Showing papers on "Shock (mechanics) published in 1998"

Shock graphs and shape matching

Abstract: We have been developing a theory for the generic representation of 2-D shape, where structural descriptions are derived from the shocks (singularities) of a curve evolution process, acting on bounding contours. We now apply the theory to the problem of shape matching. The shocks are organized into a directed, acyclic shock graph, and complexity is managed by attending to the most significant (central) shape components first. The space of all such graphs is highly structured and can be characterized by the rules of a shock graph grammar. The grammar permits a reduction of a shockgraph to a unique rooted shock tree. We introduce a novel tree matching algorithm which finds the best set of corresponding nodes between two shock trees in polynomial time. Using a diverse database of shapes, we demonstrate our system's performance under articulation, occlusion, and changes in viewpoint.

On the use of shock-capturing schemes for Large-Eddy Simulation

Abstract: Numerical simulations of freely decaying isotropic fluid turbulence were performed at various Mach numbers (from 0.2 to 1.0) using known shock-capturing Euler schemes (Jameson, TVD-MUSCL, ENO) often employed for aeronautical applications. The objective of these calculations was to evaluate the relevance of the use of such schemes in the large-eddy simulation (LES) context. The potential of the monotone integrated large-eddy simulation (MILES) approach was investigated by carrying out computations without viscous diffusion terms. Although some known physical trends were respected, it is found that the small scales of the simulated flow suffer from high numerical damping. In a quasi-incompressible case, this numerical dissipation is tentatively interpreted in terms of turbulent dissipation, yielding the evaluation of equivalent Taylor micro-scales. The Reynolds numbers based on these are found between 30 and 40, depending on the scheme and resolution (up to 1283). The numerical dissipation is also interpreted in terms of subgrid-scale dissipation in a LES context, yielding equivalent Smagorinsky “constants” which do not level off with time and which remain larger than the commonly accepted values of the classical Smagorinsky constant. On the grounds of tests with either the Smagorinsky or a dynamic model, the addition of explicit subgrid-scale (SGS) models to shock-capturing Euler codes is not recommended.

Fermi Acceleration at the Fast Shock in a Solar Flare and the Impulsive Loop-Top Hard X-Ray Source

Abstract: Because of its high injection energy, Fermi acceleration has not been considered to be viable to explain nonthermal electrons (20-100 keV) produced in solar flares. Here we propose that nonthermal electrons are efficiently accelerated by the first-order Fermi process at the fast shock, as a natural consequence of the new magnetohydrodynamic picture of the flaring region revealed with Yohkoh. An oblique fast shock is naturally formed below the reconnection site and boosts the acceleration to significantly decrease the injection energy. The slow shocks attached to the reconnection X-point heat the plasma up to 10-20 MK, exceeding the injection energy. The combination of the oblique shock configuration and the preheating by the slow shock allows bulk electron acceleration from the thermal pool. The accelerated electrons are trapped between the two slow shocks due to the magnetic mirror downstream of the fast shock, thus explaining the impulsive loop-top hard X-ray source discovered with Yohkoh. The acceleration timescale is ~0.3-0.6 s, which is consistent with the timescale of impulsive bursts. When these electrons stream away from the region enclosed by the fast shock and the slow shocks, they are released toward the footpoints and may form the simultaneous double-source hard X-ray structure at the footpoints of the reconnected field lines.

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.

Formation of a jump by the dam-break wave over a granular bed

Abstract: A previously unreported shock feature associated with the scouring of a horizontal granular bed by a dam-break wave is discussed. Near the wave centre, the present study shows, the free surface breaks backward and a hydraulic jump forms. This behaviour is described from the standpoint of shallow-water theory, suitably extended to deal with non-equilibrium sediment transport. The shock formation involves a particularly strong coupling between flow free-surface evolution and bed morphodynamics. Support for our conclusions is sought through experimental and numerical approaches. In order to magnify the observed phenomena, measurements were performed for the case of light bed particles moving in sheet and debris flow modes. A detailed picture of the transient two-phase flow is presented, based on whole field acquisition of the grain motions by particle tracking techniques. Corresponding shallow-water solutions are constructed numerically using a shock capturing scheme. Finally, an interpretation of the jump formation is proposed based on the theory of characteristics.

Fermi acceleration at fast shock in a solar flare and impulsive loop-top hard X-ray source

Abstract: We propose that non-thermal electrons are efficiently accelerated by first-order Fermi process at the fast shock, as a natural consequence of the new magnetohydrodynamic picture of the flaring region revealed with Yohkoh. An oblique fast shock is naturally formed below the reconnection site, and boosts the acceleration to significantly decrease the injection energy. The slow shocks attached to the reconnection X-point heat the plasma up to 10--20 MK, exceeding the injection energy. The combination of the oblique shock configuration and the pre-heating by the slow shock allows bulk electron acceleration from the thermal pool. The accelerated electrons are trapped between the two slow shocks due to the magnetic mirror downstream of the fast shock, thus explaining the impulsive loop-top hard X-ray source discovered with Yohkoh. Acceleration time scale is ~ 0.3--0.6 s, which is consistent with the time scale of impulsive bursts. When these electrons stream away from the region enclosed by the fast shock and the slow shocks, they are released toward the footpoints and may form the simultaneous double-source hard X-ray structure at the footpoints of the reconnected field lines.

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.

Shock absorbing component and construction method

Abstract: A shock absorbing component having a pair of surfaces with a plurality of inwardly extending indentations in the top and bottom surfaces. The indentations extend between the surfaces to provide support members for the shock absorbing component. At least some of the indentations are hemispherical. The surfaces may be formed of mesh material to allow the passage of gas or fluid therethrough. One or more inserts may be placed in the indentations. The shock absorbing component can be constructed by molding upper and lower shock absorbing component halves wherein the molds are configured to provide indentations in the top and bottom surfaces. The upper and lower halves are then joined to complete the shock absorbing component.

On the shock induced failure of brittle solids

Abstract: The response of brittle materials to uniaxial compressive shock loading has been the subject of much recent discussion. The physical interpretation of the yield point of brittle materials, the Hugoniot elastic limit (HEL), the dependence of this threshold on propagation distance and the effect of polycrystalline microstructure remain to be comprehensively explained. Evidence of failure occurring in glasses behind a travelling boundary that follows a shock front has been accumulated and verified in several laboratories. Such a boundary has been called a failure wave. The variations of properties across this front include complete loss of tensile strength, partial loss of shear strength, reduction in acoustic impedance, lowered sound speed and opacity to light. Recently we have reported a similar behaviour in the polycrystalline ceramics silicon carbide and alumina. It is the object of this work to present our observations of these phenomena and their relation to failure and the HEL in brittle materials.

Laser-shock processing of aluminium-coated 55C1 steel in water-confinement regime, characterization and application to high-cycle fatigue behaviour

Abstract: 55C1 steel was irradiated with a high-power neodymium-glass laser with application to induce plastic shock waves within targets, through the expansion of a laser-induced surface plasma. Laser-shock processing experiments were conduced in the plasma-confined regime with water to increase the laser-induced peak stresses. Physical, mechanical and processings aspects were reviewed, such as the characterization of stress waves in coated steels with a VISAR velocimeter system, and the mechanical changes induced in 55Cl in terms of compressive residual stresses or work-hardening levels. With the use of convenient protective coatings, some 7-8 GPa peak stress levels could be achieved which authorized the generation of high compressive residual stress levels (nearly 80% of the compressive yield strength), but preserved the surface integrity from detrimental roughening. Surface modifications performed under different shock conditions were shown to display some 30% increase on the bending fatigue limits of 55C1 at R=0.1.

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.

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.

A Continuum Limit of the Toda Lattice

Abstract: Introduction Analysis of Log formula An example Monotone initial data Shock 1 Shock 2 Shock 3 Shock 4 Symmetric data Global description Large time calculations Appendix I--WKB Appendix II Bibliography.

Shock-resistant semiconductor device and method for producing same

Abstract: A plurality of substrates 1 to which have been flip-chip mounted semiconductor chips 2 are laminated by means of solder bumps 7 provided for the purpose of lamination. A elastic resin is caused to fill the space between the chip upper surface 9 and the substrate 1, thus providing a shock-absorbing material layer 8. By adopting this type of three-dimensional semiconductor modular structure, the shock-absorbing material layer 8 absorbs externally applied vibration and shock, thereby improving the immunity to vibration and shock.

Radio Emission and Particle Acceleration in SN 1993J

Abstract: The radio light curves of SN 1993J are found to be well fit by a synchrotron spectrum, suppressed by external free-free absorption and synchrotron self-absorption. A standard r^-2 circumstellar medium is assumed, and found to be adequate. The magnetic field and number density of relativistic electrons behind the shock are determined. The strength of the magnetic 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 and Coulomb cooling dominate the losses of the electrons. The injected electron spectrum has a power law index -2.1, consistent with diffusive shock acceleration, and the number density scales with the thermal electron energy density. The total energy density of the relativistic electrons is, if extrapolated to gamma ~ 1, ~ 5x10^-4 of the thermal energy density. The free-free absorption required is consistent with previous calculations of the circumstellar temperature of SN 1993J, T_e ~ (2-10)x10^5 K. The relative importance of free-free absorption, Razin suppression, and the synchrotron self-absorption effect for other supernovae are briefly discussed. Guidelines for the modeling and interpretation of VLBI observations are given.

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

Accurate measurement of laser-driven shock trajectories with velocity interferometry

Abstract: We describe a velocity interferometer used to measure the velocity and trajectory of laser driven shocks in liquid deuterium accurately and continuously. This demonstration of velocity interferometry to measure shock velocity and shock trajectory in condensed matter shows strong potential for future studies of laser-driven shocks in transparent media. Accuracy of this technique can be better than 1% in velocity and ±0.2 μm in position during a 10 ns interval.

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.

Experimental and numerical investigation of the shock-induced fluidization of a particles bed

Abstract: An experimental study on unsteady two phase flow is conducted in a vertical shock tube. Shock Mach numbers range from 1.3 to 1.5 in 1 bar. The particles are initially positioned in horizontal beds of various thicknesses. Our research covers a large domain of void fraction from 1 (single particles) to 0.35 (compact beds). The experiments provide shadowgraph images for the recording of particle trajectories (effect of the gas on the particles) and side-wall pressures (action of the particles on the gas). A dense two phase flow model has been elaborated and numerically solved using a finite difference scheme with pseudoviscosity. The simulated shock-induced fluidization of a 2 cm thick bed of 1.5 mm diameter glass particles is compared to the experiment.

Shock Compression of Condensed Materials

Abstract: Preface 1 Introduction 2 Measuring kinematic parameters of shock waves 3 Shock compression of metals 4 Compression of metal compounds 5 Minerals 6 Rocks 7 Compression of organic solids 8 Liquids 9 Conclusion References

Laser shock processing of 2024-T62 aluminum alloy

Abstract: Laser shock processing (LSP) is a relatively new technique for strengthening metals. A method developed for optimizing the LSP parameters is reported in this paper. The effects of LSP on the microstructure, hardness, surface roughness, residual stress, fatigue life, fatigue crack growth (FCG) of 2024-T62 aluminum alloy were investigated. The fatigue life of the laser-shocked specimens was two times greater than that of the unshocked specimens. The fatigue crack growth rates (FCGRs) at a given stress intensity were reduced by over one order of magnitude. The fatigue behavior improvements were attributed to a combination of increased dislocation density, decreased surface roughness and compressive residual stress induced by the laser shock waves.

Velocity measurements in turbulent gaseous mixtures induced by Richtmyer–Meshkov instability

Abstract: Instantaneous velocity measurements in a gaseous mixture arising from the shock wave-induced Richtmyer–Meshkov instability are conducted for the first time in a shock tube. Laser Doppler anemometry gives us the evolution of the axial velocity fluctuations of the mixing zone, before and after its interactions with reflected shock waves from the shock tube end wall. Experimental results demonstrate that a turbulent mixing zone is generated by the incident shock wave. Afterwards, the axial variance decreases before being amplified by the first reshock interaction through a baroclinic effect. Before the second reshock arrival, we measure once again a decrease of the turbulence level which is explained by both diffusion and dissipation.

Evaluation of Estimation Methods for High Unsteady Heat Fluxes from Surface Measurements

Abstract: Shock interactions such as those that occur during atmospheric re-entry can produce extreme thermal loads on aerospace structures. These interactions are reproduced experimentally in hypersonic wind tunnels to study how the flow structures relate to the deleterious heat fluxes. In these studies, the fluid jets created by shock interactions impinge on a test cylinder, where the temperature resulting from the heat flux is measured. These measurements are used to estimate the heat flux on the surface as a result of the shock interactions. Finding the boundary flux from discrete unsteady temperature measurements is characterized by instabilities in the solution. The purpose of this work is to evaluate existing methodologies for the determination of the unsteady heat flux and to introduce a new approach based on an inverse technique. The performance of these methods is measured in terms of accuracy and their ability to handle inherently unstable or highly dynamic data such as step fluxes and high-frequency oscillating fluxes. The inverse methods proved to be the most accurate and stable of the methods examined.

Experimental Investigation of Air Radiation from Behind a Strong Shock Wave

Abstract: Radiation phenomena behind a strong shock wave have been experimentally investigated using a freepiston double-diaphragm shock tube. The shock front is detected by pressure sensors, whose rise time is calibrated by a laser schlieren technique. Spatial distribution of emission spectra correlated with the shock front is obtained for a 270-520 nm wavelength range by means of one-dimensional imaging spectroscopy, in which a wavelength vs. position image is taken by an ICCD camera with the gate time of 100 ns. Molecular spectra of N2 second positive and N^~ first negative band are observed immediately after the shock front, while atomic line spectra from N become dominant shortly after it. The rotational and vibrational temperatures of molecules, and the electronic excitation temperature of atoms are evaluated, and their spatial profiles in the streamwise direction are obtained for a shock velocity of 11.9 km/s and an ambient pressure of 0.3 torr, using N2 as the test gas. The measured temperatures seem to be in significant nonequilibrium with the translational temperature. Also, the relaxation rates for Njj" seem to be much higher than those for Ng.

Localized shock- and friction-induced melting in response to hypervelocity impact

Abstract: Abstract The distribution of shock veins, friction melts and cataclastic rocks in complex impact craters reflects the response of target lithologies to varying rates of strain. Within the Sudbury impact structure, thin (<2 mm), anastomosing veins, which can define shatter cone surfaces, permeate the target rocks in a c. 15 km wide zone around the Sudbury igneous complex (SIC). A similar relationship exists within the Vredefort impact structure, with the additional association of the high-pressure SiO2 polymorphs coesite and stishovite. These S- (shock-dominant) type pseudotachylytes are comparable with shock veins developed in meteorites. Both are considered to form primarily by shock compression-decompression during the contact and compression stage of the collision process. Larger, thicker (up to 1 km wide) friction melt bodies constitute pseudotachylytes formed by the extreme comminution of fault walls during rebound and gravitational collapse of the transient cavity. They also appear to define the concentric fault systems of multi-ring impact basins. These E- (endogenic-) type pseudotachylytes form during the modification stage of the cratering process and post-date S-type pseudotachylytes. At Sudbury, they occur up to 80 km beyond the SIC. E-type pseudotachylytes are formed by the same mechanism as pseudotachylytes in non-impact-related fault systems: frictional melting of fault walls during seismogenic slip. Those in impact structures can be large because the extreme displacements (several kilometres) facilitated by superfaults generate massive volumes of friction melt.

Fluid Dynamics of Semiradiative Blast Waves

Abstract: We derive a self-similar solution for the propagation of an extreme relativistic (or Newtonian) radiative spherical blast wave into a surrounding cold medium. The solution is obtained under the assumption that the radiation process is fast, that it takes place only in the vicinity of the shock, and that it radiates away a fixed fraction of the energy generated by the shock. We find that the energy of the blast wave behaves as a power law of the location of the shock. The power-law index depends on the fraction of the energy emitted by the shock. We obtain an analytic solution for the interior of the blast wave. In the Newtonian regime these solutions generalize the Sedov-Taylor adiabatic solution and the pressure-driven fully radiative solution. In the extreme relativistic case, these solutions generalize the Blandford-McKee adiabatic solution. They provide a new fully radiative extreme relativistic solution that is different from the Blandford-McKee fully radiative relativistic solution. This new solution develops a hot interior that causes it to cool faster than previous estimates. These new solutions might be applicable to the study of γ-ray burst afterglow or supernova remnants.

Control of Shock Structure and Secondary Flow Field Inside Transonic Compressor Rotors Through Aerodynamic Sweep

Abstract: The present paper reports a numerical study on the effects of aerodynamic sweep applied to a low-aspect-ratio, high-through-flow, state-of-the-art, axial transonic compressor design. Numerical analyses based on the Reynolds-averaged Navier-Stokes equations were used to obtain the performance of a conventional unswept rotor, a forward swept rotor, and an aft-swept rotor, at both design and off-design operating conditions. The numerical analyses predicted that the forward-swept rotor has a higher peak efficiency and a substantially larger stall margin than the baseline unswept rotor, and that the aft-swept rotor has a similar peak efficiency as the unswept rotor with a significantly smaller stall margin. The rig test confirmed the numerical assessment of the effects of aerodynamic sweep on the low-aspect-ratio, high-through-flow, transonic compressor rotor. Detailed analyses of the measured and calculated flow fields indicate that two mechanisms are primarily responsible for the differences in aerodynamic performance among these rotors. The first mechanism is a change in the radial shape of the passage shock near the casing by the endwall effect, and the second is the radial migration of low-momentum fluid to the blade tip region. Aerodynamic sweep can be used to control the shock structure near the endwall and the migration of secondary flows and, consequently, flow structures near the tip area for improved performance.Copyright © 1998 by ASME

Phenomenological Analysis of Shock-Wave Propagation in Weakly Ionized Plasmas

Abstract: Shock propagation into weakly ionized gases shows several features differing markedly from conventional, nonionized-gas shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: existence of an energy (momentum) flux toward the wave precursor or volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: Typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density, weakly ionized plasmas also is unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question is whether there exists phase transition within the plasma shock. It also remains unclear to what extent twodimensional thermal inhomogeneity effects contribute to the observed phenomena. Answering these fundamental questions requires additional experimental studies of the problem. p D d E E/N F h j k