Showing papers on "Shock (mechanics) published in 2018"

Primary resonance analysis and vibration suppression for the harmonically excited nonlinear suspension system using a pair of symmetric viscoelastic buffers

Abstract: To reduce the severity of high-magnitude vibrations and shock, end-stop buffers was used for the most suspension cabs or seats of work vehicles. This paper employs a pair of symmetric linear viscoelastic end-stops to improve the performance of a single-degree-of-freedom nonlinear suspension system under primary resonance conditions, which has cubic nonlinearity. Firstly, a piecewise symmetry tri-nonlinear model is introduced. The frequency response of relative displacement corresponding to the steady-state motion is obtained by applying the multiple-scale method, which is found to be the same with the averaging method solution. And it is further verified by numerical simulation. Its stability is then studied. Subsequently, a design criterion is proposed for jump avoidance, which is caused by the saddle-node bifurcation. Also, parametric studies are carried out to illustrate effects of design parameters for the end-stop on the isolation performance at primary resonance, including responses of the relative displacement and the absolute acceleration. The results show that with dynamic parameters properly designed by using viscoelastic end-stops, the relative displacement response can be effectively suppressed and the jump can be eliminated for both hardening and softening primary isolators. Besides, the end-stop can effectively attenuate the absolute acceleration response for a hardening primary isolator, while more damping is needed to attenuate that for a softening primary isolator, although the degree of the softening nonlinearity is mitigated. It is suggested that a moderate stiffness compared to that of the primary isolator and also a high damping of the end-stop be beneficial to both vibration isolation and jump avoidance under primary resonance conditions.

A Detached Shock Calculation by Second Order Finite Differences

Abstract: A detached shock problem for a symmetric curved convex cylindrical body moving parallel to its plane of symmetry was solved by using a third-order accurate Richtmyer form of the Lax-Wendroff conservation equations. One innovation is an easy to use “artificial viscosity” term which preserves the high order of accuracy of the calculation while removing the nonlinear instabilities which otherwise appear in the shock region and near boundaries. Another innovation is a simple transformation of Cartesian space which changes the curved body into a straight line, thus reducing the large number of special points and irregularly shaped mesh regions which would otherwise appear in the difference method calculation. Such transformations are shown to preserve the conservation property of the system of differential equations. Other aspects of the third-order artificial viscosity term and the transformation are discussed. The results of a numerical calculation on a CDC 6600 computer are compared with known results.

Multiscale modeling of shock wave localization in porous energetic material

Abstract: Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. The shock response of hexanitrostilbene is studied through a combination of large-scale reactive molecular dynamics and mesoscale hydrodynamic simulations. In order to extend our simulation capability at the mesoscale to include weak shock conditions ($l6$ GPa), atomistic simulations of pore collapse are used to define a strain-rate-dependent strength model. Comparing these simulation methods allows us to impose physically reasonable constraints on the mesoscale model parameters. In doing so, we have been able to study shock waves interacting with pores as a function of this viscoplastic material response. We find that the pore collapse behavior of weak shocks is characteristically different than that of strong shocks.

On magnetic field amplification and particle acceleration near non-relativistic astrophysical shocks: particles in MHD cells simulations

Abstract: We present simulations of magnetized astrophysical shocks taking into account the interplay between the thermal plasma of the shock and suprathermal particles. Such interaction is depicted by combining a grid-based magnetohydrodynamics description of the thermal fluid with particle in cell techniques devoted to the dynamics of suprathermal particles. This approach, which incorporates the use of adaptive mesh refinement features, is potentially a key to simulate astrophysical systems on spatial scales that are beyond the reach of pure particle-in-cell simulations. We consider in this study non-relativistic shocks with various Alfvenic Mach numbers and magnetic field obliquity. We recover all the features of both magnetic field amplification and particle acceleration from previous studies when the magnetic field is parallel to the normal to the shock. In contrast with previous particle-in-cell-hybrid simulations, we find that particle acceleration and magnetic field amplification also occur when the magnetic field is oblique to the normal to the shock but on larger time-scales than in the parallel case. We show that in our simulations, the suprathermal particles are experiencing acceleration thanks to a pre-heating process of the particle similar to a shock drift acceleration leading to the corrugation of the shock front. Such oscillations of the shock front and the magnetic field locally help the particles to enter the upstream region and to initiate a non-resonant streaming instability and finally to induce diffuse particle acceleration.

Diffusive shock re-acceleration

Abstract: We have performed two-dimensional hybrid simulations of non-relativistic collisionless shocks in the presence of pre-existing energetic particles (‘seeds’); such a study applies, for instance, to the re-acceleration of galactic cosmic rays (CRs) in supernova remnant (SNR) shocks and solar wind energetic particles in heliospheric shocks. Energetic particles can be effectively reflected and accelerated regardless of shock inclination via a process that we call diffusive shock re-acceleration. We find that re-accelerated seeds can drive the streaming instability in the shock upstream and produce effective magnetic field amplification. This can eventually trigger the injection of thermal protons even at oblique shocks that ordinarily cannot inject thermal particles. We characterize the current in reflected seeds, finding that it tends to a universal value , where is the seed charge density and is the shock velocity. When applying our results to SNRs, we find that the re-acceleration of galactic CRs can excite the Bell instability to nonlinear levels in less than , thereby providing a minimum level of magnetic field amplification for any SNR shock. Finally, we discuss the relevance of diffusive shock re-acceleration also for other environments, such as heliospheric shocks, galactic superbubbles and clusters of galaxies.

Dynamic linear response of a shock/turbulent-boundary-layer interaction using constrained perturbations

Abstract: Comprehensive experimental and computational investigations have revealed possible mechanisms underlying low-frequency unsteadiness observed in spanwise homogeneous shock-wave/turbulent-boundary-layer interactions (STBLI). In the present work, we extend this understanding by examining the dynamic linear response of a moderately separated Mach 2.3 STBLI to small perturbations. The statistically stationary linear response is analysed to identify potential time-local and time-mean linear tendencies present in the unsteady base flow: these provide insight into the selective amplification properties of the flow at various points in the limit cycle, as well as asymmetry and restoring mechanisms in the dynamics of the separation bubble. The numerical technique uses the synchronized large-eddy simulation method, previously developed for free shear flows, significantly extended to include a linear constraint necessary for wall-bounded flows. The results demonstrate that the STBLI fosters a global absolute linear instability corresponding to a time-mean linear tendency for upstream shock motion. The absolute instability is maintained through constructive feedback of perturbations through the recirculation: it is self-sustaining and insensitive to external forcing. The dynamics are characterized for key frequency bands corresponding to high–mid-frequency Kelvin–Helmholtz shedding along the separated shear layer , low–mid-frequency oscillations of the separation bubble and low-frequency large-scale bubble breathing and shock motion , where the Strouhal number is based on the nominal length of the separation bubble, : . A band-pass filtering decomposition isolates the dynamic flow features and linear responses associated with these mechanisms. For example, in the low-frequency band, extreme shock displacements are shown to correlate with time-local linear tendencies toward more moderate displacements, indicating a restoring mechanism in the linear dynamics. However, a disparity between the linearly stable shock position and the mean shock position leads to an observed asymmetry in the low-frequency shock motion cycle, in which upstream motion occurs more rapidly than downstream motion. This is explained through competing linear and nonlinear (mass depletion through shedding) mechanisms and discussed in the context of an oscillator model. The analysis successfully illustrates how time-local linear dynamics sustain several key unsteady broadband flow features in a causal manner.

Dry granular avalanche impact force on a rigid wall: Analytic shock solution versus discrete element simulations.

Abstract: The present paper investigates the mean impact force exerted by a granular mass flowing down an incline and impacting a rigid wall of semi-infinite height. First, this granular flow-wall interaction problem is modeled by numerical simulations based on the discrete element method (DEM). These DEM simulations allow computing the depth-averaged quantities-thickness, velocity, and density-of the incoming flow and the resulting mean force on the rigid wall. Second, that problem is described by a simple analytic solution based on a depth-averaged approach for a traveling compressible shock wave, whose volume is assumed to shrink into a singular surface, and which coexists with a dead zone. It is shown that the dead-zone dynamics and the mean force on the wall computed from DEM can be reproduced reasonably well by the analytic solution proposed over a wide range of slope angle of the incline. These results are obtained by feeding the analytic solution with the thickness, the depth-averaged velocity, and the density averaged over a certain distance along the incline rather than flow quantities taken at a singular section before the jump, thus showing that the assumption of a shock wave volume shrinking into a singular surface is questionable. The finite length of the traveling wave upstream of the grains piling against the wall must be considered. The sensitivity of the model prediction to that sampling length remains complicated, however, which highlights the need of further investigation about the properties and the internal structure of the propagating granular wave.

Effects of a nonadiabatic wall on hypersonic shock/boundary-layer interactions

Abstract: The effects of wall thermal conditions on the canonical case of an impinging shock wave interacting with a turbulent boundary layer is a topic that remains under explored. Direct numerical simulations are used to study the flow properties of hypersonic-shock--boundary-layer interactions with distinct wall thermal conditions and shock angles.

The Three‐Dimensional Bow Shock of Mars as Observed by MAVEN

Abstract: The Martian magnetosphere is a product of the interaction of Mars with the interplanetary magnetic field and the supersonic solar wind. The location of the bow shock has been previously modeled as conic sections using data from spacecraft such as Phobos 2, Mars Global Surveyor, and Mars Express. The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft arrived in orbit about Mars in November 2014 resulting in thousands of crossings to date. We identify over 1,000 bow shock crossings. We model the bow shock as a three-dimensional surface accommodating asymmetry caused by crustal magnetic fields. By separating MAVEN's bow shock encounters based on solar condition, we also investigate the variability of the surface. We find that the shock surface varies in shape and location in response to changes in the solar radiation, the solar wind Mach number, dynamic pressure of the solar wind, and the relative local time location of the strong crustal magnetic fields (i.e., whether they are on the dayside or on the nightside).

Ultra high-speed x-ray imaging of laser-driven shock compression using synchrotron light

Abstract: A high-power, nanosecond pulsed laser impacting the surface of a material can generate an ablation plasma that drives a shock wave into it; while in situ x-ray imaging can provide a time-resolved probe of the shock-induced material behaviour on macroscopic length scales. Here, we report on an investigation into laser-driven shock compression of a polyurethane foam and a graphite rod by means of single-pulse synchrotron x-ray phase-contrast imaging with MHz frame rate. A 6 J, 10 ns pulsed laser was used to generate shock compression. Physical processes governing the laser-induced dynamic response such as elastic compression, compaction, pore collapse, fracture, and fragmentation have been imaged; and the advantage of exploiting the partial spatial coherence of a synchrotron source for studying low-density, carbon-based materials is emphasized. The successful combination of a high-energy laser and ultra high-speed x-ray imaging using synchrotron light demonstrates the potentiality of accessing complementary information from scientific studies of laser-driven shock compression.

Shock wave boundary layer interaction controlled by surface arc plasma actuators

Abstract: An array of 16 surface arc plasma actuators (SAPAs) is employed to control the shock wave boundary layer interaction (SWBLI) at a 26° compression ramp in a Mach 2.0 flow. A new electrical circuit is used to actuate all 16 SAPAs. The electrical measurement reveals significant augmentation in peak current (200 A) and an energy deposition of 1.05 J, which are the nominal characteristics of the setup. The SAPA array is later applied for SWBLI control. The actuator array is placed upstream of the SWBLI and operates at four different frequencies, namely, 500 Hz, 1 kHz, 2 kHz, and 5 kHz. In the wind tunnel experiment, high-speed schlieren at 25 000 frames per second is used for flow visualization. The shock wave system is modified significantly by the controlling gas blobs (CGBs) or controlling gas bulbs (CGBUs) generated by SAPAs. The foot portion of the separation shock wave disappears, and the oblique shock wave bifurcates when the CGBs pass through the interaction region. The shock weakening effect is further verified through the rms of the schlieren intensity of the same phase.

Reliability for systems with self-healing effect under shock models

Abstract: In the paper, the concept of self-healing effects, permanent or limited duration on system health, is introduced by building a class of cumulative damage shock models in terms of the integral for c...

After-coal diamonds: an enigmatic type of impact diamonds

Abstract: 17 Impact diamonds were discovered in the 70s and usually accepted as being paramorphs 18 after graphite, resulting in grains of extremely high mechanical quality. A diffusion-less 19 mechanism for the graphite-to-diamond transition under huge pressure has been experimentally 20 realized and theoretically explained. Besides, another type of impact product has received much 21 less attention, namely diamonds formed after coal as a result of the impact. Here we describe 22 after-coal impact diamonds from the giant Kara astrobleme (Pay-Khoy, Russia), which resulted 23 from a large asteroid impact about 70 Ma ago. The impact created a large number of unusual 24 impact diamonds, which are described here for the first time using high-resolution techniques 25 including visible and UV Raman spectroscopy, scanning electron microscopy (SEM), atomic 26 force microscopy (AFM) and transmission electron microscopy (TEM). Two main varieties of 27 after-coal diamonds occur: micrograined (sugar-like, subdivided into coherent and friable) and, 28 as a new type, paramorphs after organic relics. After-coal diamonds differ from after-graphite 29 impact diamonds by the texture, the absence of lonsdaleite, a microand nanoporous structure. 30

Application of massive laser shock processing for improvement of mechanical and tribological properties

Abstract: The present paper aims to investigate topographical, microstructural, mechanical and tribological behaviour of precipitation hardened Al alloy subjected to massive laser shock processing (LSP) without protective coating at 2500 pulses/cm2, using three beam diameters. Wear tests under dry sliding conditions resulted in severe wear, whereas the main wear mechanisms were adhesion accompanied by abrasive wear. Nevertheless, LSP with optimal processing parameters reduce the friction coefficient and wear rate with lower degrees of adhesion and abrasion inside the wear track in comparison to the untreated sample. The enhanced tribological performance is attributed to the positive influence of LSP induced surface topography, surface compressive residual stresses (RS) and dense dislocation arrangements, as the result of high-pressure shock waves. Nonetheless, due to the narrow window of optimal parameters reduced wear resistance as a consequence of undesired thermal/softening effect due to laser ablation and melting was detected with non-optimal processing parameters.