Showing papers on "Shock wave published in 2009"
TL;DR: In this article, the authors present two-dimensional hydrodynamic simulations of stellar core collapse and develop the framework for a detailed analysis of the energetic aspects of neutrino-powered supernova explosions.
Abstract: We present two-dimensional hydrodynamic simulations of stellar core collapse and develop the framework for a detailed analysis of the energetic aspects of neutrino-powered supernova explosions. Our results confirm that the neutrino-heating mechanism remains a viable explanation of the explosion of a wider mass range of supernova progenitors with iron cores, but the explosion sets in later and develops differently than thought so far. The calculations were performed with an energy-dependent treatment of the neutrino transport based on the ray-by-ray plus approximation, in which the neutrino number, energy, and momentum equations are closed with a variable Eddington factor obtained by iteratively solving a model Boltzmann equation. We focus here on the evolution of a 15 M ☉ progenitor and provide evidence that shock revival and an explosion are initiated at about 600 ms after core bounce, powered by neutrino energy deposition. This is significantly later than previously found for an 11.2 M ☉ star, for which we also present a continuation of the explosion model published by Buras et al. The onset of the blast is fostered in both cases by the standing accretion-shock instability. This instability exhibits highest growth rates for the dipole and quadrupole modes, which lead to large-amplitude bipolar shock oscillations and push the shock to larger radii, thus increasing the time accreted matter is exposed to neutrino heating in the gain layer. As a consequence, also convective overturn behind the shock is strengthened, which otherwise is suppressed or damped because of the small shock stagnation radius. When the explosion sets in, the shock reveals a pronounced global deformation with a dominant dipolar component. In both the 11.2 M ☉ and 15 M ☉ explosions long-lasting equatorial downflows supply the gain layer with fresh gas, of which a sizable fraction is heated by neutrinos and leads to the build-up of the explosion energy of the ejecta over possibly hundreds of milliseconds. A soft nuclear equation of state that causes a rapid contraction, and a smaller radius of the forming neutron star and thus a fast release of gravitational binding energy, seems to be more favorable for the development of an explosion. Rotation has the opposite effect because in the long run it leads to a more extended and cooler neutron star and thus lower neutrino luminosities and mean energies and overall less neutrino heating. Neutron star g-mode oscillations, although we see their presence, and the acoustic mechanism play no important role in our simulations. While numerical tests show that our code is also well able to follow large-amplitude core g-modes if they are instigated; the amplitude of such oscillations remains small in our supernova runs and the acoustic energy flux injected by the ringing neutron star and by the deceleration of supersonic downflows near the neutron star surface is small compared to the neutrino energy deposition.
465 citations
TL;DR: In this paper, a large-eddy simulation of the interaction between an impinging oblique shock and a Mach 2.3 turbulent boundary layer is presented, which does not introduce any energetic low frequencies into the domain, hence avoiding possible interference with the shock/boundary layer interaction system.
Abstract: The need for better understanding of the low-frequency unsteadiness observed in shock wave/turbulent boundary layer interactions has been driving research in this area for several decades. We present here a large-eddy simulation investigation of the interaction between an impinging oblique shock and a Mach 2.3 turbulent boundary layer. Contrary to past large-eddy simulation investigations on shock/turbulent boundary layer interactions, we have used an inflow technique which does not introduce any energetically significant low frequencies into the domain, hence avoiding possible interference with the shock/boundary layer interaction system. The large-eddy simulation has been run for much longer times than previous computational studies making a Fourier analysis of the low frequency possible. The broadband and energetic low-frequency component found in the interaction is in excellent agreement with the experimental findings. Furthermore, a linear stability analysis of the mean flow was performed and a stationary unstable global mode was found. The long-run large-eddy simulation data were analyzed and a phase change in the wall pressure fluctuations was related to the global-mode structure, leading to a possible driving mechanism for the observed low-frequency motions.
455 citations
TL;DR: In this article, the authors present a detailed explanation of the physical mechanism of the nanosecond pulsed surface dielectric barrier discharge (SDBD) effect on the flow and show that the resulting pulsed-periodic disturbance causes an efficient transversal momentum transfer into the boundary layer and further flow attachment to the airfoil surface.
Abstract: This paper presents a detailed explanation of the physical mechanism of the nanosecond pulsed surface dielectric barrier discharge (SDBD) effect on the flow. Actuator-induced gas velocities show near-zero values for nanosecond pulses. The measurements performed show overheating in the discharge region on fast (? 1??s) thermalization of the plasma input energy. The mean values of such heating of the plasma layer can reach 70?K, 200?K and even 400?K for 7?ns, 12?ns and 50?ns pulse durations, respectively. The emerging shock wave together with the secondary vortex flows disturbs the main flow. The resulting pulsed-periodic disturbance causes an efficient transversal momentum transfer into the boundary layer and further flow attachment to the airfoil surface. Thus, for periodic pulsed nanosecond dielectric barrier discharge, the main mechanism of impact is the energy transfer and heating of the near-surface gas layer. The following pulse-periodic vortex movement stimulates redistribution of the main flow momentum.
321 citations
TL;DR: In this paper, a range of inclination angles between the pre-shock magnetic field and the shock normal was explored, and it was shown that only magnetic inclinations corresponding to subluminal shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration.
Abstract: We investigate shock structure and particle acceleration in relativistic magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell simulations. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. We find that only magnetic inclinations corresponding to subluminal shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration. The downstream spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail with exponential cutoff. For increasing magnetic inclination in the subluminal range, the high-energy tail accounts for an increasing fraction of particles (from ~1% to ~2%) and energy (from ~4% to ~12%). The spectral index of the power law increases with angle from –2.8 ± 0.1 to –2.3 ± 0.1. For nearly parallel shocks, particle energization mostly proceeds via the diffusive shock acceleration process; the upstream scattering is provided by oblique waves which are generated by the high-energy particles that escape upstream. For larger subluminal inclinations, shock-drift acceleration is the main acceleration mechanism, and the upstream oblique waves regulate injection into the acceleration process. For superluminal shocks, self-generated shock turbulence is not strong enough to overcome the kinematic constraints, and the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor γ0 5) magnetized (σ 0.03) flows exists only for a very small range of magnetic inclination angles (34°/γ0), so relativistic astrophysical pair shocks have to be either nearly parallel or weakly magnetized to generate nonthermal particles. These findings place constraints on the models of pulsar wind nebulae, gamma-ray bursts, and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized shocks.
304 citations
TL;DR: In this paper, the potential of microramp sub-boundary-layer vortex generators for flow control in supersonic engine inlets is investigated, focusing on the ability of these devices to beneficially affect oblique shockwave/ boundary-layer interactions.
Abstract: The potential of microramp sub-boundary-layer vortex generators for flow control in supersonic engine inlets is investigated. In particular, the study focuses on the ability of these devices to beneficially affect oblique shock-wave/ boundary-layer interactions. Experiments have been conducted at Mach 2.5 to determine the nature of flow controlled by microramps and to investigate their ability to delay separation in a reflected shock interaction. Various ramp heights between 30 and 90% of the boundary-layer thickness were investigated. The details of the vortical flow generated by such devices were identified. The general flow features were found to scale with device height and it is suggested that smaller devices need to be placed closer to the expected adverse pressure gradients. When applied to a separated oblique shock-wave/boundary-layer interaction generated with a 7 degree wedge, microramps were not able to completely eliminate flow separation, although they were shown to break up separated regions. Other performance indicators across the shock-wave/boundary-layer interaction were also improved through the application of the devices.
303 citations
TL;DR: The dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed to better understand the damage caused by collapsing bubbles.
Abstract: A high-order accurate shock- and interface-capturing scheme is used to simulate the collapse of a gas bubble in water. In order to better understand the damage caused by collapsing bubbles, the dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed. Collapse times, bubble displacements, interfacial velocities and surface pressures are quantified as a function of the pressure ratio driving the collapse and of the initial bubble stand-off distance from the wall; these quantities are compared to the available theory and experiments and show good agreement with the data for both the bubble dynamics and the propagation of the shock emitted upon the collapse. Non-spherical collapse involves the formation of a re-entrant jet directed towards the wall or in the direction of propagation of the incoming shock. In shock-induced collapse, very high jet velocities can be achieved, and the finite time for shock propagation through the bubble may be non-negligible compared to the collapse time for the pressure ratios of interest. Several types of shock waves are generated during the collapse, including precursor and water-hammer shocks that arise from the re-entrant jet formation and its impact upon the distal side of the bubble, respectively. The water-hammer shock can generate very high pressures on the wall, far exceeding those from the incident shock. The potential damage to the neighbouring surface is quantified by measuring the wall pressure. The range of stand-off distances and the surface area for which amplification of the incident shock due to bubble collapse occurs is determined.
284 citations
TL;DR: If such a phase transition happens in a future galactic supernova, its existence and properties should become observable as a second peak in the neutrino signal that is accompanied by significant changes in the energy of the emitted neutrinos.
Abstract: We explore the implications of the QCD phase transition during the postbounce evolution of core-collapse supernovae. Using the MIT bag model for the description of quark matter, we model phase transitions that occur during the early postbounce evolution. This stage of the evolution can be simulated with general relativistic three-flavor Boltzmann neutrino transport. The phase transition produces a second shock wave that triggers a delayed supernova explosion. If such a phase transition happens in a future galactic supernova, its existence and properties should become observable as a second peak in the neutrino signal that is accompanied by significant changes in the energy of the emitted neutrinos. This second neutrino burst is dominated by the emission of antineutrinos because the electron degeneracy is reduced when the second shock passes through the previously neutronized matter.
250 citations
TL;DR: In this paper, the authors describe a new experiment executed in the ONERA S3Ch transonic wind tunnel on shock oscillations over the OAT15A supercritical profile, which has allowed the precise definition of the conditions for buffet onset and the characterization of the properties of the periodic motion from unsteady surface pressure measurements.
Abstract: Shock wave/turbulent boundary-layer interaction and flow separation may induce self-sustained large-scale oscillations on a profile at transonic Mach number. This phenomenon, known as transonic buffet, is at the origin of intense pressure fluctuations which can have detrimental effects, both in external and internal aerodynamics. The present paper describes a new experiment executed in the ONERA S3Ch transonic wind tunnel on shock oscillations over the OAT15A supercritical profile. These experiments have allowed the precise definition of the conditions for buffet onset and the characterization of the properties of the periodic motion from unsteady surface pressure measurements. The flowfield behavior has been described in great detail thanks to high-speed schlieren cinematography and surveys with a two-component laser Doppler velocimetry along with a conditional sampling technique. The first aim of this study was to provide the computational fluid dynamics community with well-documented test cases to validate advanced computing methods. Concerning the physics of the phenomenon, it is suggested that it is mediated by acoustic waves which are produced at the trailing edge and which travel on the two sides of the airfoil. Also, the experimental results strongly suggest that the phenomenon is essentially two-dimensional, even if three-dimensional effects are also detected.
235 citations
TL;DR: In this article, a two-dimensional self-consistent numerical model of the discharge and gas dynamics in conditions similar to those of these experiments has been developed, which couples fluid discharge equations with compressible Navier?Stokes equations including momentum and thermal transfer from the plasma to the neutral gas.
Abstract: Surface dielectric barrier discharges (SDBDs) can modify the boundary layer of a flow and are studied as a possible means to control the flow over an airfoil. In SDBDs driven by sinusoidal voltages in the 1?10?kHz range, momentum is transferred from ions to the neutral gas, as in a corona discharge (ion wind), and the resulting electrohydrodynamic force can generate a flow of several m?s?1 in the boundary layer along the surface. In this paper we are interested in a different regime of SDBDs where nanosecond voltage pulses are applied between the electrodes. Recent experiments by the group of Starikovskii have demonstrated that such discharges are able to modify a flow although no significant ion wind can be detected.A two-dimensional self-consistent numerical model of the discharge and gas dynamics in conditions similar to those of these experiments has been developed. The model couples fluid discharge equations with compressible Navier?Stokes equations including momentum and thermal transfer from the plasma to the neutral gas. This is a difficult multi-scale problem and special care has been taken to accurately solve the equations over a large simulation domain and at a relatively low computational cost. The results show that under the conditions of the simulated experiments, fast gas heating takes place in the boundary layer, leading to the generation of a 'micro' shock wave, in agreement with the experiments.
221 citations
TL;DR: In this paper, two types of core configurations, with identical areal density, were subjected to the shock wave loading, and the results showed that configuration 1 outperformed configuration 2 in regards to their blast resistance.
214 citations
TL;DR: In this paper, the authors examined magnetohydrodynamic simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry.
Abstract: We examine magnetohydrodynamic simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry. The preshock two-phase medium is provided as a natural consequence of the thermal instability that is expected to be ubiquitous in the interstellar medium. We show that the shock-compressed shell becomes turbulent owing to the preshock density inhomogeneity, and magnetic field amplification takes place in the shell. The maximum field strength is determined by the condition that plasma β ~ 1, which gives the field strength on the order of 1 mG in the case of shock velocity ~103 km s–1. The strongly magnetized region shows filamentary and knotlike structures in two-dimensional simulations. The spatial scale of the regions with a magnetic field of ~1 mG in our simulation is roughly 0.05 pc, which is comparable to the spatial scale of the X-ray hot spots recently discovered in supernova remnants where the magnetic field strength is indicated to be amplified up to the order of 1 mG. This result may also suggest that the turbulent region with a locally strong magnetic field is expected to be spread out in the region with frequent supernova explosions, such as in the Galactic center and starburst galaxies.
TL;DR: In this article, the authors examined MHD simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry.
Abstract: We examine MHD simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry. The pre-shock two-phase medium is provided as a natural consequence of the thermal instability that is expected to be ubiquitous in the interstellar medium. We show that the shock-compressed shell becomes turbulent owing to the preshock density inhomogeneity and magnetic field amplification takes place in the shell. The maximum field strength is determined by the condition that plasma beta ~ 1, which gives the field strength on the order of 1 mG in the case of shock velocity ~ 1,000 km/s. The strongly magnetized region shows filamentary and knot-like structures in two-dimensional simulations. The spatial scale of the regions with magnetic field of 1 mG in our simulation is roughly 0.05 pc which is comparable to the spatial scale of the X-ray hot spots recently discovered in supernova remnants where the magnetic field strength is indicated to be amplified up to the order of 1 mG. This result may also suggest that the turbulent region with locally strong magnetic field is expected to be spread out in the region with frequent supernova explosions, such as in the Galactic center and starburst galaxies.
TL;DR: In this paper, a set of direct numerical simulations of isotropic turbulence passing through a nominally normal shock wave is presented, and the instantaneous structure of the shock/turbulence interaction is examined using averages conditioned on the instantaneous shock strength.
Abstract: A set of direct numerical simulations of isotropic turbulence passing through a nominally normal shock wave is presented. Upstream of the shock, the microscale Reynolds number is 40, the mean Mach number is 1.3–6.0, and the turbulence Mach number is 0.16–0.38. It is shown that the Kolmogorov scale decreases during the shock interaction, which implies that the grid resolution needed to resolve the viscous dissipation is finer than that used in previous studies. This leads to some qualitative differences with previous work, e.g., a rapid increase in the streamwise vorticity variance behind the shock and large anisotropy of the postshock Reynolds stresses. The instantaneous structure of the shock/turbulence interaction is examined using averages conditioned on the instantaneous shock strength. For locally strong compressions, the flow is characterized by overcompression, followed by an expansion. At points where the shock is locally weak, the profiles differ qualitatively depending on the strength of the inc...
TL;DR: In this paper, a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-Alfvenic fast-growing mode which has the same dispersion relation as that previously found by Bell in 2004 by treating the plasma in the magnetohydrodynamic approximation.
Abstract: We show that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-Alfvenic fast-growing mode which has the same dispersion relation as that previously found by Bell in 2004 by treating the plasma in the magnetohydrodynamic approximation. The kinetic approach allows us to investigate the dependence of the dispersion relation of these waves on the microphysics of the current which compensates the cosmic ray flow. We also show that a resonant and a non-resonant mode may appear at the same time and one of the two may become dominant on the other depending on the conditions in the acceleration region. We discuss the role of the unstable modes for magnetic field amplification and particle acceleration in supernova remnants at different stages of the remnant evolution.
TL;DR: In this paper, an experimental study was carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry.
Abstract: An experimental study is carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid, similar to what has been reported in other supersonic boundary layers. Three-dimensional vortical structures are found to be associated with the low-speed regions, in a way that can be explained by the hairpin packet model. The instantaneous reflected shock wave pattern is observed to conform to the low- and high-speed regions as they enter the interaction, and its organization may be qualitatively decomposed into streamwise translation and spanwise rippling patterns, in agreement with what has been observed in direct numerical simulations. The results are used to construct a conceptual model of the three-dimensional unsteady flow organization of the interaction.
TL;DR: In this article, a nanoparticle based planar laser scattering method (NPLS) is developed, where the nanoparticles are used as tracer, and pulseplanar laser is used as light source in NPLS; by recording images of particles in flow field with CCD, high spatiotemporal resolution supersonic flow imaging is realized.
Abstract: Due to influence of compressibility, shock wave, instabilities, and turbulence on supersonic flows, current flow visualization and imaging techniques encounter some problems in high spatiotemporal resolution and high signal-to-noise ratio (SNR) measurements. Therefore, nanoparticle based planar laser scattering method (NPLS) is developed here. The nanoparticles are used as tracer, and pulse planar laser is used as light source in NPLS; by recording images of particles in flow field with CCD, high spatiotemporal resolution supersonic flow imaging is realized. The flow-following ability of nanoparticles in supersonic flows is studied according to multiphase flow theory and calibrating experiment of oblique shock wave. The laser scattering characteristics of nanoparticles are analyzed with light scattering theory. The results of theoretical and experimental studies show that the dynamic behavior and light scattering characteristics of nanoparticles highly enhance the spatiotemporal resolution and SNR of NPLS, with which the flow field involving shock wave, expansion, Mach disk, boundary layer, sliding-line, and mixing layer can be imaged clearly at high spatiotemporal resolution.
TL;DR: In this article, the authors present basic and well-established results on the physics of SWBLI corresponding to a description in terms of an average two-dimensional steady flow, and some emphasis is placed on unsteadiness, which constitutes a salient feature of this phenomenon.
Abstract: When the flow past a vehicle flying at high velocity becomes supersonic, shock waves form, caused either by a change in the slope of a surface, a downstream obstacle or a back pressure constraining the flow to become subsonic In modern aerodynamics, one can cite a large number of circumstances where shock waves are present The encounter of a shock wave with a boundary layer results in complex phenomena because of the rapid retardation of the boundary layer flow and the propagation of the shock in a multilayered structure The consequence of shock wave/boundary layer interaction (SWBLI) are multiple and often critical for the vehicle or machine performance The shock submits the boundary layer to an adverse pressure gradient which may strongly distort its velocity profile At the same time, in turbulent flows, turbulence production is enhanced which amplifies the viscous dissipation leading to aggravated performance losses In addition, shock-induced separation most often results in large unsteadiness which can damage the vehicle structure or, at least, severely limit its performance The article first presents basic and well-established results on the physics of SWBLI corresponding to a description in terms of an average two-dimensional steady flow Such a description allows apprehending the essential properties of SWBLIs and drawing the main features of the overall flow structure associated with SWBLI Then, some emphasis is placed on unsteadiness in SWBLI which constitutes a salient feature of this phenomenon In spite of their importance, fluctuations in SWBLI have been considered since a relatively recent date although they represent a domain which deserves a special attention because of its importance for a clear physical understanding of interactions and of its practical consequences as in aeroelasticity
TL;DR: Measurements of the quartz Hugoniot curve from 0.1-1.6 TPa reveal substantial errors in the standard and have immediate ramifications for the equations of state of deuterium, helium, and carbon at pressures relevant to giant planets and other high-energy density conditions.
Abstract: Evaluation of models and theory of high-pressure material response is largely made through comparison with shock wave data, which rely on impedance match standards. The recent use of quartz as a shock wave standard has prompted a need for improved data. We report here on measurements of the quartz Hugoniot curve from 0.1-1.6 TPa. The new data, in agreement with our ab initio calculations, reveal substantial errors in the standard and have immediate ramifications for the equations of state of deuterium, helium, and carbon at pressures relevant to giant planets and other high-energy density conditions.
TL;DR: In this article, the authors demonstrate that coronal mass ejection (CME)-driven shocks can be detected in white light coronagraph images and in which properties such as the density compression ratio and shock direction can be deduced via simple modeling.
Abstract: In this paper, we demonstrate that coronal mass ejection (CME)-driven shocks can be detected in white light coronagraph images and in which properties such as the density compression ratio and shock direction can be measured. Also, their propagation direction can be deduced via simple modeling. We focused on CMEs during the ascending phase of solar cycle 23 when the large-scale morphology of the corona was simple. We selected events which were good candidates to drive a shock due to their high speeds (V > 1500 km s–1). The final list includes 15 CMEs. For each event, we calibrated the LASCO data, constructed excess mass images, and searched for indications of faint and relatively sharp fronts ahead of the bright CME front. We found such signatures in 86% (13/15) of the events and measured the upstream/downstream densities to estimate the shock strength. Our values are in agreement with theoretical expectations and show good correlations with the CME kinetic energy and momentum. Finally, we used a simple forward modeling technique to estimate the three-dimensional shape and orientation of the white light shock features. We found excellent agreement with the observed density profiles and the locations of the CME source regions. Our results strongly suggest that the observed brightness enhancements result from density enhancements due to a bow-shock structure driven by the CME.
TL;DR: In this article, a suite of progenitor models, neutrino luminosities, and two-dimensional simulations were used to investigate the matter gravitational wave (GW) emission from postbounce phases of neutrinos-driven core-collapse supernovae.
Abstract: Using a suite of progenitor models, neutrino luminosities, and two-dimensional simulations, we investigate the matter gravitational wave (GW) emission from postbounce phases of neutrino-driven core-collapse supernovae. These phases include prompt and steady-state convection, the standing accretion shock instability (SASI), and asymmetric explosions. For the stages before explosion, we propose a model for the source of GW emission. Downdrafts of the postshock-convection/SASI region strike the protoneutron star "surface" with large speeds and are decelerated by buoyancy forces. We find that the GW amplitude is set by the magnitude of deceleration and, by extension, the downdraft's speed and the vigor of postshock-convective/SASI motions. However, the characteristic frequencies, which evolve from ~100 Hz after bounce to ~300-400 Hz, are practically independent of these speeds (and turnover timescales). Instead, they are set by the deceleration timescale, which is in turn set by the buoyancy frequency at the lower boundary of postshock convection. Consequently, the characteristic GW frequencies are dependent upon a combination of core structure attributes, specifically the dense-matter equation of state (EOS) and details that determine the gradients at the boundary, including the accretion-rate history, the EOS at subnuclear densities, and neutrino transport. During explosion, the high frequency signal wanes and is replaced by a strong low frequency, ~10s of Hz, signal that reveals the general morphology of the explosion (i.e., prolate, oblate, or spherical). However, current and near-future GW detectors are sensitive to GW power at frequencies ≳50 Hz. Therefore, the signature of explosion will be the abrupt reduction of detectable GW emission.
TL;DR: In this paper, a series of three-dimensional simulations of the interaction of a supersonic wind with a nonspherical radiative cloud was performed, and the authors investigated the importance of radiative cooling on the cloud's survival.
Abstract: We have performed a series of three-dimensional simulations of the interaction of a supersonic wind with a nonspherical radiative cloud. These simulations are motivated by our recent three-dimensional model of a starburst-driven galactic wind interacting with an inhomogeneous disk, which shows that an optically emitting filament can be formed by the breakup and acceleration of a cloud into a supersonic wind. In this study, we consider the evolution of a cloud with two different geometries (fractal and spherical) and investigate the importance of radiative cooling on the cloud's survival. We have also undertaken a comprehensive resolution study in order to ascertain the effect of the assumed numerical resolution on the results. We find that the ability of the cloud to radiate heat is crucial for its survival, with a radiative cloud experiencing a lower degree of acceleration and having a higher relative Mach number to the flow than in the adiabatic case. This diminishes the destructive effect of the Kelvin-Helmholtz instability on the cloud. While an adiabatic cloud is destroyed over a short period of time, a radiative cloud is broken up via the Kelvin-Helmholtz instability into numerous small, dense cloudlets, which are drawn into the flow to form amore » filamentary structure. The degree of fragmentation is highly dependent on the resolution of the simulation, with the number of cloudlets formed increasing as the Kelvin-Helmholtz instability is better resolved. Nevertheless, there is a clear qualitative trend, with the filamentary structure still persistent at high resolution. The geometry of the cloud affects the speed at which the cloud fragments; a wind more rapidly breaks up the cloud in regions of least density. A cloud with a more inhomogeneous density distribution fragments faster than a cloud with a more uniform structure (e.g., a sphere). We confirm the mechanism behind the formation of the Halpha emitting filaments found in our global simulations of a starburst-driven wind. Based on our resolution study, we conclude that bow shocks around accelerated gas clouds, and their interaction, are the main source of the soft X-ray emission observed in these galactic-scale winds.« less
TL;DR: In this article, a linear fit on such a plot for each ground-level event determines the initial solar particle release (SPR) time, as the intercept, and the magnetic path length traversed as the slope, of the fitted line.
Abstract: Ground-level events (GLEs) are large solar energetic-particle events with sufficiently hard spectra for GeV protons to be detected by neutron monitors at ground level. For each of 30 well-observed historic GLEs from four solar cycles, extending back to 1973, I have plotted onset times versus velocity–1 for particles observed on the IMP-7 and 8, ISEE-3, Wind, and GOES spacecraft and by neutron monitors. A linear fit on such a plot for each GLE determines the initial solar particle release (SPR) time, as the intercept, and the magnetic path length traversed, as the slope, of the fitted line. Magnetic path lengths and SPR times are well determined by the fits and cannot be used as adjustable parameters to make particle and photon emission times coincide. SPR times follow the onsets of shock-induced type II radio bursts and the coronal height of the coronal mass ejection (CME)-driven shock at SPR time can be determined for GLEs spanning an interval of solar longitude of ~140 deg. For a given GLE, all particle species and energies diverge from a single SPR point at a given coronal height and footpoint longitude of the field line to the Earth. These heights tend to increase with longitudinal distance away from the source, a pattern expected for shock acceleration. Acceleration for magnetically well-connected large GLEs begins at ~2 solar radii, in contrast to non-GLEs that have been found to be strongly associated with shocks above ~3 solar radii. The higher densities and magnetic field strengths at lower altitudes may be responsible for the acceleration of higher-energy particles in GLEs, while those GLEs that begin above 3R S may compensate by having higher shock speeds. These results support the joint dependence of maximum particle energy on magnetic field strength, injected particle density, and shock speed, all predicted theoretically.
TL;DR: In this article, a semi-analytical kinetic calculation of the process of non-linear diffusive shock acceleration is presented, which includes magnetic field amplification due to cosmic ray induced streaming instability, the dynamical reaction of the amplified magnetic field and the possible effects of turbulent heating.
Abstract: We present a semi-analytical kinetic calculation of the process of non-linear diffusive shock acceleration (NLDSA) which includes magnetic field amplification due to cosmic ray induced streaming instability, the dynamical reaction of the amplified magnetic field and the possible effects of turbulent heating. This kinetic calculation allows us to show that the net effect of the amplified magnetic field is to enhance the maximum momentum of accelerated particles while reducing the concavity of the spectra, with respect to the standard predictions of NLDSA. This is mainly due to the dynamical reaction of the amplified field on the shock, which smoothens the shock precursor. The total compression factors which are obtained for parameters typical of supernova remnants are R{sub tot} {approx} 7-10, in good agreement with the values inferred from observations. The strength of the magnetic field produced through excitation of streaming instability is found in good agreement with the values inferred for several remnants if the thickness of the X-ray rims are interpreted as due to severe synchrotron losses of high energy electrons. We also discuss the relative role of turbulent heating and magnetic dynamical reaction in smoothening the shock precursor.
University of Hertfordshire1, Harvard University2, University of Bristol3, Ohio State University4, University of Virginia5, Pontifical Catholic University of Chile6, Massachusetts Institute of Technology7, Max Planck Society8, McMaster University9, Goddard Space Flight Center10, University of Birmingham11
TL;DR: In this paper, the authors present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time) and infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies.
Abstract: We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time). The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption ‐ thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray sync hrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely t hat the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We describe a self-consistent model for the lobe dynamics and shock properties using the shell of thermal emission to the north of the lobe to estimate the lobe pressure. Based on this model, we estimate that the lobe is expanding to the southwest with a velocity of �2600 km s 1 , roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain γmax for the accelerated particles to be �10 8 at the strongest part of the shock, consistent with expectat ions from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strength s.
01 Jan 2009
TL;DR: In this paper, the effects of facility-dependent gas dynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen-oxygen ignition delay times for low-temperature conditions.
Abstract: Recent literature has indicated that experimental shock tube ignition delay times for hydrogen combustion at low-temperature conditions may deviate significantly from those predicted by current detailed kinetic models. The source of this difference is uncertain. In the current study, the effects of shock tube facility-dependent gasdynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen–oxygen ignition delay times. Shock tube hydrogen–oxygen ignition delay time data were taken behind reflected shock waves at temperatures between 908 to 1118 K and pressures between 3.0 and 3.7 atm for two test mixtures: 4% H2, 2% O2, balance Ar, and 15% H2, 18% O2, balance Ar. The experimental ignition delay times at temperatures below 980 K are found to be shorter than those predicted by current mechanisms when the normal idealized constant volume (V) and internal energy (E) assumptions are employed. However, if non-ideal effects associated with facility performance and energy release are included in the modeling (using CHEMSHOCK, a new model which couples the experimental pressure trace with the constant V, E assumptions), the predicted ignition times more closely follow the experimental data. Applying the new CHEMSHOCK model to current experimental data allows refinement of the reaction rate for H + O2 + Ar ↔ HO2 + Ar, a key reaction in determining the hydrogen–oxygen ignition delay time in the low-temperature region.
TL;DR: In this article, the authors proposed a model for diffusive shock acceleration in which stochastic magnetic fields in the shock precursor are generated through purely fluid mechanisms of a so-called small-scale dynamo.
Abstract: We propose a model for diffusive shock acceleration (DSA) in which stochastic magnetic fields in the shock precursor are generated through purely fluid mechanisms of a so-called small-scale dynamo. This contrasts with previous DSA models that considered magnetic fields amplified through cosmic ray (CR) streaming instabilities, i.e., either by way of individual particles resonant scattering in the magnetic fields, or by macroscopic electric currents associated with large-scale CR streaming. Instead, in our picture, the solenoidal velocity perturbations that are required for the dynamo to work are produced through the interactions of the pressure gradient of the CR precursor and density perturbations in the inflowing fluid. Our estimates show that this mechanism provides fast growth of magnetic field and is very generic. We argue that for supernovae shocks the mechanism is capable of generating upstream magnetic fields that are sufficiently strong for accelerating CRs up to around 1016 eV. No action of any other mechanism is necessary.
TL;DR: In this paper, the authors investigate the nature of nonlinear fast magnetoacoustic waves about a 2D magnetic X-point and observe the formation of both fast and slow oblique magnetic shocks.
Abstract: Context. This paper extends the models of Craig & McClymont ( 1991, ApJ, 371, L41) and McLaughlin & Hood ( 2004, A& A, 420, 1129) to include finite beta and nonlinear effects. Aims. We investigate the nature of nonlinear fast magnetoacoustic waves about a 2D magnetic X-point.
Methods. We solve the compressible and resistive MHD equations using a Lagrangian remap, shock capturing code (Arber et al. 2001, J. Comp. Phys., 171, 151) and consider an initial condition in v x B . (z) over cap (a natural variable of the system).
Results. We observe the formation of both fast and slow oblique magnetic shocks. The nonlinear wave deforms the X-point into a "cusp-like" point which in turn collapses to a current sheet. The system then evolves through a series of horizontal and vertical current sheets, with associated changes in connectivity, i.e. the system exhibits oscillatory reconnection. Our final state is non-potential (but in force balance) due to asymmetric heating from the shocks. Larger amplitudes in our initial condition correspond to larger values of the final current density left in the system.
Conclusions. The inclusion of nonlinear terms introduces several new features to the system that were absent from the linear regime.
TL;DR: In this paper, relativistic unmagnetized collisionless shocks are used to drive magnetic field evolution on a timescale 104 plasma times and accelerate high-energy particles accelerated by the shock.
Abstract: We study relativistic unmagnetized collisionless shocks using unprecedentedly large particle-in-cell simulations of two-dimensional pair plasma. High-energy particles accelerated by the shock are found to drive magnetic field evolution on a timescale 104 plasma times. Progressively stronger magnetic fields are generated on larger scales in a growing region around the shock. Shock-generated magnetic fields and accelerated particles carry 1% and 10% of the downstream energy flux, respectively. Our results suggest limits on the magnetization of relativistic astrophysical flows.
TL;DR: The self-steepening of a nonlinear dust acoustic wave (DAW) into a saw-tooth wave with sharp gradient in dust density, very similar to those found in numerical solutions of the fully nonlinear fluid equations for a nondispersive DAW.
Abstract: Repeated, self-excited dust acoustic shock waves (DASWs) have been observed in a dc glow discharge dusty plasma using high-speed video imaging. Two major observations are reported: (1) The self-steepening of a nonlinear dust acoustic wave (DAW) into a saw-tooth wave with sharp gradient in dust density, very similar to those found in numerical solutions of the fully nonlinear fluid equations for a nondispersive DAW [B. Eliasson and P. K. Shukla, Phys. Rev. E 69, 067401 (2004)], and (2) the collision and confluence of two DASWs.
TL;DR: The role of magnetic fields in gamma-ray burst (GRB) flows remains debated in this paper, where the authors attempt to understand quantitatively the dynamical effect and observational signatures of GRB ejecta magnetization on the onset of the afterglow.
Abstract: Context. The role of magnetic fields in gamma-ray burst (GRB) flows remains debated. If of sufficient strength, they can leave their signature on the initial phases of the afterglow by substantially changing the backreaction of the flow as a consequence of its interaction with the external medium.Aims. We attempt to understand quantitatively the dynamical effect and observational signatures of GRB ejecta magnetization on the onset of the afterglow.Methods. We perform ultrahigh-resolution, one-dimensional, relativistic MHD simulations of the interaction between a radially expanding, magnetized ejecta with the interstellar medium. We require ultrahigh numerical resolution because of the extreme jump conditions in the region of interaction between the ejecta and the circumburst medium. We study the complete evolution of an ultrarelativistic shell to the self-similar asymptotic phase.Results. Our simulations demonstrate that the complete evolution can be characterized in terms of two parameters, the ξ parameter introduced by Sari and Piran and the magnetization . We use this fact in producing numerical models in which the shell Lorentz factor is between 10 and 20 and rescaling the results to arbitrarily large values of . We find that the reverse shock is typically weak or absent for ejecta characterized by . The onset of the forward shock emission is strongly dependent on the magnetization. On the other hand, the magnetic energy of the shell is transferred into the external medium on a short timescale (of several times the duration of the burst). The later forward shock emission contains no information about the initial magnetization of the flow. The asymptotic evolution of strongly magnetized shells, after experiencing significant deceleration, resembles that of hydrodynamic shells, i.e. they enter fully into the Blandford-McKee self-similar regime.