Showing papers on "Shock (mechanics) published in 2009"

Large-eddy simulation of low-frequency unsteadiness in a turbulent shock-induced separation bubble

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.

Particle Acceleration in Relativistic Magnetized Collisionless Pair Shocks: Dependence of Shock Acceleration on Magnetic Obliquity

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.

Numerical simulations of non-spherical bubble collapse

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.

External forward shock origin of high energy emission for three GRBs detected by Fermi

Abstract: We analyze the >100MeV data for 3 GRBs detected by Fermi (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock. We arrive at this conclusion by four different methods as follows. (1) We check the light curve and spectral behavior of the >100MeV data, and late time X-ray and optical data, and find them consistent with the closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100MeV, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100MeV data, and with these we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data. (4) We calculate the external forward shock model parameters using only the late time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high energy data obtained by Fermi/LAT. The ability of a simple external forward shock, to fit the entire data from the end of the burst (1-50s) to about a week, covering more than eight-decades in photon frequency provides compelling confirmation of the external forward shock synchrotron origin of the >100MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shock-compressed magnetic field in the circum-stellar medium with pre-shocked values of a few tens of micro-Gauss.

Particle Acceleration in Relativistic Magnetized Collisionless Pair Shocks: Dependence of Shock Acceleration on Magnetic Obliquity

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. Particle energization is driven by the Diffusive Shock Acceleration process for nearly parallel shocks, and switches to Shock-Drift Acceleration for larger subluminal inclinations. For "superluminal" shocks, the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor gamma0 > 5) magnetized (sigma > 0.03) flows exists only for a very small range of magnetic inclination angles (< 34/gamma0 degrees), 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 AGN jets, Pulsar Wind Nebulae and Gamma Ray Bursts that invoke particle acceleration in relativistic magnetized shocks. (Abridged)

Ion dynamics and acceleration in relativistic shocks

Abstract: Ab initio numerical study of collisionless shocks in electron-ion unmagnetized plasmas is performed with fully relativistic particle in cell simulations. The main properties of the shock are shown, focusing on the implications for particle acceleration. Results from previous works with a distinct numerical framework are recovered, including the shock structure and the overall acceleration features. Particle tracking is then used to analyze in detail the particle dynamics and the acceleration process. We observe an energy growth in time that can be reproduced by a Fermi-like mechanism with a reduced number of scatterings, in which the time between collisions increases as the particle gains energy, and the average acceleration efficiency is not ideal. The in depth analysis of the underlying physics is relevant to understanding the generation of high-energy cosmic rays, the impact on the astrophysical shock dynamics, and the consequent emission of radiation.

Direct numerical simulation of canonical shock/turbulence interaction

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...

Shock compression of quartz to 1.6 TPa: redefining a pressure standard.

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.

Dynamical feedback of self‐generated magnetic fields in cosmic ray modified shocks

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.

Time-resolved dynamics of nanosecond laser-induced phase explosion

Abstract: Visualization of Nd : YAG laser ablation of aluminium targets was performed by a shadowgraph apparatus capable of imaging the dynamics of ablation with nanosecond time resolution. Direct observations of vaporization, explosive phase change and shock waves were obtained. The influence of vaporization and phase explosion on shock wave velocity was directly measured. A significant increase in the shock wave velocity was observed at the onset of phase explosion. However, the shock wave behaviour followed the form of a Taylor–Sedov spherical shock below and above the explosive phase change threshold. The jump in the shock wave velocity above phase explosion threshold is attributed to the release of stored enthalpy in the superheated liquid surface. The energy released during phase explosion was estimated by fitting the transient shock wave position to the Taylor scaling rules. Results of temperature calculations indicate that the vapour temperature at the phase explosion threshold is slightly higher than the critical temperature at the early stages of the shock wave formation. The shock wave pressure nearly doubled when transitioning from normal vaporization to phase explosion.

On the escape of particles from cosmic ray modified shocks

Abstract: The solution of the problem of particle acceleration in the non-linear regime, when the dynamical reaction of the accelerated particles cannot be neglected, shows strong shock modification.When stationarity is imposed by hand, the solution may show a prominent energy flux away from the shock towards upstream infinity. This feature is peculiar of cosmic ray modified shocks, while being energetically insignificant in the test particle regime. The escape flux appears also in situations in which it is physically impossible to have particle escape towards upstream infinity, thereby leading to question its interpretation.We show here that the appearance of an escape flux is due to the unphysical assumption of stationarity of the problem, and in a realistic situation it translates to an increase of the value of the maximum-momentum when the shock velocity is constant. On the other hand, when the shock velocity decreases (for instance during the Sedov-Taylor phase of a supernova explosion), escape to upstream infinity is possible for particles with momenta in a narrow range close to the maximum momentum.

Weibel instability and associated strong fields in a fully three-dimensional simulation of a relativistic shock

Abstract: Plasma instabilities (e.g., Buneman, Weibel, and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new three-dimensional relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the active galactic nuclei and gamma-ray burst context.

Proper Motions and Brightness Variations of Nonthermal X-Ray Filaments in the Cassiopeia a Supernova Remnant

Abstract: We present Chandra ACIS X-ray observations of the Galactic supernova remnant Cassiopeia A taken in 2007 December. Combining these data with previous archival Chandra observations taken in 2000, 2002, and 2004, we estimate the remnant's forward shock velocity at various points around the outermost shell to range between 4200 and 5200 ± 500 km s–1. Using these results together with previous analyses of Cas A's X-ray emission, we present a model for the evolution of Cas A and find that it's expansion is well fit by a ρej ∝ r –(7–9) ejecta profile running into a circumstellar wind. We further find that while the position of the reverse shock in this model is consistent with that measured in the X-rays, in order to match the forward shock velocity and radius we had to assume that ~ 30% of the explosion energy has gone into accelerating cosmic rays at the forward shock. The new X-ray images also show that brightness variations can occur for some forward shock filaments like that seen for several nonthermal filaments seen projected in the interior of the remnant. Spectral fits to exterior forward shock filaments and interior nonthermal filaments show that they exhibit similar spectra. This together with similar flux variations suggests that interior nonthermal filaments might be simply forward shock filaments seen in projection and not located at the reverse shock as has been recently proposed.

Time-Lapse Changes of Seismic Velocity in the Shallow Ground Caused by Strong Ground Motion Shock of the 2000 Western-Tottori Earthquake, Japan, as Revealed from Coda Deconvolution Analysis

Abstract: A large earthquake shock often drops the seismic velocity of the shallow ground. However, it is not clear whether the dropped velocity recovers shortly after the earthquake shock or not. The purpose of this article is to report the time-lapse changes of seismic velocity in the shallow ground after the 2000 Western-Tottori earthquake, Japan. We deconvolve the coda record of small earthquakes registered on the ground surface by that registered at the 100 m depth in a borehole at a station that experienced a strong shock from the mainshock. Because coda waves are mostly composed of randomly scattered S waves, deconvolution of the two coda records enables us to obtain a robust image of the ground structure. Assuming that the shear modulus was reduced at the depth of 0–11 m, we estimate the shear modulus change in each time period after the mainshock by fitting synthetic coda deconvolution to the observed one from 1 to 16 Hz. As a result, the shear modulus dropped to 52% of the value obtained before the mainshock a few minutes after the strong earthquake shock. This caused a decrease in the S -wave velocity of 30% and an increase in S -wave travel time of 17 msec. The shear modulus continued to recover for over 1 yr following the logarithm of the lapse time. It recovered to 69%, 83%, 87%, and 97% of the value obtained before the mainshock in the periods of 0 to 1 week, 1 week to 1 month, 1 month to 1 yr, and 1 to 4 yr after the mainshock, respectively.

Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock

Abstract: Plasma instabilities (e.g., Buneman, Weibel and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new 3-D relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic (HD) like shock structure. In the leading shock, electron density increases by a factor of 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the AGN and GRB context.

Shock ignition: an alternative scheme for HiPER

Abstract: Two main paths are now under investigation that aim at thermonuclear ignition of hydrogen isotopes using lasers: central hot spot self-ignition and externally driven fast ignition of preassembled fuel. A third, intermediate, scheme is shock ignition, which combines the simplicity of self-ignition capsules to the hydrodynamic robustness of the fast ignition fuel assembly. This study addresses the potential of shock ignition for the HiPER project and provides a preliminary assessment of possible detrimental effects. Monodimensional simulations are performed to study the robustness of the ignition scheme in terms of shock launching time and laser power. Bidimensional simulations address the sensitivity of shock ignition to irradiation nonuniformity and to low mode asymmetries of the fuel assembly.

Adaptive shock detection

Abstract: Adaptive shock detection systems are provided according to various embodiments of the present invention. In one embodiment, an adaptive shock detection system comprises a shock sensing circuit, and a controller configured to receive shock detection signals from the shock sensing circuit and to dynamically adjust a sensitivity of the shock sensing circuit based the received shock detection signals.

The use of driver inserts to reduce non-ideal pressure variations behind reflected shock waves

Abstract: Non-ideal shock tube facility effects, such as incident shock wave attenuation, can cause variations in the pressure histories seen in reflected shock wave experiments. These variations can be reduced, and in some cases eliminated, by the use of driver inserts. Driver inserts, when designed properly, act as sources of expansion waves which can counteract or compensate for gradual increases in reflected shock pressure profiles. An algorithm for the design of these inserts is provided, and example pressure measurements are presented that demonstrate the success of this approach. When these driver inserts are employed, near- ideal, constant-volume performance in reflected shock wave experiments can be achieved, even at long test times. This near-ideal behavior simplifies the interpretation of shock tube chemical kinetics experiments, particularly in experiments which are highly sensitive to temperature and pressure changes, such as measurements of ignition delay time of exothermic reactions.

Electron shock surfing acceleration in multidimensions: two-dimensional particle-in-cell simulation of collisionless perpendicular shock

Abstract: Electron acceleration mechanisms in high-Mach-number collisionless shocks propagating in a weakly magnetized medium is investigated using a self-consistent two-dimensional particle-in-cell simulation. Simulation results show that strong electrostatic waves are excited via the electron-ion electrostatic two-stream instability at the leading edge of the shock transition region as in the case of earlier one-dimensional simulations. We observe strong electron acceleration that is associated with the turbulent electrostatic waves in the shock transition region. The electron energy spectrum in the shock transition region exhibits a clear power-law distribution with spectral index of 2.0-2.5. By analyzing the trajectories of accelerated electrons, we find that the acceleration mechanism is very similar to shock-surfing acceleration of ions. In contrast to the ion shock surfing, however, the energetic electrons are reflected by electron-scale electrostatic fluctuations in the shock transition region and not by the ion-scale cross-shock electrostatic potential. The reflected electrons are then accelerated by the convective electric field in front of the shock. We conclude that the multidimensional effects as well as the self-consistent shock structure are essential for the strong electron acceleration at high-Mach-number shocks.

Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube

Abstract: We report on new characterization capabilities recently implemented in the NASA Ames Electric Arc Shock Tube (EAST) facility. A new optical configuration, completely enclosed within a high vacuum chamber and attached to the shock tube, has enabled observation of spatially (and hence, temporally) resolved radiation through the shock layer. These imaging optics are coupled with four spectrometers covering the complete wavelength range of 1201700 nm, allowing for simultaneous measurement, at the same axial location, of spectral features over a broad range at various spectral resolutions. Measurements in the new system have addressed several of the discrepancies between model and experiment in prior EAST testing. The presence of CN impurity emission has been nearly eliminated, while atomic C and H emissions have been reduced. The presence of background continuum radiation has been confirmed as a real effect in the shock tube. Stark broadening measurements have been performed on Hydrogen Balmer-α line and show the electron number densities in the shock to be somewhat higher than model predictions. Experimental artifacts in the old configuration have been discovered and explain disagreements in the measured absolute magnitude of radiance.

Shock compression of reactive powder mixtures

Abstract: The shock compression of reactive powder mixtures can yield varied chemical behaviour with occurrence of mechanochemical reactions in the timescale of the high pressure state, or thermochemical reactions in the timescale of temperature equilibration, or simply the creation of dense packed highly reactive state of material. The principal challenge has been to understand the processes that distinguish between mechanochemical (shock induced) and thermochemical (shock assisted) reactions, which has broad implications for the synthesis of novel metastable or non-equilibrium materials, or the design of highly configurable next generation energetic materials. In this paper, the process of shock compression in reactive powder mixtures and the associated role of various intrinsic and extrinsic characteristics of reactants in the triggering of ultrafast shock induced chemical reactions are discussed. Experimental techniques employing time resolved diagnostics and results which identify the occurren...

A self- similar solution of a shock propagation in a mixture of a non-ideal gas and small solid particles

Abstract: Similarity solutions are obtained for unsteady, one-dimensional self-similar flow behind a strong shock wave, driven by a moving piston, in a dusty gas. The dusty gas is assumed to consist of a mixture of small solid particles and a non-ideal gas, in which solid particles are continuously distributed. It is assumed that the equilibrium flow-condition is maintained and variable energy input is continuously supplied by the piston. Solutions are obtained under both the isothermal and adiabatic conditions of the flow-field. The spherical case is worked out in detail to investigate to what extent the flow-field behind the shock is influenced by the non-idealness of the gas in the mixture as well as by the mass concentration of the solid particles, by the ratio of density of the solid particles to the initial density of the mixture and by the energy input due to moving piston. A comparison is also made between isothermal and adiabatic cases.

Ion Superdiffusion at the Solar Wind Termination Shock

Abstract: We investigate the propagation of 0.54-3.5 MeV ions accelerated at the termination shock of the solar wind. Data are from Voyager 2 and refer to a time interval about one year long, just before the Voyager 2 termination shock crossing at the end of 2007 August, at roughly 83.7 AU. A recently developed technique, which allows to unravel the transport properties from an analysis of the energetic particle time profiles, is used. The ion time profiles exhibit a power-law decay from a few days to 200 days before the shock front, so that transport is found to be superdiffusive, with a mean square deviation growing like Δx 2 ∝ t α, with α ~ 1.3. This means that ion propagation in the heliosphere can be intermediate between normal diffusion and ballistic motion. The implication of ion superdiffusion on particle acceleration mechanisms at the termination shock is discussed, as well as some observational evidence coming from both Voyager 1 and Voyager 2, which questions diffusive shock acceleration.