passing through a stellar atmosphere is developed which allows predictions to be made of the onset of instability of the system against mass loss by hydrodynamic ejection. This diagnostic method for determining when shock-driven mass loss may be expected is compared to several numerical isothermal hydrodynamical models. The predictions by the analytical theory of the onset of mass loss are in good agreement with the numerical hydrodynamical models. The role of random aperiodicities in enhancing mass loss is investigated in the numerical models and is found to be minor. Effects of atmospheric density gradients and postshock heating are also investigated numerically and are found to be critical. We conclude that the observed mass loss rates for the long-period variables can be produced by the shock wave mechanism alone if proper account is taken of the high pressures in the hot postshock hydrogen recombination zone.
TL;DR: In this article, two limiting treatments, namely the "adiabatic" and the "kink" treatments, have been developed to study the macroscopic acceleration process for non-thermal particles at the front of MHD shock waves.
TL;DR: In this article, a transition from nonsteady to steady waves where the Rankine-Hugoniot relations are obeyed is the partial relaxation of compressive shear stress behind the shock front which accompanies small, but permanent, transverse strains in atomic positions.
Abstract: Molecular-dynamics calculations of shock waves in perfect three-dimensional solids at nonzero initial temperatures reveal a transition in the nature of the asymptotic shockwave structure as a function of shock strength. The key to this transition from nonsteady to steady waves where the Rankine-Hugoniot relations are obeyed is the partial relaxation of compressive shear stress behind the shock front which accompanies small, but permanent, transverse strains in atomic positions.
TL;DR: In this article, the collision of interplanetary shock waves with the bow-shock-wave-magnetopause system is considered both in the gas-dynamic and magnetohydrodynamic approximations.
Abstract: The collision of interplanetary shock waves with the bow-shock-wave-magnetopause system is considered both in the gas-dynamic and magnetohydrodynamic approximations. It is shown that the shock wave is reflected from the magnetopause as a rarefaction wave which, in turn, is reflected from the rearward side of the bow shock. This secondary rarefaction wave arrives at the magnetosphere after a time interval of 3--5 min after the interplanetary shock wave's arrival. The rarefaction wave decreases the flow pressure on the magnetosphere and causes the reverse (i.e., outward) motion of the magnetopause. With the help of an approximate solution of the differential equations the problem of rarefaction wave reflection from the magnetosphere is considered. The law of the subsolar point motion of the magnetosphere during the abrupt shock-like perturbation is obtained. The experimental data confirm the theoretical results.
TL;DR: In this paper, measurements of energetic protons between about 28 and about 145 keV on ISEE-1 upstream and downstream of the earth's bow shock during times of a radial interplanetary magnetic field are presented.
Abstract: Measurements of energetic protons between about 28 and about 145 keV on ISEE-1 upstream and downstream of the earth's bow shock during times of a radial interplanetary magnetic field are presented. The protons exhibited a slight antisunward flow in the spacecraft system in front of the bow shock and a strong flow along the magnetic field in the spacecraft system in the magnetosheath. Compton-Getting transformations of these anisotropies into the plasma frame of reference have been performed. It is argued that the observations can be explained best in terms of particle acceleration by multiple reflection between the bow shock and upstream scattering centers.
TL;DR: In this paper, the authors reported the results of experiments on shock reflexion in a wind tunnel and a shock tube and further results are presented here, showing that transition to Mach reflexion takes place continuously at the shock wave incidence angle ω 0 corresponding to the normal shock point ω0 = ωN, unless the downstream boundaries form a throat.
Abstract: Our 1975 paper reported the results of experiments on shock reflexion in a wind tunnel and a shock tube; further results are presented here. For strong shocks it is shown that transition to Mach reflexion takes place continuously at the shock wave incidence angle ω0 corresponding to the normal shock point ω0 = ωN, unless the downstream boundaries form a throat. In this event transition can be promoted anywhere within the range ω0 [les ] ωN, and it is even possible to suppress regular reflexion altogether! However when ω0 ωN is spurious. For weak shocks the transition condition is not known but it is found that even for regular reflexion a marked discrepancy between theory and experiment develops as the shocks become progressively weaker. Also when weak shocks diffract over single concave corners there is a somewhat surprising discontinuity in the regular reflexion range. It seems that none of these phenomena can be adequately explained by real gas effects such as viscosity and variation of specific heats.
TL;DR: In this paper, the pyrolysis of NH3 was studied behind reflected shock waves at temperatures 2500-3000 K, using mass spectrometric analysis of dynamically sampled gas.
Abstract: The pyrolysis of NH3 was studied behind reflected shock waves at temperatures 2500–3000 K, using mass spectrometric analysis of dynamically sampled gas. The initial mixtures contained 0.14% to 6% N...
TL;DR: In this paper, the authors deal with interplanetary shocks, detected and analyzed to date, from the Helios 1 and 2 spacecraft in eccentric solar orbits, and the plasma wave turbulence associated with the shock observed on March 30, 1976 is studied in detail.
Abstract: The present paper deals with interplanetary shocks, detected and analyzed to date, from the Helios 1 and 2 spacecraft in eccentric solar orbits. The plasma wave turbulence associated with the shock observed on March 30, 1976 is studied in detail. This event is of particular interest because it represents a clearly defined burst of turbulence against a quiet solar wind background both upstream and downstream of the shock. The shock itself is an oblique shock with upstream parameters characterized by a low Mach number, a low beta, and an abnormally large electron to ion temperature ratio. The types of plasma wave detected are discussed.
TL;DR: In this article, the central, high energy, heavy ion collisions are studied in a hydrodynamic model, which contains a phase transition at low densities of the matter, and includes the π -meson production and resonance excitations at high temperatures.
TL;DR: In this article, the authors detected 69 energetic proton events (EPE) (in the 0.6-3.4 MeV energy range) during 1973-1974 in the heliocentric radial range 1-5 AU.
Abstract: The University of Iowa instrument aboard Pioneer 11 detected 69 energetic proton events (EPE) (in the 0.6-3.4 MeV energy range) during 1973-1974 in the heliocentric radial range 1-5 AU. Sixty percent of the EPE peak within plus or minus 5 hours of a corotating interaction region (CIR) boundary, while 19% peak inside and 21% peak outside the interaction regions. Of the CIR boundaries at which an EPE peaks with plus or minus 5 hours, 80% have associated shocks. The observed intensities and pitch angle distributions of protons near shock fronts are consistent with a theoretical simulation of the acceleration of protons by a drift in the electric field at the shock front.
TL;DR: In this paper, a Fermi-type acceleration model is constructed to explain the origin of energetic protons (30 ∼ 100 keV) which have been observed upstream of the bow shock.
TL;DR: In this article, the authors established that the length scale governing the detachment of a shock wave from a wedge is the distance from the leading edge to the sonic line, and by considering the view of observers with different length scales, they predicted that the detachment distance increases gradually with wedge angle for relaxing flow and more rapidly in a perfect gas.
Abstract: By establishing that the length scale governing the detachment of a shock wave from a wedge is the distance from the leading edge to the sonic line, and by considering the view of observers with different length scales, it is predicted that the detachment distance increases gradually with wedge angle for relaxing flow and more rapidly in a perfect gas. Both of these features are confirmed by experiments in the free-piston shock tunnel. The influence of other length scales is discussed. The phenomenon is related to a relaxation effect in which a subsonic layer grows from the translational-rotational shock as the wedge inclination is increased beyond the frozen sonic point.