Showing papers on "Shock (mechanics) published in 2016"

Tailored Buckling Microlattices as Reusable Light‐Weight Shock Absorbers

Abstract: Structures and materials absorbing mechanical (shock) energy commonly exploit either viscoelasticity or destructive modifications. Based on a class of uniaxial light-weight geometrically nonlinear mechanical microlattices and using buckling of inner elements, either a sequence of snap-ins followed by irreversible hysteretic - yet repeatable - self-recovery or multistability is achieved, enabling programmable behavior. Proof-of-principle experiments on three-dimensional polymer microstructures are presented.

The microphysics of collisionless shock waves

Abstract: Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulae, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.

The microphysics of collisionless shock waves

Abstract: Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulae, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in-situ observations, analytical and numerical developments. A particular emphasize is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics

Progenitor-dependent Explosion Dynamics in Self-consistent, Axisymmetric Simulations of Neutrino-driven Core-collapse Supernovae

Abstract: We present self-consistent, axisymmetric core-collapse supernova simulations performed with the Prometheus-Vertex code for 18 pre-supernova models in the range of 11-28 solar masses, including progenitors recently investigated by other groups. All models develop explosions, but depending on the progenitor structure, they can be divided into two classes. With a steep density decline at the Si/Si-O interface, the arrival of this interface at the shock front leads to a sudden drop of the mass-accretion rate, triggering a rapid approach to explosion. With a more gradually decreasing accretion rate, it takes longer for the neutrino heating to overcome the accretion ram pressure and explosions set in later. Early explosions are facilitated by high mass-accretion rates after bounce and correspondingly high neutrino luminosities combined with a pronounced drop of the accretion rate and ram pressure at the Si/Si-O interface. Because of rapidly shrinking neutron star radii and receding shock fronts after the passage through their maxima, our models exhibit short advection time scales, which favor the efficient growth of the standing accretion-shock instability. The latter plays a supportive role at least for the initiation of the re-expansion of the stalled shock before runaway. Taking into account the effects of turbulent pressure in the gain layer, we derive a generalized condition for the critical neutrino luminosity that captures the explosion behavior of all models very well. We validate the robustness of our findings by testing the influence of stochasticity, numerical resolution, and approximations in some aspects of the microphysics.

Rippled Quasiperpendicular Shock Observed by the Magnetospheric Multiscale Spacecraft.

Abstract: Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.

Shock compression response of high entropy alloys

Abstract: In this work, we studied the shock response of two typical equiatomic high entropy alloys (HEAs) (i.e. FCC-structured CrMnFeCoNi alloy and BCC-structured NiCoFeCrAl alloy). The experimental results show that these two HEAs exhibit a relatively high Hugoniot elastic limit and high-phase transition threshold stress. We attribute this anomalous dynamic response of HEAs to their intrinsic chemically disordered structures. This work may provide new insight into shock compression behavior of HEAs.IMPACT STATEMENT This is the first work to demonstrate that high entropy alloys (HEAs) behave ‘super-stably’ under shock loading, which may provide new insight into shock compression behavior of HEAs.

Quick -witted damping device is glued to full -automatic system

Abstract: The utility model discloses a quick -witted damping device is glued to full -automatic system, including mounting panel and bottom plate, the both sides of bottom plate are provided with fixing bolt, are provided with the fastening rubber pad on the lower surface of bottom plate, be equipped with vibration -isolating rubber pad on the upper surface of bottom plate, be equipped with magnetism shock attenuation unit on vibration -isolating rubber pad, magnetism shock attenuation unit removes end and stiff end and all sets up in the shell including shell, removal end, stiff end and bracing piece, and remove end and stiff end and be the permanent magnet, and the magnetic pole syntropy, a fixed support bar is served in the removal, vibration -isolating rubber pad's both sides still are equipped with the spring respectively. The utility model discloses a fix pneumatic diaphragm pump on the mounting panel, its vibrations that produce at the operation in -process are absorbed by magnetism shock attenuation unit and spring, can not produce the striking to ground, and the interelectrode interact of magnetism can not take place deformation because of long -time the use, and the shock attenuation effect remains unchanged, reduces and safeguards the frequency, keeps good shock attenuation effect.

Reliability analysis of multiple-component series systems subject to hard and soft failures with dependent shock effects

Abstract: New reliability models have been developed for systems subject to competing hard and soft failure processes with shocks that have dependent effects. In the new model, hard failure occurs when transmitted system shocks are large enough to cause any component in a series system to fail immediately, soft failure occurs when any component deteriorates to a certain failure threshold, and system shocks affect both failure processes for all components. Our new research extends previous reliability models that had dependent failure processes, where the dependency was only because of the shared number of shock exposures and not the shock effects associated with individual system shocks. Dependency of transmitted shock sizes and shock damages to the specific failure processes for all components has not been sufficiently considered, and yet for some actual examples, this can be important. In practice, the effects of shock damages to the multiple failure processes among components are often dependent. In this...

A shock front at the radio relic of Abell 2744

Abstract: Radio relics are Mpc-scale diffuse radio sources at the peripheries of galaxy clusters which are thought to trace outgoing merger shocks. We present XMM-Newton and Suzaku observations of the galaxy cluster Abell 2744 (z = 0.306), which reveal the presence of a shock front 1.5 Mpc east of the cluster core. The surface-brightness jump coincides with the position of a known radio relic. Although the surface-brightness jump indicates a weak shock with aMach number M = 1.7(-0.3)(+0.5), the plasma in the post-shock region has been heated to a very high temperature (similar to 13 keV) by the passage of the shock wave. The low-acceleration efficiency expected from such a weak shock suggests that mildly relativistic electrons have been re-accelerated by the passage of the shock front.

21.2 A 4µW-to-1mW parallel-SSHI rectifier for piezoelectric energy harvesting of periodic and shock excitations with inductor sharing, cold start-up and up to 681% power extraction improvement

Abstract: A piezoelectric energy harvester (PEH) converts vibration-induced mechanical stress into electrical charge. This conversion is optimized for cantilever-based generators when excited continuously in resonance. However, this is rarely achieved using ambient vibrations, where changes in excitation frequencies (fEX) and magnitudes, or shock excitations are more common [1]. The implemented system (Fig. 21.2.1) is a fully autonomous energy-harvesting interface based on the parallel-synchronized-switch harvesting-on-inductor (SSHI) technique, also known as bias-flip [2], and can work with periodic as well as shock excitations. It allows enhanced ambient energy extraction by operating at an ideal rectified voltage (VBUF) [2] set by means of an optimal power point circuit (OPP). It operates with different harvesters and a wide variation of accelerations and excitation frequencies. The system achieves cold start-up and provides a configurable output voltage (VLDO) that can power systems such as wireless sensor nodes, biomedical or hand-held devices. It can use a single inductor by means of an inductor-sharing block (IS), and has over-voltage protection (OVP).

FORMATION OF H i CLOUDS IN SHOCK-COMPRESSED INTERSTELLAR MEDIUM: PHYSICAL ORIGIN OF ANGULAR CORRELATION BETWEEN FILAMENTARY STRUCTURE AND MAGNETIC FIELD

Abstract: Recent observations of neutral Galactic interstellar medium showed that filamentary structures of HI clouds are aligned with the interstellar magnetic field. Many interesting applications are proposed based on the alignment such as measurement of magnetic field strength through the Chandrasekhar-Fermi method and removal of polarized foreground dust emissions for the detection of inflationary polarized emission in the cosmic microwave background radiation. However, the physical origin of the alignment remains to be explained. To understand the alignment mechanism, we examine formation of HI clouds triggered by shock compression of diffuse warm neutral medium using three-dimensional magnetohydrodynamic simulations with the effects of optically thin cooling and heating. We show that the shock-compressed diffuse interstellar medium of density n~1 cm^-3 evolves into HI clouds with typical density n~50 cm^-3 via thermal instability driven by cooling, which is consistent with previous studies. We apply a machine vision transformation developed by Clark et al.(2014) to the resulting column density structures obtained by the simulations in order to measure angle correlation between filamentary structures of HI clouds and magnetic field. We find that the orientation of HI filaments depends on the environmental turbulent velocity field, particularly on the strength of shear strain in the direction of the magnetic field, which is controlled by the angle between the shock propagation direction and upstream magnetic field. When the strain along the magnetic field is weak, filamentary components of HI clouds basically lie perpendicular to the magnetic field. However, the filaments have come to align with the magnetic field, if we enhance the turbulent strain along the magnetic field or if we set turbulence in the preshock medium.

Impacts into quartz sand: Crater formation, shock metamorphism, and ejecta distribution in laboratory experiments and numerical models

Abstract: We investigated the ejection mechanics by a complementary approach of cratering experiments, including the microscopic analysis of material sampled from these experiments, and 2-D numerical modeling of vertical impacts. The study is based on cratering experiments in quartz sand targets performed at the NASA Ames Vertical Gun Range. In these experiments, the preimpact location in the target and the final position of ejecta was determined by using color-coded sand and a catcher system for the ejecta. The results were compared with numerical simulations of the cratering and ejection process to validate the iSALE shock physics code. In turn the models provide further details on the ejection velocities and angles. We quantify the general assumption that ejecta thickness decreases with distance according to a power-law and that the relative proportion of shocked material in the ejecta increase with distance. We distinguish three types of shock metamorphic particles (1) melt particles, (2) shock lithified aggregates, and (3) shock-comminuted grains. The agreement between experiment and model was excellent, which provides confidence that the models can predict ejection angles, velocities, and the degree of shock loading of material expelled from a crater accurately if impact parameters such as impact velocity, impactor size, and gravity are varied beyond the experimental limitations. This study is relevant for a quantitative assessment of impact gardening on planetary surfaces and the evolution of regolith layers on atmosphereless bodies.

Mechanism and Prediction for Occurrence of Shock-Train Sharp Forward Movement

Abstract: A numerical simulation was conducted using the commercial computational fluid dynamics software FLUENT to study the influence of inner flow structures on the motion characteristics of a separation oblique shock train caused by backpressure. Simulation results indicated there was a separation point of the boundary layer at the head of the shock train. The separated flow expanded in the isolator and led to movements of the shock train. With the shock train moving forward in the isolator, the leading edge of the shock train passed through the peaks and valleys of pressure along the forward path. As the pressure gradient at the separation point switched between positive and negative gradients, the state properties of the shock train movements transformed between a slow forward movement and a sudden sharp forward movement. The sudden sharp forward movement of the shock train always occurred when the separation point surmounted reflection points of the background wave system, which implies that the forward moti...

Multiobjective crashworthiness optimization of multi-layer honeycomb energy absorber panels under axial impact

Abstract: In this study, the design and optimization of a multi-layer configuration of hexagonal metal honeycomb energy absorber is performed using the genetic algorithm. It is assumed that the body structure with a predefined velocity impacts with barrier and the design objectives are to absorb whole kinetic energy besides limiting impact shock force. The response surfaces of honeycomb impact characteristics are extracted using finite element approach and then a honeycomb energy absorber is sized for case of a presumed impact problem. A multi-objective optimization technique is adopted to maximize the energy absorption capacity and to minimize the impact shock level while minimizing the total absorber size. A factorial design of experiment and response surface method is utilized to solve the optimization problem. The geometric specifications of honeycomb panels including the cell size, foil thickness, height and absorber face area for each layer of honeycomb panel are assumed as the design variables. Some optimization problems are handled and the optimized designs are compared to those from the literature wherever available.

Spectral evolution of flaring blazars from numerical simulations

Abstract: Context. High-resolution Very Long Baseline Interferometry (VLBI) observations of active galactic nuclei revealed traveling and stationary or quasi-stationary radio components in several blazar jets. The traveling radio components are, in general, interpreted as shock waves generated by pressure perturbations injected at the jet nozzle. The stationary features can be interpreted as recollimation shocks in nonpressure matched jets if they show a quasi-symmetric bump in the spectral index distribution. In some jets there may be interactions between the two kinds of shocks. These shock-shock interactions are observable with VLBI techniques and their signature should also be imprinted on the single-dish light curves.Aims. In this paper, we investigate the spectral evolution produced by the interaction between a recollimation shock with traveling shock waves to address the question of whether these interactions contribute to the observed flares and what their signature in both single-dish and VLBI observations looks like.Methods. We performed relativistic hydrodynamic simulations of overpressured and pressure-matched jets. To simulate the shock interaction we injected a perturbation at the jet nozzle once a steady state was reached. We computed the nonthermal emission, including adiabatic and synchotron losses, resulting from the simulation.Results. We show that the injection of perturbations in a jet can produce a bump in emission at GHz frequencies previous to the main flare, which is produced when the perturbation fills the jet in the observer’s frame. The detailed analysis of our simulations and the nonthermal emission calculations show that interaction between a recollimation shock and traveling shock produce a typical and clear signature in both the single-dish light curves and in the VLBI observations: the flaring peaks are higher and delayed with respect to the evolution of a perturbation through a conical jet. This fact can allow us to detect such interactions for stationary components lying outside of the region in which the losses are dominated by inverse Compton scattering.

Early Decay Mechanism of Shocked ε-CL-20: A Molecular Dynamics Simulation Study

Abstract: e-2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is currently the most powerful explosive commercially available. Nevertheless, the early decay events of shocked e-CL-20 still remain unclear. We perform quantum based self-consistent charge density-functional tight-binding molecular dynamics simulations, in combination with the multiscale shock simulation technique, to reveal the events with four specified shock velocities (Us) of 8 to 11 km/s. We find that the temperature and pressure increases and that the volume reduction is enhanced with increasing shock strength. The ring opening is observed to trigger molecular decay at all four shock conditions; while the sufficient NO2 fission is observed at Us = 8 and 9 km/s, and strongly inhibited at Us = 10 and 11 km/s. Moreover, the evolution of main chemical species, such as active intermediates, stable products, and clusters, is strongly dependent on the shock strength. NO2 and H are dominant in the primary intermediates, responsible for w...

Hydraulic energy transfer

Abstract: A regenerative shock absorber that include a housing and a piston that moves at least partially through the housing when the shock is compressed or extended from a rest position. When the piston moves, hydraulic fluid is pressurized and drives a hydraulic motor. The hydraulic motor, in turn, drives an electric generator that produced electric energy. The electric energy may be provided to a vehicle, among other things. The regenerative shock absorber may also provide ride performance that comparable to or exceeds that of conventional shock absorbers.