Showing papers on "Shock (mechanics) published in 2015"

Three-dimensional Core-collapse Supernova Simulated Using a 15 M⊙ Progenitor

Abstract: We have performed ab initio neutrino radiation hydrodynamics simulations in three and two spatial dimensions (3D and 2D) of core-collapse supernovae from the same 15 M⊙ progenitor through 440 ms after core bounce. Both 3D and 2D models achieve explosions; however, the onset of explosion (shock revival) is delayed by ~100 ms in 3D relative to the 2D counterpart and the growth of the diagnostic explosion energy is slower. This is consistent with previously reported 3D simulations utilizing iron-core progenitors with dense mantles. In the ~100 ms before the onset of explosion, diagnostics of neutrino heating and turbulent kinetic energy favor earlier explosion in 2D. During the delay, the angular scale of convective plumes reaching the shock surface grows and explosion in 3D is ultimately lead by a single, large-angle plume, giving the expanding shock a directional orientation not dissimilar from those imposed by axial symmetry in 2D simulations. Finally, we posit that shock revival and explosion in the 3D simulation may be delayed until sufficiently large plumes form, whereas such plumes form more rapidly in 2D, permitting earlier explosions.

Three-dimensional core-collapse supernova simulated using a 15 $M_\odot$ progenitor

Abstract: We have performed ab initio neutrino radiation hydrodynamics simulations in three and two spatial dimensions (3D and 2D) of core-collapse supernovae from the same 15 $M_\odot$ progenitor through 440 ms after core bounce. Both 3D and 2D models achieve explosions, however, the onset of explosion (shock revival) is delayed by $\sim$100 ms in 3D relative to the 2D counterpart and the growth of the diagnostic explosion energy is slower. This is consistent with previously reported 3D simulations utilizing iron-core progenitors with dense mantles. In the $\sim$100 ms before the onset of explosion, diagnostics of neutrino heating and turbulent kinetic energy favor earlier explosion in 2D. During the delay, the angular scale of convective plumes reaching the shock surface grows and explosion in 3D is ultimately lead by a single, large-angle plume, giving the expanding shock a directional orientation not dissimilar from those imposed by axial symmetry in 2D simulations. We posit that shock revival and explosion in the 3D simulation may be delayed until sufficiently large plumes form, whereas such plumes form more rapidly in 2D, permitting earlier explosions.

Neutrino-driven supernova of a low-mass iron-core progenitor boosted by three-dimensional turbulent convection

Abstract: We present the first successful simulation of a neutrino-driven supernova explosion in three dimensions (3D), using the Prometheus-Vertex code with an axis-free Yin–Yang grid and a sophisticated treatment of three-flavor, energy-dependent neutrino transport. The progenitor is a nonrotating, zero-metallicity 9.6 star with an iron core. While in spherical symmetry outward shock acceleration sets in later than 300 ms after bounce, a successful explosion starts at ∼130 ms postbounce in two dimensions (2D). The 3D model explodes at about the same time but with faster shock expansion than in 2D and a more quickly increasing and roughly 10% higher explosion energy of >1050 erg. The more favorable explosion conditions in 3D are explained by lower temperatures and thus reduced neutrino emission in the cooling layer below the gain radius. This moves the gain radius inward and leads to a bigger mass in the gain layer, whose larger recombination energy boosts the explosion energy in 3D. These differences are caused by less coherent, less massive, and less rapid convective downdrafts associated with postshock convection in 3D. The less violent impact of these accretion downflows in the cooling layer produces less shock heating and therefore diminishes energy losses by neutrino emission. We thus have, for the first time, identified a reduced mass accretion rate, lower infall velocities, and a smaller surface filling factor of convective downdrafts as consequences of 3D postshock turbulence that facilitate neutrino-driven explosions and strengthen them compared to the 2D case.

Relativistic Shocks: Particle Acceleration and Magnetization

Abstract: We review the physics of relativistic shocks, which are often invoked as the sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources of ultra-high energy cosmic-rays. We focus on particle acceleration and magnetic field generation, and describe the recent progress in the field driven by theory advances and by the rapid development of particle-in-cell (PIC) simulations. In weakly magnetized or quasi parallel-shocks (i.e. where the magnetic field is nearly aligned with the flow), particle acceleration is efficient. The accelerated particles stream ahead of the shock, where they generate strong magnetic waves which in turn scatter the particles back and forth across the shock, mediating their acceleration. In contrast, in strongly magnetized quasi-perpendicular shocks, the efficiencies of both particle acceleration and magnetic field generation are suppressed. Particle acceleration, when efficient, modifies the turbulence around the shock on a long time scale, and the accelerated particles have a characteristic energy spectral index of \(s_{\gamma}\simeq2.2\) in the ultra-relativistic limit. We discuss how this novel understanding of particle acceleration and magnetic field generation in relativistic shocks can be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and GRB afterglows.

Diffusive shock acceleration and reconnection acceleration processes

Abstract: Shock waves, as shown by simulations and observations, can generate high levels of downstream vortical turbulence, including magnetic islands. We consider a combination of diffusive shock acceleration (DSA) and downstream magnetic-island-reconnection-related processes as an energization mechanism for charged particles. Observations of electron and ion distributions downstream of interplanetary shocks and the heliospheric termination shock (HTS) are frequently inconsistent with the predictions of classical DSA. We utilize a recently developed transport theory for charged particles propagating diffusively in a turbulent region filled with contracting and reconnecting plasmoids and small-scale current sheets. Particle energization associated with the anti-reconnection electric field, a consequence of magnetic island merging, and magnetic island contraction, are considered. For the former only, we find that (i) the spectrum is a hard power law in particle speed, and (ii) the downstream solution is constant. For downstream plasmoid contraction only, (i) the accelerated spectrum is a hard power law in particle speed; (ii) the particle intensity for a given energy peaks downstream of the shock, and the distance to the peak location increases with increasing particle energy, and (iii) the particle intensity amplification for a particular particle energy, f(x,c/c_0)/f(0,c/c_0), is not 1, as predicted by DSA, but increases with increasing particle energy. The general solution combines both the reconnection-induced electric field and plasmoid contraction. The observed energetic particle intensity profile observed by Voyager 2 downstream of the HTS appears to support a particle acceleration mechanism that combines both DSA and magnetic-island-reconnection-related processes.

Particle acceleration by a solar flare termination shock.

Abstract: Solar flares--the most powerful explosions in the solar system--are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.

How bad or good are the external forward shock afterglow models of gamma-ray bursts?

Abstract: The external forward shock models have been the standard paradigm to interpret the broadband afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal ...

Stochastic electron acceleration during spontaneous turbulent reconnection in a strong shock wave

Abstract: Explosive phenomena such as supernova remnant shocks and solar flares have demonstrated evidence for the production of relativistic particles. Interest has therefore been renewed in collisionless shock waves and magnetic reconnection as a means to achieve such energies. Although ions can be energized during such phenomena, the relativistic energy of the electrons remains a puzzle for theory. We present supercomputer simulations showing that efficient electron energization can occur during turbulent magnetic reconnection arising from a strong collisionless shock. Upstream electrons undergo first-order Fermi acceleration by colliding with reconnection jets and magnetic islands, giving rise to a nonthermal relativistic population downstream. These results shed new light on magnetic reconnection as an agent of energy dissipation and particle acceleration in strong shock waves.

Stability, Receptivity, and Sensitivity Analyses of Buffeting Transonic Flow over a Profile

Abstract: A transonic flow over the OAT15A supercritical profile is considered. The interaction between the shock wave and the turbulent boundary layer is investigated through numerical simulation and global stability analysis for a wide range of angles of attack. Numerical simulations are in good agreement with previous studies and manage to reproduce the high-amplitude self-sustained shock oscillations known as shock buffet. In agreement with previous results, it is found that the buffet phenomenon is driven by an unstable global mode of the linearized Navier–Stokes equations. Analysis of the adjoint global mode reveals that the flow is most receptive to harmonic forcings on the suction side of the profile, within the boundary layer upstream of the shock foot, in the recirculation bubble downstream of the shock foot, and on the right characteristic that impinges the shock foot. An eigenvalue sensitivity analysis shows that a steady streamwise force applied either in the boundary layer or in the recirculation regi...

Neutrino-driven supernova of a low-mass iron-core progenitor boosted by three-dimensional turbulent convection

Abstract: We present the first successful simulation of a neutrino-driven supernova explosion in three dimensions (3D), using the Prometheus-Vertex code with an axis-free Yin-Yang grid and a sophisticated treatment of three-flavor, energy-dependent neutrino transport. The progenitor is a nonrotating, zero-metallicity 9.6 Msun star with an iron core. While in spherical symmetry outward shock acceleration sets in later than 300 ms after bounce, a successful explosion starts at ~130 ms postbounce in two dimensions (2D). The 3D model explodes at about the same time but with faster shock expansion than in 2D and a more quickly increasing and roughly 10 percent higher explosion energy of >10^50 erg. The more favorable explosion conditions in 3D are explained by lower temperatures and thus reduced neutrino emission in the cooling layer below the gain radius. This moves the gain radius inward and leads to a bigger mass in the gain layer, whose larger recombination energy boosts the explosion energy in 3D. These differences are caused by less coherent, less massive, and less rapid convective downdrafts associated with postshock convection in 3D. The less violent impact of these accretion downflows in the cooling layer produces less shock heating and therefore diminishes energy losses by neutrino emission. We thus have, for the first time, identified a reduced mass accretion rate, lower infall velocities, and a smaller surface filling factor of convective downdrafts as consequences of 3D postshock turbulence that facilitate neutrino-driven explosions and strengthen them compared to the 2D case.

Relativistic Shocks: Particle Acceleration and Magnetization

Abstract: We review the physics of relativistic shocks, which are often invoked as the sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources of ultra-high energy cosmic-rays. We focus on particle acceleration and magnetic field generation, and describe the recent progress in the field driven by theory advances and by the rapid development of particle-in-cell (PIC) simulations. In weakly magnetized or quasi parallel-shocks (where the magnetic field is nearly aligned with the flow), particle acceleration is efficient. The accelerated particles stream ahead of the shock, where they generate strong magnetic waves which in turn scatter the particles back and forth across the shock, mediating their acceleration. In contrast, in strongly magnetized quasi-perpendicular shocks, the efficiencies of both particle acceleration and magnetic field generation are suppressed. Particle acceleration, when efficient, modifies the turbulence around the shock on a long time scale, and the accelerated particles have a characteristic energy spectral index of ~ 2.2 in the ultra-relativistic limit. We discuss how this novel understanding of particle acceleration and magnetic field generation in relativistic shocks can be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and GRB afterglows.

Modeling zoned shock effects on stochastic degradation in dependent failure processes

Abstract: This article studies a system that experiences two dependent competing failure processes, in which shocks are categorized into different shock zones. These two failure processes, a stochastic degradation process and a random shock process, are dependent because arriving shocks can cause instantaneous damage on the degradation process. In existing studies, every shock causes an abrupt damage on degradation. However, this may not be the case when shock loads are small and within the tolerance of system resistance. In the proposed model, only shock loads that are larger than a certain level are considered to cause abrupt damage on degradation, which makes this new model realistic and challenging. Shocks are divided into three zones based on their magnitudes: safety zone, damage zone, and fatal zone. The abrupt damage is modeled using an explicit function of shock load exceedances (differences between load magnitudes and a given threshold). Due to the complexity in modeling these two dependent stochastic fail...

Gigabar spherical shock generation on the OMEGA laser.

Abstract: A two-step scheme for inertial confinement fusion generates gigabar shock pressures in a fuel target.

Numerical Study of Shock Buffet on Three-Dimensional Wings

Abstract: The paper presents a computational study of the transonic shock-buffet flow instability phenomenon on three-dimensional wings. Reynolds-averaged Navier–Stokes simulations were conducted on three wing configurations, all based on the RA16SC1 airfoil, at shock-buffet flow conditions. Numerical validation is presented for the OAT15A and RA16SC1 swept wings based on wind-tunnel experiments. The simulated configurations include infinite-straight, infinite-swept, and finite-swept three-dimensional wing models of several sweep angles and span lengths. Based on the results, the effects of three-dimensional flow, wing sweep, and span length on the shock-buffet characteristics are identified. For small wing-sweep angles, the fundamental shock-buffet instability mechanism remains similar to the two-dimensional mechanism, which is characterized mainly by chordwise shock oscillations. For moderate sweep angles, a phenomenon of lateral pressure disturbance propagation is observed. This phenomenon is essentially differe...

Experimental investigation on the strain-rate effect and inertia effect of closed-cell aluminum foam subjected to dynamic loading

Abstract: In this paper, a series of experiments were conducted to clarify the effects of the strain rate and inertia on the deformation behavior of closed-cell aluminum foams under impact. The quartz-crystal technique was employed to analyze the stress uniformity of aluminum foam samples under split Hopkinson pressure bar (SHPB) loading. It was revealed that the condition of stress uniformity is hard to reach for a thicker foam sample, and the strength of aluminum foam is sensitive to strain rate. Two different direct-impact Hopkinson pressure bar (DHPB) methods were introduced to validate the three deformation modes, i.e. homogeneous mode, transitional mode and shock mode. Results displayed that the stress at the front surface increased dramatically than that at the back surface as the impact speeds increased from 16 m/s to 113 m/s. The axial-inertia effect became more important than the strain rate effect under high speed impact. The dynamic deformation processes were recorded by a Phantom-675 camera and were analyzed through the digital imaging correlation (DIC) method. The deformation of aluminum foam in homogeneous mode was presented by the evolution of global distributed failure, but it was dominated by the local failure in shock mode.

Three-dimensional simulations of SASI- and convection-dominated core-collapse supernovae

Abstract: We investigate the effect of dimensionality on the transition to explosion in neutrino-driven core-collapse supernovae. Using parameterized hydrodynamic simulations of the stalled supernova shock in one-, two- (2D), and three spatial dimensions (3D), we systematically probe the extent to which hydrodynamic instabilities alone can tip the balance in favor of explosion. In particular, we focus on systems that are well into the regimes where the Standing Accretion Shock Instability (SASI) or neutrino-driven convection dominate the dynamics, and characterize the difference between them. We find that SASI-dominated models can explode with up to ~20% lower neutrino luminosity in 3D than in 2D, with the magnitude of this difference decreasing with increasing resolution. This improvement in explosion conditions is related to the ability of spiral modes to generate more non-radial kinetic energy than a single sloshing mode, increasing the size of the average shock radius, and hence generating better conditions for the formation of large-scale, high-entropy bubbles. In contrast, convection-dominated explosions show a smaller difference in their critical heating rate between 2D and 3D (<8%), in agreement with previous studies. The ability of our numerical implementation to maintain arbitrary symmetries is quantified with a set of SASI-based tests. We discuss implications for the diversity of explosion paths in a realistic supernova environment.

Condition-Based Maintenance for Repairable Deteriorating Systems Subject to a Generalized Mixed Shock Model

Abstract: Based on reliability analysis for a system subject to $s$ -dependent competing risks of internal degradation and external shocks, we propose a condition-based maintenance policy considering imperfect repair for complex systems. The internal degradation (e.g., crack growth, erosion, or corrosion) can be modeled using a stochastic deterioration process. External shocks arriving at random times are divided into two classes based on their impacts on the system: 1) fatal shocks that can cause the system to fail immediately, if a shock belongs to any of three classic shock models (i.e., extreme shock model, run shock model, and $\delta $ -shock model), or the generalized mixed shock model; and 2) non-fatal shocks that can damage the system by randomly increasing the degradation level. Using the proposed condition-based maintenance policy, the system is inspected at fixed time intervals, and a decision for an appropriate maintenance action (i.e., no action, imperfect repair, preventive or corrective replacement) is made based on the actual health condition of the system detected through inspection. The imperfect repair restores the system by lowering the degradation amount to a certain level. The objective is to determine the optimal inspection interval that minimizes the expected long-run maintenance cost rate. A micro-electro-mechanical system example is used to evaluate the efficiency of the developed reliability and condition-based maintenance models.

Imaging shock waves in diamond with both high temporal and spatial resolution at an XFEL

Abstract: The advent of hard x-ray free-electron lasers (XFELs) has opened up a variety of scientific opportunities in areas as diverse as atomic physics, plasma physics, nonlinear optics in the x-ray range, and protein crystallography. In this article, we access a new field of science by measuring quantitatively the local bulk properties and dynamics of matter under extreme conditions, in this case by using the short XFEL pulse to image an elastic compression wave in diamond. The elastic wave was initiated by an intense optical laser pulse and was imaged at different delay times after the optical pump pulse using magnified x-ray phase-contrast imaging. The temporal evolution of the shock wave can be monitored, yielding detailed information on shock dynamics, such as the shock velocity, the shock front width, and the local compression of the material. The method provides a quantitative perspective on the state of matter in extreme conditions.

A Compact Variable Stiffness and Damping Shock Absorber for Vehicle Suspension

Abstract: A shock absorber is an important device for vehicle suspension. The semi-active suspension requires the damping or stiffness of the shock absorber to be controllable. This paper proposed a novel compact shock absorber with both damping and stiffness variable characteristics. The shock absorber is developed based on MR fluid through an innovative design. A prototype is tested by MTS to characterize the variable damping and stiffness properties. A mathematical model that incorporated the Bingham model is established and an optimization method is adopted to identify the parameters. The coherence of experiments and the proposed model verified the control ability of dual damping and stiffness of the shock absorber.

Precursors to interstellar shocks of solar origin

Abstract: On or about 2012 August 25, the Voyager 1 spacecraft crossed the heliopause into the nearby interstellar plasma. In the nearly three years that the spacecraft has been in interstellar space, three notable particle and field disturbances have been observed, each apparently associated with a shock wave propagating outward from the Sun. Here, we present a detailed analysis of the third and most impressive of these disturbances, with brief comparisons to the two previous events, both of which have been previously reported. The shock responsible for the third event was first detected on 2014 February 17 by the onset of narrowband radio emissions from the approaching shock, followed on 2014 May 13 by the abrupt appearance of intense electron plasma oscillations generated by electrons streaming outward ahead of the shock. Finally, the shock arrived on 2014 August 25, as indicated by a jump in the magnetic field strength and the plasma density. Various disturbances in the intensity and anisotropy of galactic cosmic rays were also observed ahead of the shock, some of which are believed to be caused by the reflection and acceleration of cosmic rays by the magnetic field jump at the shock, and/or by interactions with upstream plasma waves. Comparisons to the two previous weaker events show somewhat similar precursor effects, although differing in certain details. Many of these effects are very similar to those observed in the region called the "foreshock" that occurs upstream of planetary bow shocks, only on a vastly larger spatial scale.

Shock wave formation in the collapse of a vapor nanobubble.

Abstract: In this Letter, the dynamics of a collapsing vapor bubble is addressed by means of a diffuse-interface formulation The model cleanly captures, through a unified approach, all the critical features of the process, such as phase change, transition to supercritical conditions, thermal conduction, compressibility effects, and shock wave formation and propagation Rather unexpectedly for pure vapor bubbles, the numerical experiments show that the process consists in the oscillation of the bubble associated with the emission of shock waves in the liquid, and with the periodic disappearance and reappearance of the liquid-vapor interface due to transition to super- or subcritical conditions The results identify the mechanism of shock wave formation as strongly related to the transition of the vapor to the supercritical state, with a progressive steepening of a focused compression wave evolving into a shock which is eventually reflected as an outward propagating wave in the liquid

Mesoscale simulation of reactive pressed energetic materials under shock loading

Abstract: Shock load analysis of two different samples of pressed HMX energetic material is performed using the Eulerian compressible multimaterial code SCIMITAR3D. The numerical framework uses an image to computation approach to perform shock analysis on real microstructures of the energetic samples. Image processing algorithms are applied on SEM images of both samples to implicitly represent the microstructures using level set functions. The chemical decomposition of HMX is modeled using the Henson-Smilowitz multi-step kinetic mechanism. It is observed that microstructural characteristics play a crucial role in determining the ignition behavior of the energetic materials. For the applied shock loads and for the particular samples investigated, class III sample leads to initiation of chemical reaction and the class V sample does not ignite. It is also shown that the orientation of elongated voids with respect to incident shock load is an important factor contributing to the initiation of chemical reactions in the class III sample. This is explained by performing numerical experiments of elongated void oriented at different angles with respect to the shock load. Results show the importance of microstructural details, such as void size, distribution, and orientation for initiation.

Interpreting observations of molecular outflow sources: the MHD shock code mhd_vode

Abstract: The planar MHD shock code mhd_vode has been developed in order to simulate both continuous (C) type shock waves and jump (J) type shock waves in the interstellar medium. The physical and chemical state of the gas in steady-state may also be computed and used as input to a shock wave model. The code is written principally in FORTRAN 90, although some routines remain in FORTRAN 77. The documented program and its input data are described and provided as supplementary material, and the results of exemplary test runs are presented. Our intention is to enable the interested user to run the code for any sensible parameter set and to comprehend the results. With applications to molecular outflow sources in mind, we have computed, and are making available as supplementary material, integrated atomic and molecular line intensities for grids of C- and J-type models; these computations are summarized in the Appendices.

Unsteadiness in transonic shock-wave/boundary-layer interactions: experimental investigation and global stability analysis

Abstract: A transonic interaction between a shock wave and a turbulent boundary layer is experimentally and theoretically investigated. The configuration is a transonic channel flow over a bump, where a shock wave causes the separation of the boundary layer in the form of a recirculating bubble downstream of the shock foot. Different experimental techniques allow for the identification of the main unsteadiness features. As recognised in similar shock-wave/boundary-layer interactions, the flow field exhibits two distinct characteristic frequencies, whose origins are still controversial: a low-frequency motion which primarily affects the shock wave; and medium-frequency perturbations localised in the shear layer. A Fourier analysis of a series of Schlieren snapshots is performed to precisely characterise the structure of the perturbations at low- and medium-frequencies. Then, the Reynolds-averaged Navier–Stokes (RANS) equations closed with a Spalart–Allmaras turbulence model are solved to obtain a mean flow, which favourably compares with the experimental results. A global stability analysis based on the linearization of the full RANS equations is then performed. The eigenvalues of the Jacobian operator are all damped, indicating that the interaction dynamic cannot be explained by the existence of unstable global modes. The input/output behaviour of the flow is then analysed by performing a singular-value decomposition of the Resolvent operator; pseudo-resonances of the flow may be identified and optimal forcings/responses determined as a function of frequency. It is found that the flow strongly amplifies both medium-frequency perturbations, generating fluctuations in the mixing layer, and low-frequency perturbations, affecting the shock wave. The structure of the optimal perturbations and the preferred frequencies agree with the experimental observations.

Particle Acceleration by a Solar Flare Termination Shock

Abstract: Solar flares - the most powerful explosions in the solar system - are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well-reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.

Shock and Vibration

Abstract: ed/Indexed in Applied Mechanics Reviews, Aquatic Sciences and Fisheries Abstracts part 2 (ASFA-2), Compendex, CSA Illumina, Current