Showing papers on "Shock (mechanics) published in 2017"

Supernova simulations from a 3D progenitor model - Impact of perturbations and evolution of explosion properties

Abstract: We study the impact of large-scale perturbations from convective shell burning on the core-collapse supernova explosion mechanism using three-dimensional (3D) multi-group neutrino hydrodynamics simulations of an 18 solar mass progenitor. Seed asphericities in the O shell, obtained from a recent 3D model of O shell burning, help trigger a neutrino-driven explosion 330ms after bounce whereas the shock is not revived in a model based on a spherically symmetric progenitor for at least another 300ms. We tentatively infer a reduction of the critical luminosity for shock revival by ~20% due to pre-collapse perturbations. This indicates that convective seed perturbations play an important role in the explosion mechanism in some progenitors. We follow the evolution of the 18 solar mass model into the explosion phase for more than 2s and find that the cycle of accretion and mass ejection is still ongoing at this stage. With a preliminary value of 0.77 Bethe for the diagnostic explosion energy, a baryonic neutron star mass of 1.85 solar masses, a neutron star kick of ~600km/s and a neutron star spin period of ~20ms at the end of the simulation, the explosion and remnant properties are slightly atypical, but still lie comfortably within the observed distribution. Although more refined simulations and a larger survey of progenitors are still called for, this suggests that a solution to the problem of shock revival and explosion energies in the ballpark of observations are within reach for neutrino-driven explosions in 3D.

Revising the shock classification of meteorites

Abstract: The current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock-forming minerals such as olivine and plagioclase. The S6 stage (55–90 GPa; 850–1750 °C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high-pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high-pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high-pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable high-pressure phases. High-pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s−1) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.

Stability and modal analysis of shock/boundary layer interactions

Abstract: The dynamics of oblique shock wave/turbulent boundary layer interactions is analyzed by mining a large-eddy simulation (LES) database for various strengths of the incoming shock. The flow dynamics is first analyzed by means of dynamic mode decomposition (DMD), which highlights the simultaneous occurrence of two types of flow modes, namely a low-frequency type associated with breathing motion of the separation bubble, accompanied by flapping motion of the reflected shock, and a high-frequency type associated with the propagation of instability waves past the interaction zone. Global linear stability analysis performed on the mean LES flow fields yields a single unstable zero-frequency mode, plus a variety of marginally stable low-frequency modes whose stability margin decreases with the strength of the interaction. The least stable linear modes are grouped into two classes, one of which bears striking resemblance to the breathing mode recovered from DMD and another class associated with revolving motion within the separation bubble. The results of the modal and linear stability analysis support the notion that low-frequency dynamics is intrinsic to the interaction zone, but some continuous forcing from the upstream boundary layer may be required to keep the system near a limit cycle. This can be modeled as a weakly damped oscillator with forcing, as in the early empirical model by Plotkin (AIAA J 13:1036–1040, 1975).

Efficient Parallelization of a Shock Capturing for Discontinuous Galerkin Methods using Finite Volume Sub-cells

Abstract: We present a shock capturing procedure for high order Discontinuous Galerkin methods, by which shock regions are refined in sub-cells and treated by finite volume techniques. Hence, our approach combines the good properties of the Discontinuous Galerkin method in smooth parts of the flow with the perfect properties of a total variation diminishing finite volume method for resolving shocks without spurious oscillations. Due to the sub-cell approach the interior resolution on the Discontinuous Galerkin grid cell is nearly preserved and the number of degrees of freedom remains the same. This structure allows the interpretation of the data either as DG solution or as finite volume solution on the subgrid. In this paper we explain the efficient implementation of this coupled method on massively parallel computers and show some numerical results.

Electron Surfing and Drift Accelerations in a Weibel-Dominated High-Mach-Number Shock.

Abstract: How electrons get accelerated to relativistic energies in a high-Mach-number quasiperpendicular shock is presented by means of ab initio particle-in-cell simulations in three dimensions. We found that coherent electrostatic Buneman waves and ion-Weibel magnetic turbulence coexist in a strong-shock structure whereby particles gain energy during shock surfing and subsequent stochastic drift accelerations. Energetic electrons that initially experienced the surfing acceleration undergo pitch-angle diffusion by interacting with magnetic turbulence and continuous acceleration during confinement in the shock transition region. The ion-Weibel turbulence is the key to the efficient nonthermal electron acceleration.

Dispersive Dam-Break Flow of a Photon Fluid

Abstract: We investigate the temporal photonic analogue of the dam-break phenomenon for shallow water by exploiting a fiber optics setup. We clearly observe the decay of the steplike input (photonic dam) into a pair of oppositely propagating rarefaction wave and dispersive shock wave. Our results show evidence for a critical transition of the dispersive shock into a self-cavitating state. The detailed observation of the cavitating state dynamics allows for a fully quantitative test of the Whitham modulation theory applied to the universal defocusing nonlinear Schrodinger equation.

MMS Observations and Hybrid Simulations of Surface Ripples at a Marginally Quasi-Parallel Shock

Abstract: Simulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, that is, surface waves or ripples, are expected to propagate in the ra ...

Prediction dynamic model of shock train with complex background waves

Abstract: Oblique shock waves are unavoidable in a rectangular hypersonic inlet, leading to a non-uniform flow field. While a significant body of the literature exists regarding the shock train modeling in a uniform incoming flow condition, few efforts have focused on the shock train behavior considering the influence of the shock wave boundary layer interactions. A low-order dynamic model of the shock train has been constructed with the help of the free interaction theory and a 1-D analysis approach. Experimental and numerical investigations have been carried out to evaluate the low-order model. The results show that the model has the capability of qualitatively analyzing the shock train behavior. In the cases with incident shocks, the rapid forward movement of the shock train has been observed by experiment. Besides this phenomenon was also modeled using the low-order model. Schlieren images show that when the shock train approaches the interaction zone, its behavior is characterized by oscillation and then follo...

Plastic wire coil containing box with shock absorption function

Abstract: The invention discloses a plastic wire coil containing box with a shock absorption function The plastic wire coil containing box with the shock absorption function comprises a box body and a box cover The box cover is located at the top of the box body and is in threaded connection with the box body Two fixing blocks and a base are fixedly connected to the bottom of the inner wall of the box body, and the base is located between the two fixing blocks The top of the base is fixedly connected with two damping springs Through the plastic wire coil containing box with the shock absorption function, when a vehicle vibrates in the plastic wire coil transportation process, a storage plate exerts pressure on an arch shock abrasion plate, the arch shock abrasion plate can relieve part of shock force effectively and transmits part of shock force to the damping springs in the downwards pressed process, the damping springs can effectively relieve part of shock force while the rest of shock force is transferred to a connecting rod, vertical shock force is decomposed into horizontal force through the connecting rod, and thus plastic wire coils are prevented from being damaged

Persistence of Precursor Waves in Two-dimensional Relativistic Shocks

Abstract: We investigated the efficiency of coherent upstream large-amplitude electromagnetic wave emission via synchrotron maser instability in relativistic magnetized shocks using two-dimensional particle-in-cell simulations. We considered a purely perpendicular shock in an electron–positron plasma. The coherent wave emission efficiency was measured as a function of the magnetization parameter σ, which is defined as the ratio of the Poynting flux to the kinetic energy flux. The wave amplitude was systematically smaller than that observed in one-dimensional simulations. However, it continued to persist, even at a considerably low magnetization rate, where the Weibel instability dominated the shock transition. The emitted electromagnetic waves were sufficiently strong to disturb the upstream medium, and transverse filamentary density structures of substantial amplitude were produced. Based on this result, we discuss the possibility of the wakefield acceleration model to produce nonthermal electrons in a relativistic magnetized ion–electron shock.

Large-Eddy and Direct Numerical Simulations of the Bachalo-Johnson Flow with Shock-Induced Separation

Abstract: The Bachalo-Johnson experiment on an axisymmetric bump has been a primary validation case for turbulence models in shock-boundary-layer interactions since the 1980’s. In the present work, Wall-Modelled Large-Eddy Simulations (WMLES) of this flow were conducted using Improved Delayed Detached-Eddy Simulation (IDDES) as the sub-grid-scale (SGS) and wall model, with a synthetic turbulence generator, expecting close enough agreement with experiment. However, the WMLES results are disappointing, even in terms of the shock position, even though the results from two grids with 4.7 × 108 and 1.6 × 109 cells respectively agree well with each other. This failure of grid refinement to warn of an inaccurate simulation is of great concern, and the reasons for it are explored. We then conducted a Direct Numerical Simulation (DNS) embedded in the LES over a reduced domain, with 8 × 109 grid cells. The DNS has a far more accurate shock position and overall pressure distribution. The skin friction in the favourable pressure gradient is also much higher than in the LES; thus, wide differences appear upstream of the shock wave, most probably caused by the rapid acceleration which leads to atypical shear-stress profiles. Other SGS models were tried, and performed worse than IDDES. The DNS essentially fulfils the initial expectations although in a reduced domain and provides data for turbulence-modelling studies, for instance by extracting an effective eddy viscosity from it. The most noticeable remaining disagreement with experiment is over the Reynolds shear stress.

Numerical Investigation of Shock-Train Response to Inflow Boundary-Layer Variations

Abstract: A dataset of normal shock trains in a rectangular cross-section channel has been created from direct numerical simulations in an effort to quantify the impact of inflow confinement ratio on the sho

Numerical study of a planar shock interacting with a cylindrical water column embedded with an air cavity

Abstract: This paper performs a numerical study on the interaction of a planar shock wave with a water column embedded with/without a cavity of different sizes at high Weber numbers. The conservative-type Euler and non-conservative scalar two-equations representing the transportation of two-phase properties consist of the diffusion interface capture models. The numerical fluxes are computed by the Godunov-type Harten-Lax–van Leer contact Riemann solver coupled with an incremental fifth-order weighted essentially non-oscillatory (WENO) scheme. A third-order total variation diminishing (TVD) Runge–Kutta scheme is used to advance the solution in time. The morphology and dynamical characteristics are analysed qualitatively and quantitatively to demonstrate the breakup mechanism of the water column and formation of transverse jets under different incident shock intensities and embedded-cavity sizes. The jet tip velocities are extracted by analysing the interface evolution. The liquid column is prone to aerodynamic breakup with the formation of micro-mist at later stages instead of liquid evaporation because of the weakly heating effects of the surrounding air. It is numerically confirmed that the liquid-phase pressure will drop below the saturated vapour pressure, and the low pressure can be sustained for a certain time because of the focusing of the expansion wave, which accounts for the cavitation inside the liquid water column. The geometrical parameters of the deformed water column are identified, showing that the centreline width decreases but the transverse height increases nonlinearly with time. The deformation rates are nonlinearly correlated under different Mach numbers. The first transverse jet is found for a water column with an embedded cavity, whereas the water hammer shock and second jet do not occur under the impact of low intensity incident shock waves. The -velocity component recorded at the rear stagnation point can remain unchanged for a comparable time after a declined evolution, which indicates that the downstream wall of the shocked water ring somehow moves uniformly. It can be explained that the acceleration of the downstream wall is balanced by the trailing shedding vortex, and this effect is more evident under higher Mach numbers. The increased enstrophy, mainly generated at the interface, demonstrates the competition of the baroclinic effects of the shock wave impact over dilatation.

Transient response for a half-space with variable thermal conductivity and diffusivity under thermal and chemical shock

Abstract: The present work aims to investigate the transient thermoelastic diffusive response for a half-space with variable thermal conductivity and diffusivity in the context of the generalized thermoelastic diffusion theory. The boundary plane of the half-space is assumed to be traction free and subjected to a time-dependent thermal and chemical shock. The governing equations of the problem are formulated by using Kirchhoff’s transformation. Due to the complexity of the equations, Laplace transformation method is applied to solve them. Numerical results are obtained and illustrated graphically. Parameter studies are performed to evaluate the effects of variable thermal conductivity and diffusivity on the response. The present investigation could be helpful for better understanding the multifield coupling effect of mechanical and thermal fields in real materials.

Equilibrium Radiative Heating from 9.5 to 15.5 km/s for Earth Atmospheric Entry

Abstract: This paper presents an overview of the analysis and measurements of equilibrium radiation obtained in NASA Ames Research Center’s Electric Arc Shock Tube facility as a part of recent testing aimed at reaching shock velocities up to 15.5 km/s. The goal of these experiments was to measure the level of radiation encountered during high-speed Earth entry conditions, such as would be relevant for an asteroid, interplanetary, or lunar return mission. These experiments provided the first spectrally and spatially resolved data for high-speed Earth entry and cover conditions ranging from 9.5 to 15.5 km/s at 13.3 and 26.6 Pa (0.1 and 0.2 torr). The present analysis endeavors to provide a detailed comparison of shock tube radiation measurements and simulations at high-speed conditions. A comprehensive comparison between the spectrally resolved absolute equilibrium radiance measured in the Electric Arc Shock Tube facility and NASA’s predictive tools is presented. To provide a more accurate representation of the agr...

Evolution of dislocation mechanisms in single-crystal Cu under shock loading in different directions

Abstract: Even though there are numerous experiments and molecular dynamic simulations of Cu under shock loading, there appears to be no literature on the evolution of different types of dislocation mechanisms and their mutual interactions during the process of shock loading, which this article addresses through molecular dynamic simulations using the Mishin EAM potential for Cu. Three different directions , , and that have been considered in this article are subjected to shock compression with piston velocities ranging between 0.3–3 km s−1. The evolution of Hirth locks, Lomer–Cottrell locks, cross-slips, jogs, and dislocation-originated stacking-fault tetrahedra are demonstrated in this article for different direction shock loading of single-crystal Cu.

Numerical study of variable density turbulence interaction with a normal shock wave

Abstract: Accurate numerical simulations of shock–turbulence interaction (STI) are conducted with a hybrid monotonicity-preserving–compact-finite-difference scheme for a detailed study of STI in variable density flows. Theoretical and numerical assessments of data confirm that all turbulence scales as well as the STI are well captured by the computational method. Linear interaction approximation (LIA) convergence tests conducted with the shock-capturing simulations exhibit a similar trend of converging to LIA predictions to shock-resolving direct numerical simulations (DNS). The effects of density variations on STI are studied by comparing the results corresponding to an upstream multi-fluid mixture with the single-fluid case. The results show that for the current parameter ranges, the turbulence amplification by the normal shock wave is much higher and the reduction in turbulence length scales is more significant when strong density variations exist. Turbulent mixing enhancement by the shock is also increased and stronger mixing asymmetry in the postshock region is observed when there is significant density variation. The turbulence structure is strongly modified by the shock wave, with a differential distribution of turbulent statistics in regions having different densities. The dominant mechanisms behind the variable density STI are identified by analysing the transport equations for the Reynolds stresses, vorticity, normalized mass flux and density specific volume covariance.

Equation of state and shock compression of warm dense sodium-A first-principles study.

Abstract: As one of the simple alkali metals, sodium has been of fundamental interest for shock physics experiments, but knowledge of its equation of state (EOS) in hot, dense regimes is not well known. By combining path integral Monte Carlo (PIMC) results for partially ionized states [B. Militzer and K. P. Driver, Phys. Rev. Lett. 115, 176403 (2015)] at high temperatures and density functional theory molecular dynamics (DFT-MD) results at lower temperatures, we have constructed a coherent equation of state for sodium over a wide density-temperature range of 1.93-11.60 g/cm3 and 103-1.29×108 K. We find that a localized, Hartree-Fock nodal structure in PIMC yields pressures and internal energies that are consistent with DFT-MD at intermediate temperatures of 2×106 K. Since PIMC and DFT-MD provide a first-principles treatment of electron shell and excitation effects, we are able to identify two compression maxima in the shock Hugoniot curve corresponding to K-shell and L-shell ionization. Our Hugoniot curves provide a benchmark for widely used EOS models: SESAME, LEOS, and Purgatorio. Due to the low ambient density, sodium has an unusually high first compression maximum along the shock Hugoniot curve. At beyond 107 K, we show that the radiation effect leads to very high compression along the Hugoniot curve, surpassing relativistic corrections, and observe an increasing deviation of the shock and particle velocities from a linear relation. We also compute the temperature-density dependence of thermal and pressure ionization processes.

Regimes describing shock boundary layer interaction and ignition in shock tubes

Abstract: Regimes of shock boundary layer interaction are proposed in consideration of shock tube kinetic experiments. For this, we examine three ways that the reflected shock wave interacts with the boundary layer: incipient separation occurs when the shock is just strong enough to subject the flow to an adverse pressure gradient leading to flow reversal; shear layer instabilities manifest after a certain length of time and can cause inhomogeneities in the test gas; and shock bifurcation occurs when the back pressure of the test gas is sufficient to contain the boundary layer fluid within a stagnation bubble causing severe inhomogeneities in the test gas. Theory delineating these regimes is developed, and these delineations are compared to simulations of shock tube experiments as well as experimental data, where reasonable agreement is found. Through the theory applied to the incipient separation regime, it is determined that boundary layer separation likely occurs in most shock tube experiments; however, separation is unlikely to affect a chemical kinetic experiment except at long test times. To quantify the effect of the boundary layer, a bifurcation Damkohler number is introduced, which is found to perform sensibly well at classifying strong and weak ignition in shock tubes, implying that these combustion phenomena are determined by a competition of physical and chemical timescales. Finally, simulations suggest that tailoring the incident shock Mach number for a given experiment could provide opportunities for mitigating inhomogeneities in the test gas.

Grid-converged Solution and Analysis of the Unsteady Viscous Flow in a Two-dimensional Shock Tube

Abstract: The flow in a shock tube is extremely complex with dynamic multi-scale structures of sharp fronts, flow separation, and vortices due to the interaction of the shock wave, the contact surface, and the boundary layer over the side wall of the tube. Prediction and understanding of the complex fluid dynamics is of theoretical and practical importance. It is also an extremely challenging problem for numerical simulation, especially at relatively high Reynolds numbers. Daru & Tenaud (Daru, V. & Tenaud, C. 2001 Evaluation of TVD high resolution schemes for unsteady viscous shocked flows. Computers & Fluids 30, 89-113) proposed a two-dimensional model problem as a numerical test case for high-resolution schemes to simulate the flow field in a square closed shock tube. Though many researchers have tried this problem using a variety of computational methods, there is not yet an agreed-upon grid-converged solution of the problem at the Reynolds number of 1000. This paper presents a rigorous grid-convergence study and the resulting grid-converged solutions for this problem by using a newly-developed, efficient, and high-order gas-kinetic scheme. Critical data extracted from the converged solutions are documented as benchmark data. The complex fluid dynamics of the flow at Re = 1000 are discussed and analysed in detail. Major phenomena revealed by the numerical computations include the downward concentration of the fluid through the curved shock, the formation of the vortices, the mechanism of the shock wave bifurcation, the structure of the jet along the bottom wall, and the Kelvin-Helmholtz instability near the contact surface.