Showing papers on "Shock wave published in 1996"

Shock wave emission and cavitation bubble generation by picosecond and nanosecond optical breakdown in water

Abstract: Shock wave emission and cavitation bubble expansion after optical breakdown in water with Nd:YAG laser pulses of 30‐ps and 6‐ns duration is investigated for energies between 50 μJ and 10 mJ which are often used for intraocular laser surgery. Time‐resolved photography is applied to measure the position of the shock front and the bubble wall as a function of time. The photographs are used to determine the shock front and bubble wall velocity as well as the shock wave pressure as a function of time or position. Calculations of the bubble formation and shock wave emission are performed using the Gilmore model of cavitation bubble dynamics and the Kirkwood–Bethe hypothesis. The calculations are based on the laser pulse duration, the size of the plasma, and the maximally expanded cavitation bubble, i.e., on easily measurable parameters. They yield the dynamics of the bubble wall, the pressure evolution inside the bubble, and pressure profiles in the surrounding liquid at fixed times after the start of the laser...

Exact Analytic Solutions for Stellar Wind Bow Shocks

Abstract: Stellar wind bow shocks have been seen in association with a wide variety of stellar objects, from pulsars to young stars. A new solution method is presented for bow shocks in the thin-shell limit, stressing the importance of the conserved momentum within the shell. This method leads to exact analytic solutions to the classical problem of Baranov, Krasnobaev, & Kulikovskii. Simple formulae are given for the shell's shape, mass column density, and velocity of shocked gas at all points in the shell. These solutions will facilitate detailed comparison between observed sources and bow shock models.

Comparison of eddy viscosity-transport turbulence models for three-dimensional, shock-separated flowfields

Abstract: An evaluation of four one-equation eddy viscosity-transport turbulence closure models as applied to three-dimensional shock wave/boundary-layer interactions is presented herein. Comparisons of two versions of the Baldwin-Barth model, an approach of Edwards and McRae, and a modified form of the Spalart-Allmaras model are presented for two test cases, one involving Mach 8 flow over a flat plate/sharp fin apparatus and the other involving Mach 3 flow over a cylinder-offset-cone geometry. Strengths and weaknesses of the one-equation approaches are highlighted through direct comparison with experimental data, and the effect of grid refinement is examined.

Far-Infrared Water Emissions from Magnetohydrodynamic Shock Waves

Abstract: Nondissociative, magnetohydrodynamic, C-type shock waves are expected to be a prodigious source of far-infrared water emissions in dense interstellar regions. We have constructed a model to calculate the farinfrared H20 line spectra that emerge from such shocks. Using the best estimates currently available for the radiative cooling rate and the degree of ion-neutral coupling within the shocked gas, we modeled the temperature structure of MHD shocks using standard methods in which the charged and neutral particles are treated separately as two weakly coupled, interpenetrating fluids. Then we solved the equations of statistical equilibrium to find the populations of the lowest 179 and 170 rotational states of ortho- and para-H2O We have completed an extensive parameter study to determine the emergent H2O line luminosities as a function of preshock density in the range n(H2) equals 10(exp 4) - 10(sup 6.5)/cc and shock velocity in the range upsilon(sub s) = 5 - 40 km/ s. We find that numerous rotational transitions of water are potentially observable using the Infrared Space Observatory and the Submillimeter Wave Astronomy Satellite and may be used as diagnostics of the shocked gas. We have also computed the rotational and ro-vibrational emissions expected from H2, CO, and OH, and we discuss how complementary observations of such emissions may be used to further constrain the shock conditions. In common with previous studies, we come close to matching the observed H2, and high-J CO emissions from the Orion-KL star-forming region on the basis of a single shock model. We present our predictions for the strengths of H2O line emission from the Orion shock, and we show how our results may be scaled to other regions where molecular shocks are likely to be present.

Pickup ion energization by shock surfing

Abstract: Energization at a quasi-perpendicular shock is described for ions which approach the shock with a speed much less than that of the incoming plasma. These ions may be trapped between the shock electrostatic potential and the upstream Lorentz force and accelerated by “surfing” along the shock surface, before eventually escaping the shock into the upstream or downstream plasma. The process is described in detail, extending previous work on the mechanism, and energy gains are calculated. It is pointed out that pickup ions in the solar wind are ideally configured, so that a reasonable fraction of the ions can be accelerated by this mechanism at cometary bow shocks, the solar wind termination shock, and interplanetary traveling shocks. The mechanism may provide the required “injection” or preacceleration at quasi-perpendicular shocks for subsequent diffusive shock acceleration.

Resonance Oscillation of Radiative Shock Waves in Accretion Disks around Compact Objects

Abstract: We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power law processes: $\Lambda \propto T^{\alpha} \rho^2$ to mimic cooling in our simulation. We employ {\it Smoothed Particle Hydrodynamics} technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period is roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the post-shock cooling time. The amplitude of oscillation could be up to ten percent of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. Due to the oscillation, the energy output from the disk is also seen to vary quasi-periodically. We believe that these oscillations might be responsible for the quasi periodic oscillation (QPO) behaviors seen in several black hole candidates, in neutron star systems as well as dwarf novae outbursts such as SS Cygni and VW Hyi.

Cooling Timescales and Temporal Structure of Gamma-Ray Bursts

Abstract: A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultrarelativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in two shocks, a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is then radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling timescale of the shocked material in the forward and reverse shocks for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts, the energy density of the magnetic field in the shocked material must be greater than about 1% of the thermal energy density. In addition, the electrons must be nearly in equipartition with the protons, since otherwise we do not have reasonable radiative efficiencies of GRBs. Inverse Compton scattering can increase the cooling rate of the relevant electrons, but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, they provide a plausible explanation for several observations. The model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the presence of a characteristic break energy in GRB spectra.

Interaction of the solar wind with the local interstellar medium: A multifluid approach

Abstract: A distinct fluid description for neutral hydrogen originating from either the interstellar medium, the heliosheath, or the solar wind has been developed to describe the detailed interaction of the solar wind with the local interstellar medium (LISM) [Williams et al., 1995]. Such a multifluid description for the hydrogen serves to capture the highly anisotropic nature of the neutral distribution. The neutral multifluid is coupled to a hydrodynamic plasma through charge exchange and the time-dependent model is solved in two spatial dimensions. This approach is used (1) to elucidate the nature of the solar wind - LISM interaction more carefully and precisely than has been done previously; (2) to identify precisely the role played by the three identifiably distinct neutral populations in determining the global structure and dynamics of the heliosphere (it is found, for example, that the heliopause is weakly time-dependent for the two-shock model); (3) to contrast these results with those obtained from the simpler, computationally more efficient model developed by Pauls et al. [1995]; and (4) to investigate the differences in global heliospheric structure and neutral properties between a two-shock (which results from a supersonic interstellar wind impinging on the heliosphere) and a one-shock (i.e., for a subsonic interstellar wind) model. In particular, observational criteria are presented that may allow one to distinguish between a one-shock and a two-shock heliosphere.

Grand Unification of Solutions of Accretion and Winds around Black Holes and Neutron Stars

Abstract: We provide the complete set of global solutions of viscous transonic flows (VTFs) around black holes and neutron stars. These solutions describe the optically thick and optically thin flows from the horizon of the black hole or from the neutron star surface to the location where the flow joins with a Keplerian disk. We study the nature of the multiple sonic points as functions of advection, rotation, viscosity, heating and cooling. Stable shock waves, which join two transonic solutions, are found to be present in a large region of the parameter space. We classify the solutions in terms of whether or not the flow can have a standing shock wave. We find no new topology of solutions other than what are observed in our previous studies of isothermal VTFs. We particularly stress the importance of the boundary conditions and argue that we have the most complete solution of accretion and winds around black holes and neutron stars.

Experimental and Computational Investigation of the Tip Clearance Flow in a Transonic Axial Compressor Rotor

Abstract: Experimental and computational techniques are used to investigate tip clearance flows in a transonic axial compressor rotor at design and part speed conditions. Laser anemometer data acquired in the endwall region are presented for operating conditions near peak efficiency and near stall at 100% design speed and at near peak efficiency at 60% design speed. The role of the passage shock/leakage vortex interaction in generating endwall blockage is discussed. As a result of the shock/vortex interaction at design speed, the radial influence of the tip clearance flow extends to 20 times the physical tip clearance height. At part speed, in the absence of the shock, the radial extent is only 5 times the tip clearance height. Both measurements and analysis indicate that under part-speed operating conditions a second vortex, which does not originate from the tip leakage flow, forms in the endwall region within the blade passage and exits the passage near midpitch. Mixing of the leakage vortex with primary flow downstream of the rotor at both design and part speed conditions is also discussed.

Rate coefficients for charge transfer between hydrogen and the first 30 elements

Abstract: We present analytic fits to charge exchange rate coefficients over the full range of temperatures which occurs in photoionized or shock-heated plasmas. We consider reactions between neutral hydrogen and all elements with parent ion charge {ital q}=1{minus}4 up to {ital Z}=30. Many rates were obtained from various sources in the literature. For reactions for which no data were available, we calculated rates using the Landau-Zener formalism. For these new reactions, we tabulate both total and state-specific rate coefficients. All are fitted with a consistent, accurate formula. These fits may be incorporated easily into spectral synthesis codes, and we make available an electronic form of our results. We draw attention to the most important reactions without high-quality rate coefficients to encourage further work. {copyright} {ital 1996 The American Astronomical Society.}

Three‐dimensional multiscale MHD model of cometary plasma environments

Abstract: First results of a three-dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super-Alfvenic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10-km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws-type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS-MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M approximately equals 2, M(sub A) approximately equals 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 x 10(exp 3) km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop-shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the 'true' diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross-tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with published Giotto observations.

Shock waves in plasmas containing variable‐charge impurities

Abstract: Shock structures in plasmas containing variable‐charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter.

Electron-ion Equilibration in Nonradiative Shocks Associated With SN 1006

Abstract: The ultraviolet spectrum of SN 1006 observed by the Hopkins Ultraviolet Telescope (HUT) is modeled to infer the degree of electron-ion equilibration at the nonradiative shocks in the remnant. Such an approach is possible since the spectrum has lines (He II λ1640) whose excitation is dominated by electrons, and others (C IV λ1550, N V λ1240, O VI λ1036) in which protons and other ions are more important, and the intensity ratio between these is sensitive to the electron-ion temperature ratio. We find substantially less than full equilibration, marginally consistent with existing plasma simulations but rather more suggestive of even lower equilibration than these studies would predict. The more stringent limits on the electron temperature resulting from this allow a more accurate determination of the distance to SN 1006. Our result is 1.8 ± 0.3 kpc.

Nonlinear Particle Acceleration in Oblique Shocks

Abstract: The solution of the nonlinear diffusive shock acceleration problem, where the pressure of the non-thermal population is sufficient to modify the shock hydrodynamics, is widely recognized as a key to understanding particle acceleration in a variety of astrophysical environments. We have developed a Monte Carlo technique for self-consistently calculating the hydrodynamic structure of oblique, steady state shocks, together with the first-order Fermi acceleration process and associated nonthermal particle distributions. This is the first internally consistent treatment of modified shocks that includes cross-field diffusion of particles. Our method overcomes the injection problem faced by analytic descriptions of shock acceleration and the lack of adequate dynamic range and artificial suppression of cross-field diffusion faced by plasma simulations; it currently provides the most broad and versatile description of collisionless shocks undergoing efficient particle acceleration. We present solutions for plasma quantities and particle distributions upstream and downstream of shocks, illustrating the strong differences observed between nonlinear and test particle cases. It is found that, for strong scattering, there are only marginal differences in the injection efficiency and resultant spectra for two extreme scattering modes, namely large-angle scattering and pitch-angle diffusion, for a wide range of shock parameters, i.e., for nonper-pendicular subluminal shocks with field obliquities less than or equal to 75° and de Hoffmann-Teller frame speeds much less than the speed of light.

Shock waves in trombones

Abstract: Based on physical models of musical instruments and of the human voice, a new generation of sound synthesizers is born: virtual instruments. The models used for wind instruments are simple feedback loops in which a nonlinear sound source drives a linear filter representing the pipe of the instrument. While very rewarding musical sounds have been obtained with these models, it has become obvious that some essential phenomena escape such a description. In particular the brightness of the sound generated by trombones is expected to be due to the essential nonlinearity of the wave propagation in the pipe. At fortissimo levels this leads to shock wave formation observed in our experiments both from pressure measurements and flow visualization. A modest modification of the physical model could already take this phenomenon into account. The key idea is that the nonlinear effect is essential for the transfer of sound from the source toward the listener, but can be ignored in a model of the generation of the pipe ...

Interaction of a shock with a longitudinal vortex

Abstract: In this paper we study the shock/longitudinal vortex interaction problem in axisymmetric geometry. Linearized analysis for small vortex strength is performed, and compared with results from a high order axisymmetric shock-fitted Euler solution obtained for this purpose. It is confirmed that for weak vortices, predictions from linear theory agree well with results from nonlinear numerical simulations at the shock location. To handle very strong longitudinal vortices, which may ultimately break the shock, we use an axisymmetric high order essentially non-oscillatory (ENO) shock capturing scheme. Comparison of shock-captured and shock-fitted results are performed in their regions of common validity. We also study the vortex breakdown as a function of Mach number ranging from 1.3 to 10, thus extending the range of existing results. For vortex strengths above a critical value, a triple point forms on the shock and a secondary shock forms to provide the necessary deceleration so that the fluid velocity can adjust to downstream conditions at the shock.

Gas dynamics and radiation heat transfer in the vapor plume produced by pulsed laser irradiation of aluminum

Abstract: The interaction of pulsed laser irradiation of nanosecond duration with a metal surface is studied by numerical simulation. The heat transfer in the solid substrate and the melted liquid is modeled as one‐dimensional transient heat conduction using the enthalpy formulation for the solution of phase change problems. A discontinuity layer is assumed just above the liquid surface. Mass, momentum, and energy conservation are expressed across this layer, while the vapor across the discontinuity is modeled as an ideal gas. The compressible gas dynamics is computed numerically by solving the system of Euler equations for mass, momentum, and energy, supplemented with an isentropic equation of state in a two‐dimensional axisymmetric system of coordinates. The excimer laser‐beam absorption and radiation transport in the vapor phase are modeled using the discrete ordinates method. The rates for ionization are computed using the Saha–Eggert equation assuming conditions of local thermal equilibrium. The inverse bremsstrahlung mechanism is considered as the main mechanism of plasma absorption. Results show that a thin, submicron vapor layer is formed above the target surface in the duration of laser pulse while thermal radiation plays the key role for plume cooling during the period of strong absorption by the plasma. The release of a very strong shock wave, propagating with a speed of 104 m/s, is observed in the evaporating plume.

Shock waves in a liquid containing small gas bubbles

Abstract: Numerical and experimental studies of the transient shock wave phenomena in a liquid containing non‐condensable gas bubbles are presented. In the numerical analysis, individual bubbles are tracked to estimate the effect of volume oscillations on the wave phenomena. Thermal processes inside each bubble, which have significant influence on the volume oscillation, are calculated directly using full equations for mass, momentum and energy conservation, and those results are combined with the averaged conservation equations of the bubbly mixture to simulate the propagation of the shock wave. A silicone oil/nitrogen bubble mixture, in which the initial bubble radius is about 0.6 mm and the gas volume fraction is 0.15% – 0.4%, is used in the shock tube experiments. The inner diameter of the shock tube is chosen to be 18 mm and 52 mm in order to investigate the multidimensional effects on the wave phenomena. In a fairly uniform bubbly mixture, the experimental results agree well with the numerical ones computed using a uniform spatial distribution of bubbles. On the other hand, in all the other experiments, the bubbles in the shock tubes are not distributed uniformly, being relatively concentrated along the axis of the tube. This non‐uniformity substantially alters the profile of the shock waves. The numerical predictions where such a distribution is taken into account agree well with those experimental data.

On the dynamics of multi-dimensional detonation

Abstract: We present an asymptotic theory for the dynamics of detonation when the radius of curvature of the detonation shock is large compared to the one-dimensional, steady, Chapman-Jouguet (CJ) detonation reaction-zone thickness. The analysis considers additional time-dependence in the slowly varying reaction zone to that considered ill previous works. The detonation is assumed to have a sonic point in the reaction- zone structure behind the shock, and is referred to as an eigenvalue detonation. A new, iterative method is used to calculate the eigenvalue relation, which ultimately is expressed as an intrinsic, partial differential equation (PDE) for the motion of the shock surface. Two cases are considered for an ideal equation of state. The first corresponds to a model of a condensed-phase explosive, with modest reaction rate sensitivity, and the intrinsic shock surface PDE is a relation between the normal detonation shock velocity, Dn, the first normal time derivative of the normal shock velocity, Dn, and the shock curvature, K. The second case corresponds to a gaseous explosive mixture, with the large reaction rate sensitivity of Arrhenius kinetics, and the intrinsic shock surface PDE is a relation between the normal detonation shock velocity, Dn, its first and second normal time derivatives of the normal shock velocity, Dn, Dn, and the shock curvature, K, and its first normal time derivative of the curvature, k. For the second case, one obtains a one-dimensional theory of pulsations of plane CJ detonation and a theory that predicts the evolution of self-sustained cellular detonation. Versions of the theory include the limits of near-CJ detonation, and when the normal detonation velocity is significantly below its CJ value. The curvature of the detonation can also be of either sign, corresponding to both diverging and converging geometries.

GPS detection of ionospheric perturbations following a space shuttle ascent

Abstract: The exhaust plume of the Space Shuttle during its ascent is a very powerful source of energy that excites atmospheric acoustic perturbations. Because of the coupling between neutral particles and electrons at ionospheric altitudes, these low frequency acoustic perturbations induce variations of the ionospheric electron density. We computed ionospheric electron content time series using Global Positioning System data collected on Bermuda island during the STS-58 Space Shuttle launch. The analysis of these time series shows a perturbation of the ionospheric electron content following the launch and lasting for 35 mn, with periods less than 10 mn. The perturbation is complex and shows two sub-events separated by about 15 mn at 200 km from the source. The phase velocities and waveform characteristics of the two sub-events lead us to interpret the first impulsive arrival as the direct propagation of the shock wave front, followed by oscillatory guided waves probably excited by the primary shock wave and propagating along horizontal atmospheric interfaces at 120 km altitude and below.

Experimental study of a normal shock/homogeneous turbulence interaction

Abstract: A new method is used to generate a quasihomogeneous isotropic turbulent supersonic flow at Mach number 3. The interaction with a normal shock wave is analyzed by means of hot-wire anemometry and laser Doppler velocimetry. Preliminary results show that the usual behavior of unperturbed turbulence is observed (isotropy, spectra, integral scales, and decay). The shock wave increases the longitudinal fluctuating velocities in agreement with Ribner's theory. Increase of the high-frequency components and decrease of the integral longitudinal scales are observed at the traverse of the shock.

Richtmyer-Meshkov instability with strong radiatively driven shocks

Abstract: The Richtmyer–Meshkov instability is investigated with strong radiatively driven shocks (Mach≳20, 5×compression) using the Nova laser. The target consists of a solid density ablator and lower‐density plastics (Atwood number A<0) in planar geometry to facilitate in‐flight radiographic diagnosis. Perturbations η=η0 cos kx are imposed at the interface to seed the instability. The experiments agree with full hydrodynamic simulations over a wide variety of conditions. For small initial amplitudes ‖A‖kη0<1, the growth rate agrees with a linear impulsive model using the average of the pre‐ and post‐shock initial amplitudes. For ‖A‖kη0≳1, the growth rate is limited to the difference between the transmitted shock speed and the interface speed.

Method for the comminution of concretions

Abstract: The invention relates to a method for the comminution of concretions in vivo by controlled, concentrated cavitation energy. This method utilizes two shock wave pulses (7, 8) with a specified time delay (3) and pressure relationship, with the first shock wave pulse (7) being used to induce a transient cavitation bubble cluster (9) near the target concretion (4), and the second shock wave pulse (8) to control and force the collapse of the cavitation bubble cluster towards the target concretion (4) with concentrated energy disposition, while avoiding injury to surrounding tissue caused by random collapse of the cavitation bubbles (9). This invention makes it possible to significantly enhance the fragmentation efficiency of the concretion using shock waves while reducing potential deleterious injury to surrounding tissue.

Indirect and direct laser driven shock waves and applications to copper equation of state measurements in the 10-40 Mbar pressure range.

Abstract: High quality shock waves with direct- and indirect-laser drive were generated. We used the phase zone plate smoothing technique in the case of direct drive and thermal x rays from laser heated cavities in the case of indirect drive. The possibility of producing homogeneous, steady shock waves without significant preheating effects with both methods has been proved. By using such shocks, copper equation of state measurements have been performed up to 40 Mbar, which was previously obtained only with nuclear explosions. \textcopyright{} 1996 The American Physical Society.