Showing papers on "Shock wave published in 1998"

Radio emission from the unusual supernova 1998bw and its association with the γ-ray burst of 25 April 1998

Abstract: Data accumulated over the past year strongly favour the idea that γ-ray bursts lie at cosmological distances, although the nature of the power source remains unclear. Here we report radio observations of the supernova SN1998bw, which exploded at about the same time, and in about the same direction, as the γ-ray burst GRB980425. At its peak, the supernova was unusually luminous at radio wavelengths. A simple interpretation of the data requires that the source expanded with an apparent velocity of at least twice the speed of light, indicating that the supernova was accompanied by a shock wave moving at relativistic speeds (the ejects of supernovae are typically characterized by non-relativistic velocities). The energy of the shock is at least 10^(49)erg, with an inferred ejecta mass of 10^(-5) solar masses, and we suggest that the early phase of this shock wave produced the burst of γ-rays. Although in general the properties of supernovae are very different from those of γ-ray bursts, we argue that this unusual supernova establishes a second class of γ-ray burst, which is distinctly different from the cosmological kind.

Plasticity Induced by Shock Waves in Nonequilibrium Molecular-Dynamics Simulations

Abstract: Nonequilibrium molecular-dynamics simulations of shock waves in three-dimensional 10-million atom face-centered cubic crystals with cross-sectional dimensions of 100 by 100 unit cells show that the system slips along all of the available {111} slip planes, in different places along the nonplanar shock front. Comparison of these simulations with earlier ones on a smaller scale not only eliminates the possibility that the observed slippage is an artifact of transverse periodic boundary conditions, but also reveals the richness of the nanostructure left behind. By introducing a piston face that is no longer perfectly flat, mimicking a line or surface inhomogeneity in the unshocked material, it is shown that for weaker shock waves (below the perfect-crystal yield strength), stacking faults can be nucleated by preexisting extended defects.

The gap lemma and geometric criteria for instability of viscous shock profiles

Abstract: An obstacle in the use of Evans function theory for stability analysis of traveling waves occurs when the spectrum of the linearized operator about the wave accumulates at the imaginary axis, since the Evans function has in general been constructed only away from the essential spectrum. A notable case in which this difficulty occurs is in the stability analysis of viscous shock profiles. Here we prove a general theorem, the “gap lemma,” concerning the analytic continuation of the Evans function associated with the point spectrum of a traveling wave into the essential spectrum of the wave. This allows geometric stability theory to be applied in many cases where it could not be applied previously. We demonstrate the power of this method by analyzing the stability of certain undercompressive viscous shock waves. A necessary geometric condition for stability is determined in terms of the sign of a certain Melnikov integral of the associated viscous profile. This sign can easily be evaluated numerically. We also compute it analytically for solutions of several important classes of systems. In particular, we show for a wide class of systems that homoclinic (solitary) waves are linearly unstable, confirming these as the first known examples of unstable viscous shock waves. We also show that (strong) heteroclinic undercompressive waves are sometimes unstable. Similar stability conditions are also derived for Lax and overcompressive shocks and for nn

Transition to the Radiative Phase in Supernova Remnants

Abstract: The evolution of a supernova remnant through the transition from an adiabatic Sedov-Taylor blast wave to a radiative pressure-driven snowplow phase is studied using one- and two-dimensional hydrodynamic simulations. This transition is marked by a catastrophic collapse of the postshock gas, forming a thin, dense shell behind the forward shock. After the transition, the shock front is characterized by a deceleration parameter, Vt/R ≈ 0.33, which is considerably higher than the analytic estimate of for a pressure-driven snowplow. In two dimensions, the catastrophic collapse is accompanied by violent dynamical instabilities of the thin, cool shell. The violence of the collapse and the subsequent instability of the shell increase with increasing ambient density. Preshock density perturbations as small as 1% in an ambient medium with density of 100 cm-3 can lead to distortions of the shock front larger than 10% of the radius of the remnant.

Measurements of the Equation of State of Deuterium at the Fluid Insulator-Metal Transition

Abstract: A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography.

Models of Synchrotron X-Rays from Shell Supernova Remnants

Abstract: The diffusive shock acceleration process can accelerate particles to a maximum energy depending on the shock speed and age and on any competing loss processes on the particles The shock waves of young supernova remnants can easily accelerate electrons to energies in excess of 1 TeV, where they can produce X-rays by the synchrotron process I describe a detailed calculation of the morphology and spectrum of synchrotron X-rays from supernova remnants Remnants are assumed to be spherical and in the Sedov evolutionary phase, though the results are insensitive to the detailed dynamics The upstream magnetic field is assumed uniform; downstream it is assumed to be compressed but not additionally turbulently amplified In all cases, spectra begin to depart from power laws somewhere in the optical to UV range and roll off smoothly through the X-ray band I show that simple approximations for the electron emissivity are not adequate; a full convolution of the individual electron synchrotron emissivity with a calculated electron distribution at each point in the remnant is required Models limited by the finite shock age, by synchrotron or inverse Compton losses on electrons, or by escape of electrons above some energy have characteristically different spectral shapes, but within each class, models resemble one another strongly and can be related by simple scalings The images and spectra depend primarily on the remnant age, the upstream magnetic field strength, and the level of magnetic turbulence near the shock in which the electrons scatter In addition, images depend on the viewing or aspect angle between the upstream magnetic field and the line of sight The diffusion coefficient is assumed to be proportional to particle energy (or mean free path proportional to gyroradius), but I investigate the possibility that the proportionality constant becomes much larger above some energy, corresponding to an absence of long-wavelength MHD waves Models producing similar spectra may differ significantly in morphology, which allows for possible discriminations I parameterize the model spectra in terms of a slope at 4 keV and a factor by which the X-ray flux density at that energy falls below the extrapolated radio spectrum Synchrotron radiation may contribute significantly to the X-ray emission of remnants up to several thousand years old

Energy spectra of cosmic rays accelerated at ultrarelativistic shock waves

Abstract: Energy spectra of particles accelerated by the first-order Fermi mechanism are investigated at ultrarelativistic shock waves, outside the range of Lorentz factors considered previously. For particle transport near the shock a numerical method involving small amplitude pitch-angle scattering is applied for flows with Lorentz factors $\gamma$ from 3 to 243. For large $\gamma$ shocks a convergence of derived energy spectral indices up to the value $\sigma_\infty \approx 2.2$ is observed for all considered turbulence amplitudes and magnetic field configurations. Recently the same index was derived for $\gamma$-ray bursts by Waxman [Astrophys. J. Lett. 485, L5 (1997)].

Energy absorption of impacts during running at various stride lengths.

Abstract: Purpose:The foot-ground impact experienced during running produces a shock wave that is transmitted through the human skeletal system. This shock wave is attenuated by deformation of the ground/shoe as well as deformation of biological tissues in the body. The goal of this study was to inves

Observations of shock waves in cloud cavitation

Abstract: This paper describes an investigation of the dynamics and acoustics of cloud cavitation, the structures which are often formed by the periodic breakup and collapse of a sheet or vortex cavity. This form of cavitation frequently causes severe noise and damage, though the precise mechanism responsible for the enhancement of these adverse effects is not fully understood. In this paper, we investigate the large impulsive surface pressures generated by this type of cavitation and correlate these with the images from high-speed motion pictures. This reveals that several types of propagating structures (shock waves) are formed in a collapsing cloud and dictate the dynamics and acoustics of collapse. One type of shock wave structure is associated with the coherent collapse of a well-defined and separate cloud when it is convected into a region of higher pressure. This type of global structure causes the largest impulsive pressures and radiated noise. But two other types of structure, termed 'crescent-shaped regions' and 'leading-edge structures' occur during the less-coherent collapse of clouds. These local events are smaller and therefore produce less radiated noise but the interior pressure pulse magnitudes are almost as large as those produced by the global events. The ubiquity and severity of these propagating shock wave structures provides a new perspective on the mechanisms reponsible for noise and damage in cavitating flows involving clouds of bubbles. It would appear that shock wave dynamics rather than the collapse dynamics of single bubbles determine the damage and noise in many cavitating flows.

Dynamical Interaction of Solar Magnetic Elements and Granular Convection: Results of a Numerical Simulation

Abstract: Nonstationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) have been simulated using a numerical code for two-dimensional MHD with radiative energy transfer. Dynamical phenomena are identified in the simulations, which may contribute to chromospheric and coronal heating. Among these are the bending and horizontal displacement of a flux sheet by convective flows and the excitation and propagation of shock waves both within and outside the magnetic structure. Observational signatures of these phenomena are derived from calculated Stokes profiles of Zeeman-sensitive spectral lines. We suggest that the extended red wings of the observed Stokes V profiles are due to downward coacceleration of magnetized material in a turbulent boundary layer between the flux sheet and the strong external downflow. Upward-propagating shocks in magnetic structures should be detectable if a time resolution of about 10 s is achieved, together with a spatial resolution that allows one to isolate individual magnetic structures. Determination of the complicated internal dynamics of magnetic elements requires observations with a spatial resolution better than 100 km in the solar photosphere.

Influence of pulse duration on mechanical effects after laser-induced breakdown in water

Abstract: The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

The Chemistry of Explosives

Abstract: Explosive devices may be mechanical, chemical, or atomic. Mechanical explosions occur when a closed system is heated—a violent pressure rupture can occur. However, this doesn’t make a heated can of soup an explosive. An explosive substance is one which reacts chemically to produce heat and gas with rapid expansion of matter. A detonation is a very special type of explosion. It is a rapid chemical reaction, initiated by the heat accompanying a shock compression, which liberates sufficient energy, before any expansion occurs, to sustain the shock wave. A shock wave propagates into the unreacted material at supersonic speed, between 1500 m/s and 9000 m/s.

Radio Emission and Particle Acceleration in SN 1993J

Abstract: The radio light curves of SN 1993J are discussed. We find that a fit to the individual spectra by a synchrotron spectrum, suppressed by external free-free absorption and synchrotron self-absorption, gives a superior fit to models based on pure free-free absorption. A standard r-2 circumstellar medium is assumed and is found to be adequate. From the flux and cutoff wavelength, the magnetic field in the synchrotron-emitting region behind the shock is determined to B ≈ 64(Rs/1015 cm)-1 G. The strength of the field argues strongly for turbulent amplification behind the shock. The ratio of the magnetic and thermal energy density behind the shock is ~0.14. Synchrotron losses dominate the cooling of the electrons, whereas inverse Compton losses due to photospheric photons are less important. For most of the time also Coulomb cooling affects the spectrum. A model where a constant fraction of the shocked, thermal electrons are injected and accelerated, and subsequently lose their energy due to synchrotron losses, reproduces the observed evolution of the flux and number of relativistic electrons well. The injected electron spectrum has dn/dγ ∝ γ-2.1, consistent with diffusive shock acceleration. The injected number density of relativistic electrons scales with the thermal electron energy density, ρV2, rather than the density, ρ. The evolution of the flux is strongly connected to the deceleration of the shock wave. The total energy density of the relativistic electrons, if extrapolated to γ ~ 1, is ~5 × 10-4 of the thermal energy density. The free-free absorption required is consistent with previous calculations of the circumstellar temperature of SN 1993J, Te ~ (2-10) × 105 K, which failed in explaining the radio light curves by pure free-free absorption. Implications for the injection of the relativistic electrons, and the relative importance of free-free absorption, Razin suppression, and the synchrotron self-absorption effect for other supernovae, are also briefly discussed. It is argued that especially the expansion velocity, both directly and through the temperature, is important for determining the relative importance of the free-free absorption and synchrotron self-absorption. Some guidelines for the modeling and interpretation of VLBI observations are also given.

Shock oscillation in underexpanded screeching jets

Abstract: The periodic oscillation of the shock waves in screeching, underexpanded, supersonic jets, issuing from a choked, axisymmetric, nozzle at fully expanded Mach numbers (Mj) of 1.19 and 1.42, is studied experimentally and analytically. The experimental part uses schlieren photography and a new shock detection technique which depends on a recently observed phenomenon of laser light scattering by shock waves. A narrow laser beam is traversed from point to point in the flow field and the appearance of the scattered light is sensed by a photomultiplier tube (PMT). The time-averaged and phase-averaged statistics of the PMT data provide significant insight into the shock motion. It is found that the shocks move the most in the jet core and the least in the shear layer. This is opposite to the intuitive expectation of a larger-amplitude shock motion in the shear layer where organized vortices interact with the shock. The mode of shock motion is the same as that of the emitted screech tone. The instantaneous profiles of the first four shocks over an oscillation cycle were constructed through a detailed phase averaged measurement. Such data show a splitting of each shock (except for the first one) into two weaker ones through a ‘moving staircase-like’ motion. During a cycle of motion the downstream shock progressively fades away while a new shock appears upstream. Spark schlieren photographs demonstrate that a periodic convection of large organized vortices over the shock train results in the above described behaviour. An analytical formulation is constructed to determine the self-excitation of the jet column by the screech sound. The screech waves, while propagating over the jet column, add a periodic pressure fluctuation to the ambient level, which in turn perturbs the pressure distribution inside the jet. The oscillation amplitude of the first shock predicted from this linear analysis shows reasonable agreement with the measured data. Additional reasons for shock oscillation, such as a periodic perturbation of the shock formation mechanism owing to the passage of the organized structures, are also discussed.

Energetic Particle Events: Efficiency of Interplanetary Shocks as 50 keV < E < 100 MeV Proton Accelerators

Abstract: We have studied the injection rate of shock-accelerated protons in long-lasting particle events by tracing back the magnetohydrodynamic conditions at the shock under which protons are accelerated. This tracing back is carried out by -tting the observed Nux and anisotropy pro-les at di†erent energies, considering the magnetic connection between the shock and the observer, and modeling the propagation of the shock and of the particles along the interplanetary magnetic -eld. A focused-di†usion transport equation that includes the e†ects of adiabatic deceleration and solar wind convection has been used to model the evolution of the particle population. The mean free path and the injection rate have been derived by requiring consistency with the observed Nux and anisotropy pro-les for di†erent energies, in the upstream region of the events. We have extended the energy range of previous models down to 50 keV and up to D100 MeV. We have analyzed four proton events, representative of west, central merid- ian, and east scenarios. The spectra of the injection rate of shock-accelerated protons derived for these events show that for energies higher than 2 MeV the shock becomes a less efficient proton accelerator. We have related the derived injection rates to the evolution of the strength of the shock, particularly to the normalized downstream-upstream velocity ratio (VR), the magnetic -eld ratio, and the angle As h Bn . a result, we have derived an empirical relation of the injection rate with respect to the normalized veloc- ity ratio (log Q P VR), but we have not succeeded with the other two parameters. The Q(VR) relation allows us to determine the injection rate of shock-accelerated particles along the shock front and throughout its dynamical expansion, reproducing multispacecraft observations for one of the simulated events. This relation allows us to analyze the inNuence of the corotation e†ect on the modeled particle Nux and anisotropy pro-les. Subject headings: acceleration of particles E interplanetary medium E MHD E shock waves E Sun: particle emission

Shock Wave Emissions of a Sonoluminescing Bubble

Abstract: A single bubble in water is excited by a standing ultrasound wave. At high intensity the bubble starts to emit light. Together with the emitted light pulse, a shock wave is generated in the liquid at collapse time. The time-dependent velocity of the outward-traveling shock is measured with an imaging technique. The pressure in the shock and in the bubble is shown to have a lower limit of 5500 bars. Visualization of the shock and the bubble at different phases of the acoustic cycle reveals previously unobserved dynamics during stable and unstable sonoluminescence.

Cellular detonation stability. Part 1. A normal-mode linear analysis

Abstract: A detailed investigation of the hydrodynamic stability to transverse linear disturbances of a steady, one-dimensional detonation in an ideal gas undergoing an irreversible, unimolecular reaction with an Arrhenius rate constant is conducted via a normal-mode analysis The method of solution is an iterative shooting technique which integrates between the detonation shock and the reaction equilibrium point Variations in the disturbance growth rates and frequencies with transverse wavenumber, together with two-dimensional neutral stability curves and boundaries for all unstable low- and higher frequency modes, are obtained for varying detonation bifurcation parameters These include the detonation overdrive, chemical heat release and reaction activation energy Spatial perturbation eigenfunction behaviour and phase and group velocities are also obtained for selected sets of unstable modes Results are presented for both Chapman–Jouguet and overdriven detonation velocities Comparisons between the earlier pointwise determination of stability and interpolated neutral stability boundaries obtained by Erpenbeck are made Possible physical mechanisms which govern the wavenumber selection underlying the initial onset of either regular or irregular cell patterns are also discussed

On the Theory of Shock Waves for an Arbitrary Equation of State

Abstract: The fundamental equations of the hydrodynamic theory of one-dimensional shock waves — that is, the equations of conservation of mass, of momentum, and of energy — are developed. These are used to calculate the velocity, massvelocity, temperature, and pressure rise in shock waves in air and in water. With one additional equation, they suffice to permit a calculation of detonation velocities in gaseous and in solid explosives. Predictions of detonation velocity as a function of loading density are thereby achieved, accurate to a few percent. Pressures, temperatures, and mass-velocities inside the explosive are also computed. The question of rarefaction waves following the detonation front in the explosive is investigated. The initial velocity, pressure, and so forth, of the shock wave produced at the end of a stick of explosive are calculated successfully. The dying away of shock waves, problems of reflection, and so forth, are also discussed briefly.

Method and apparatus for predictive diagnosis of moving machine parts

Abstract: For automatically predicting machine failure a transducer sensor, such as piezoelectric crystal, is applied to a machine for sensing machine motion and structure-borne sound, including vibration friction, and shock waves. The structure-borne sound and motion sensed is converted to electrical signals which are filtered to leave only the friction and shock waves, which waves are processed, as by detecting the envelope and integrating beneath the enveloppe, resulting in a measure of friction and shock wave energy, i.e., stress wave energy. This measure is computed and processed for producing fault progression displays for periodic and aperiodic damage. This is accomplished in a personal computer, menu-driven environment.

3D numerical simulation of gaseous flows structure in semidetached binaries

Abstract: The results of 3D hydrodynamic simulation of mass transfer in semidetached binaries of different types (cataclysmic variables and low-mass X-ray binaries) are presented. We find that taking into account of a circumbinary envelope leads to significant changes in the stream-disc morphology. In particular, the obtained steady-state self-consistent solutions show an absence of impact between gas stream from the inner Lagrangian point L1 and forming accretion disc. The stream deviates under the action of gas of circumbinary envelope, and does not cause the shock perturbation of the disc boundary (traditional `hotspot'). At the same time, the gas of circumbinary envelope interacts with the stream and causes the formation of an extended shock wave, located on the stream edge. We discuss the implication of this model without `hotspot' (but with a shock wave located outside the disc) for interpretation of observations. The comparison of synthetic light curves with observations proves the validity of the discussed hydrodynamic model without `hotspot'. We also consider the influence of a circumbinary envelope on the mass transfer rate in semidetached binaries. The obtained features of flow structure in the vicinity of L1 show that the gas of circumbinary envelope plays an important role in the flow dynamics, and that it leads to significant (in order of magnitude) increasing of the mass transfer rate. The comparison of gaseous flows structure obtained in 2D and 3D approaches is presented. We discuss the common features of the flow structures and the possible reasons of revealed differences.

On laser induced single bubble near a solid boundary: Contribution to the understanding of erosion phenomena

Abstract: Cavitation erosion is an especially destructive and complex phenomenon. In order to understand its basic mechanism, the dynamics of laser-induced vapor bubbles have been investigated. Special experimental devices have been used to record ultrafast visualizations and pressure measurements. From these measurements, the different sources of stresses, induced on the solid wall by the presence of the bubble (shock wave, microjet), have been characterized. The “water hammer” pressure associated with the microjet velocity varies up to 210 MPa. When the bubble collapses near a solid wall, the pressure emitted is less than in an infinite medium. Pressure values up to 2.5 MPa have been found. These values have been associated with the duration of the pressure applied to the solid wall, which is about 30 ns for the microjet and more than 300 ns for the shock wave. These results have been correlated with the analysis of damage created on the surface sample.

Study of Passive Control in a Transonic Shock Wave/Boundary-Layer Interaction

Abstract: Passive control applied to a turbulent shock wave/boundary-layer interaction has been investigated by considering a two-dimensional channel flow. The field has been probed in great detail by using a two-component laser Doppler velocimetry system to execute mean velocity and turbulence measurements. Four different perforated plates have been considered along with the solid wall reference case. These measurements have shown that passive control deeply modifies the inviscid flowfield structure, the single shock being replaced by a lambda shock system. This modified compression induces a substantial reduction of the wave drag associated with the interaction. On the other hand, the combined injection-suction effect taking place in the control region provokes an increase of the viscous drag, which nearly outbalances the reduction in wave drag. It was found that passive control induced a modest decrease of the total drag compared to the solid wall case. Moreover, the experimental wall transpiration velocity distribution in the control region is well represented by the usual laws

Lattice-Boltzmann models for high speed flows

Abstract: We formulate a lattice Boltzmann model to solve supersonic flows. The particle velocities are determined by the mean velocity and internal energy. The adaptive nature of particle velocities permits the mean flow to have a high Mach number. The introduction of a particle potential energy causes the model to be suitable for a perfect gas with an arbitrary specific heat ratio. The macroscopic conservation equations are derived by the Chapman-Enskog method. The simulations were carried out on the hexagonal lattice. However, the extension to both two- and three-dimensional square lattices is straightforward. As preliminary tests, we present the Sod shock-tube simulation and the two-dimensional shock reflection simulation.

Characteristics of Undular Hydraulic Jumps: Experiments and Analysis

Abstract: The writers measured velocity, pressure and energy distributions, wavelengths, and wave amplitudes along undular jumps in a smooth rectangular channel 0.25 m wide. In each case the upstream flow was a fully developed shear flow. Analysis of the data shows that the jump has strong three-dimensional features and that the aspect ratio of the channel is an important parameter. Energy dissipation on the centerline is far from negligible and is largely constrained to the reach between the start of the lateral shock waves and the first wave crest of the jump, in which the boundary layer develops under a strong adverse pressure gradient. A Boussinesq-type solution of the free-surface profile, velocity, and energy and pressure distributions is developed and compared with the data. Limitations of the two-dimensional analysis are discussed.

Single-shot spatially resolved characterization of laser-induced shock waves in water

Abstract: We have developed an optical method for single-shot spatially resolved shock-wave peak-pressure measurements. A schlieren technique and streak photography were used to follow the propagation of the shock wave. The shock position r as a function of time was extracted from the streak images by digital image-processing techniques. The resulting r(t) curves were differentiated with respect to time to yield shock-wave velocities that were converted to shock pressures with the aid of the equation of the state of the medium. Features and limitations of the technique are demonstrated and discussed on the basis of measurements of shock-wave amplitudes generated by laser-induced breakdown in water. For this purpose, laser pulses of 6-ns duration and pulse energies of 1 and 10 mJ were focused into a cuvette containing water. Complete p(t) curves were obtained with a temporal resolution in the subnanosecond range. The total acquisition and processing time for a single event is ~2 min. The shock-peak pressures at the source were found to be 8.4 ? 1.5 and 11.8 ? 1.6 GPa for pulse energies of 1 and 10 mJ, respectively. Within the first two source radii, the shock-wave pressure p(r) was found to decay on average in proportion to r(-1.3?0.2) for both pulse energies. Thereafter the pressure dropped in proportion to r(-2.2?0.1). In water the method can be used to measure shock-wave amplitudes exceeding 0.1 GPa. Because it is a single-shot technique, the method is especially suited for investigating events with large statistical variations.

Numerical Simulations of Triggered Star Formation. I. Collapse of Dense Molecular Cloud Cores

Abstract: Results from numerical simulations of shock waves impacting molecular cloud cores are presented. The three-dimensional smoothed particle hydrodynamics code used in the calculations includes effects from a varying adiabatic exponent, molecular, atomic, and dust cooling, as well as magnetic pseudofluid. The molecular cloud cores are assumed to be embedded in background cloud material and to have evolved into their preimpact state under ambipolar diffusion. The shock wave is assumed to be locally plane parallel and steady. The results are sensitive to the thermodynamics employed in the calculations, because it determines the shock structure and the stability of the core. Shocks with velocities in the range of 20-45 km s-1 are capable of triggering collapse, while those with lower speeds rarely do. The results also depend on the properties of the preimpact core. Highly evolved cores with high initial densities are easier to trigger into collapse, and they tend to collapse to a single point. Less evolved cores with lower densities and larger radii may fragment during collapse and form binaries.

Optimal and wavelet-based shock wave detection and estimation

Abstract: Detection and estimation of aeroacoustic shock waves generated by supersonic projectiles are considered. The shock wave is an N-shaped acoustic wave emanating in the form of an acoustic cone trailing the projectile. An optimal detection/estimation scheme is considered based on a parametric signal plus white Gaussian noise model. To gain robustness and reduce complexity, we then focus on gradient estimators for shock wave edge detection, exploiting the very fast shock rise and fall times. The approach is cast in terms of a wavelet transform where the level of smoothing corresponds to scale. A multiscale analysis is described, consisting of multiscale products, to enhance edge detection and estimation. This method is effective and robust with respect to unknown environmental interference that will generally not exhibit singularities as sharp as the N-wave edges. Experimental results are presented for discriminating N waves in the presence of vehicle noise. Results are also shown, as a function of miss distance, for gradient-based detection of simulated small projectile shocks inserted into recorded tank noise.

A Finite - Volume Particle Method for CompressibleFlows

Abstract: We derive a new class of particle methods for conservation laws, which are based on numerical flux functions to model the interactions between moving particles. The derivation is similar to that of classical Finite-Volume methods; except that the fixed grid structure in the Finite-Volume method is substituted by so-called mass packets of particles. We give some numerical results on a shock wave solution for Burgers equation as well as the well-known one-dimensional shock tube problem.