起爆高密度TATB炸药的飞片速度阈值

采用实验与数值模拟相合的方法研究了飞片起爆钝感TATB炸药的性能。设计了飞片可靠起爆钝感TATB炸药的起爆序列,利用全光纤位移干涉测速系统分别测出飞片可靠起爆和未起爆TATB炸药的速度,初步确定飞片起爆钝感TATB炸药的起爆速度阈值。采用DYNA2D程序对飞片起爆TATB炸药过程进行数值模拟,模拟结果与实验结果相符合。

     

Growth of hexagonal quantum dots under preferential evaporation

We perform numerical simulations of hexagonal quantum dots of AlGaN semiconductors. We show that the competition between surface mass diffusion and evaporation rules the morphology of the quantum dots. The system displays three different behaviors: presence of separated islands without a wetting layer, islands dissolving into the wetting layer, or islands that do not evolve. The first behavior is of special interest because its optoelectrical properties are significantly improved in comparison with quantum dots with a wetting layer.     

The effect of die exit curvature,die surface roughness and a fluoropolymer additive on sharkskin extrusion instabilities in polyethylene processing

This paper reports experimental observations and numerical simulations relating to sharkskin extrusion instabilities for two different types of polyethylene, a metallocene high-density polyethylene (HDPE) and a linear low-density polyethylene (LLDPE). Experimental results are presented for both the effect of die exit curvature and die surface roughness for slit die geometry. Matching polyflow numerical simulations are also reported and are shown to be qualitatively consistent with experimental observations. The onset of the sharkskin instability is correlated with the magnitude of the stress concentration at the die exit, and is found to be sensitive to both the melt/wall separation point for a curved exit die, and the level of partial slip at the die wall. Additional observations on the effect of a fluoropolymer additive also support the sensitivity of the sharkskin instability to partial slip at the wall.     

Kinetic theory molecular dynamics; numerical considerations

Typical numerical simulations of dense plasmas are limited by either an inability to treat the dynamical quantum evolution of the electrons or a difficulty with strongly-coupled ions. Yet these different physics problems are individually well-treated by particular approximations. Kinetic theory molecular dynamics (KTMD) is a hybrid approach that treats electrons via kinetic theory (KT) and ions with molecular dynamics (MD). We present a derivation suitable for classical plasmas and specialize to the Vlasov or mean-field case. In addition, we consider the limit of adiabatic electron dynamics, where the problem reduces to the Poisson–Boltzmann (PB) equations coupled to MD. An exploration of practical ways to implement KTMD within an existing MD framework. The initial goal is to develop computationally efficient solutions of the PB problem, suitable for large-scale PB or Thomas-Fermi MD simulations.     

The persistence of Maxwellian D and T distributions during burn in inertial confinement fusion

We present particle simulations of the heating of a DT plasma by fusion-produced α-particles. The D, T and α-particles are each treated as separate kinetic species while the electrons are assumed to be Maxwellian. Fusion reactions are also simulated for the D and T ions in such a way as to account correctly for any non-thermal effects. It is shown that the D and T ions remain close to Maxwellian under a wide range of conditions and correspondingly the fusion reactivity remains thermal.     

Spatiotemporal soliton clusters in strongly nonlocal media with variable potential coefficients

We study analytically and numerically spatiotemporal solitons in three-dimensional strongly nonlocal nonlinear media. A broad class of exact self-similar solutions to the strongly nonlocal Schrödinger equation with variable potential coefficients has been obtained. We find robust soliton cluster solutions of the accessible type, constructed with the help of Kummer and Hermite functions. They are characterized by the set of three quantum numbers. Dynamical features of these spatiotemporal accessible solitons are discussed. The validity of the analytical solutions and their stability is verified by means of direct numerical simulations.     

Ab initio investigation of a possible liquid–liquid phase transition in MgSiO3 at megabar pressures

We perform density functional molecular dynamics simulations of liquid and solid MgSiO₃ in the pressure range of 120–1600 GPa and for temperatures up to 20,000 K in order to provide new insight into the nature of the liquid–liquid phase transition that was recently predicted on the basis of decaying laser shock wave experiments [Phys. Rev. Lett. 108 (2012) 065701]. However, our simulations did not show any signature of a phase transition in the liquid phase. We derive the equation of state for the liquid and solid phases and compute the shock Hugoniot curves. We discuss different thermodynamic functions and by explore alternative interpretations of the experimental findings.     

Stirring by anisotropic squirming

We consider a fluid stirred by the locomotions of squirmers through it and generalize the stochastic hydrodynamic model proposed by Thiffeault and Childress, Phys. Lett. A (2010) and Lin et al., J. Fluid Mech. (2011) to the case in which the swimmers move in anisotropically random directions. A non-diagonal effective diffusivity tensor is derived with which the diffusive preference of a passive particle along any given direction can be computed to provide more details of the phenomena beyond scalar statistics. We further identify a fraction from the orthogonal decomposition of the drift-induced particle displacement to distinguish the underlying nonlinear mixing mechanism for different types of swimmers. Numerical simulations verify the analytical results with explicit examples of prescribed, anisotropic stirring motions. We also connect our formulation to several measures used in clinical medical research such as diffusion tensor imaging where anisotropic diffusion has a significant consequence.     

A lower bound on snap-through instability of curved beams under thermomechanical loads

A non-linear finite element formulation (three dimensional continuum elements) is implemented and used for modeling dynamic snap-through in beams with initial curvature. We identify a non-trivial (non-flat) configuration of the beam at a critical temperature value below which the beam will no longer experience snap-through under any magnitude of applied quasi-static load for beams with various curvatures. The critical temperature is shown to successfully eliminate snap-through in dynamic simulations at quasistatic loading rates. Thermomechanical coupling is included in order to model a physically minimal amount of damping in the system, and the resulting post-snap vibrations are shown to be thermoelastically damped. We propose a test to determine the critical snap-free temperature for members of general geometry and loading pattern; the analogy between mechanical prestress and thermal strain that holds between the static and dynamic simulations is used to suggest a simple method for reducing the vulnerability of thin-walled structural members to dynamic snap-through in members of large initial curvature via the introduction of initial pretension.     

Modeling of high-density compaction of granular materials by the Discrete Element Method

Cold compaction of metal powders is now commonly studied at a microscopic scale, to further our understanding of contact mechanics between grains. The Discrete Element Method (DEM) is therefore, a good compromise between calculation time and precision. DEM simulations are in general limited to a relative density of about 0.8, because the existing contact laws do not reproduce all the physical phenomena involved in the densification of granular media. Local contact mechanics can be studied by finite element analyses on meshed distinct elements (MDEM, Meshed Distinct Element Method). However, this method is too time-consuming when in the presence of a large number of grains. In the following work, a new analytical contact law will be formulated with MDEM which will subsequently be used to validate the DEM model. Thus, it will be possible with DEM modeling to reproduce high-density compaction of random packings up to a relative density of about 0.95. By introducing a local relative density parameter in the force–displacement relationship, the incompressibility effects which rule high-density behaviors can be introduced in the modeling of powder compaction.     

Positive Solutions to Singular Second and Third Order Differential Equations for Quantum Fluids

We analyze a quantum trajectory model given by a steady-state hydrodynamic system for quantum fluids with positive constant temperature in bounded domains for arbitrary large data. The momentum equation can be written as a dispersive third-order equation for the particle density where viscous effects are incorporated. The phenomena that admit positivity of the solutions are studied. The cases, one space dimensional dispersive or non-dispersive, viscous or non-viscous, are thoroughly analyzed with respect to positivity and existence or non-existence of solutions, all depending on the constitutive relation for the pressure law. We distinguish between isothermal (linear) and isentropic (power law) pressure functions of the density. It is proved that in the dispersive, non-viscous model, a classical positive solution only exists for “small” (positive) particle current densities, both for the isentropic and isothermal case. Uniqueness is also shown in the isentropic subsonic case, when the pressure law is strictly convex. However, we prove that no weak isentropic solution can exist for “large” current densities. The dispersive, viscous problem admits a classical positive solution for all current densities, both for the isentropic and isothermal case, with an “ultra-diffusion” condition. The proofs are based on a reformulation of the equations as a singular elliptic second-order problem and on a variant of the Stampacchia truncation technique. Some of the results are extended to general third-order equations in any space dimension. Accepted July 1, 2000?Published online February 14, 2001     

Modern topics and challenges in dynamic fracture

The field of dynamic fracture has been enlivened over the last 5 years or so by a series of remarkable accomplishments in different fields—earthquake science, atomistic (classical and quantum) simulations, novel laboratory experiments, materials modeling, and continuum mechanics. Important concepts either discovered for the first time or elaborated in new ways reveal wider significance. Here the separate streams of the literature of this progress are reviewed comparatively to highlight commonality and contrasts in the mechanics and physics.Much of the value of the new work resides in the new questions it has raised, which suggests profitable areas for research in the next few years and beyond. From the viewpoint of fundamental science, excitement is greatest in the struggle to probe the character of dynamic fracture at the atomic scale, using Newtonian or quantum mechanics as appropriate (a qualifier to be debated!). But lively interest is also directed towards modeling and experimentation at macroscales, including the geological, where the science of fracture is pulled at once by fundamental issues, such as the curious effects of friction, and the structural, where dynamic effects are essential to proper design or certification and even in manufacture.     

Bayesian inference of inaccuracies in radiation transport physics from inertial confinement fusion experiments

First principles microphysics models are essential to the design and analysis of high energy density physics experiments. Using experimental data to investigate the underlying physics is also essential, particularly when simulations and experiments are not consistent with each other. This is a difficult task, due to the large number of physical models that play a role, and due to the complex and noisy nature of the experiments. This results in a large number of parameters that make any inference a daunting task; it is also very important to consistently treat both experimental and prior understanding of the problem. In this paper we present a Bayesian method that includes both these effects, and allows the inference of a set of modifiers that have been constructed to give information about microphysics models from experimental data. We pay particular attention to radiation transport models. The inference takes into account a large set of experimental parameters and an estimate of the prior knowledge through a modified χ² function, which is minimised using an efficient genetic algorithm. Both factors play an essential role in our analysis. We find that although there is evidence of inaccuracies in off-line calculations of X-ray drive intensity and Ge L shell absorption, modifications to radiation transport are unable to reconcile differences between 1D HYDRA simulations and the experiment.     

A cost‐effective curvature calculation approach for interfacial flows on unstructured meshes

We present a simple and cost‐effective curvature calculation approach for simulations of interfacial flows on structured and unstructured grids. The interface is defined using volume fractions, and the interface curvature is obtained as a function of the gradients of volume fractions. The gradient computation is based on a recently proposed gradient recovery method that mimicks the least squares approach without the need to solve a system of equations and is quite easy to implement on arbitrary polygonal meshes. The resulting interface curvature is used in a continuum surface force formulation within the framework of a well‐balanced finite‐volume algorithm to simulate multiphase flows dominated by surface tension. We show that the proposed curvature calculation is at least as accurate as some of the existing approaches on unstructured meshes while being straightforward to implement on any mesh topology. Numerical investigations also show that spurious currents in stationary problems that are dependent on the curvature calculation methodology are also acceptably low using the proposed approach. Studies on capillary waves and rising bubbles in viscous flows lend credence to the ability of the proposed method as an inexpensive, robust, and reasonably accurate approach for curvature calculation and numerical simulation of multiphase flows.     

Suspension-like hardening behavior of HDPE and time-hardening superposition

The rheology of solidifying high-density polyethylene (HDPE) is investigated. Experiments on an HDPE were performed with a novel RheoDSC device. Results agree quantitatively with simulations for a suspension of elastic spheres in a viscoelastic matrix except for very low values of space filling (<5%), indicating that the rheological behavior of the crystallizing melt in the frequency range investigated is purely suspension like. The hardening behavior of the material is characterized in two different ways; a normalized rheological function and a time-hardening superposition (THS) master curve of rheological properties. An improvement is proposed to the procedure for performing THS that was previously used in the literature. Based on this procedure, a novel method for predicting the rheological properties of crystallizing melts is presented.     

Sharkskin instabilities and the effect of slip from gas-assisted extrusion

This paper is concerned with a polymer extrusion instability and the effect of introducing slip by means of a thin lubricating gas layer between the extrusion die wall and the flowing polymer melt. Gas-assisted extrusion (GAE) experiments were carried out using high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) for a number of different gas injection die geometries. The stress distributions within the polymer melt were monitored during extrusion using flow birefringence. Polyflow numerical simulations were used to calculate the local stress concentrations in the melt at the die exit, as these were believed to be related to the occurrence of sharkskin. Simulations were also used to observe the effect of a full slip boundary condition as imparted by GAE. A key finding of the paper is that GAE in the parallel section of the die wall simply moved the local exit stress concentration upstream to the point of gas injection, and therefore did not reduce sharkskin. Simulations indicated that for correctly designed dies, the local surface stress concentration would be reduced. However, it was found experimentally that it was not possible to obtain a stable gas layer for this die design with upstream gas injection. A numerical investigation, involving simulations of varying levels of partial slip along the die wall, indicated an optimum level of slip where the stress concentrations were reduced. It is speculated that this is the reason that coatings such as PTFE, which impart a partial slip, can reduce sharkskin while GAE does not. The findings show that a controlled level of partial slip lowers the overall stress concentrations.     

Bending-induced extension in two-dimensional crystals

We find by ab initio simulations that significant overall tensile strain can be induced by pure bending in a wide range of two-dimensional crystals perpendicular to the bending moment, just like an accordion being bent to open. This bending-induced tensile strain increases in a power law with bent curvature and can be over 20% in monolayered black phosphorus and transition metal dichalcogenides at a moderate curvature of 2 nm?1 but more than an order weaker in graphene and hexagon boron nitride. This accordion effect is found to be a quantum mechanical effect raised by the asymmetric response of chemical bonds and electron density to the bending curvature.     

碳纳米管的力学

在发现碳纳米管后不久,对于这些有趣结构的力学性质--包括高强度、高硬度、低密度和结构的完美性的理论预测,使人们认识到它们可能具有理想的科技应用价值.对这些预测的实验验证或个别验伪以及大量基于不同模型的计算机模拟方法,使得逾10年来对碳纳米管力学的理解日趋深入但远未达到尽头.本文回顾了理论预测,并对这种微小结构的观察和操作中经常用到的富有挑战性的实验技术进行了讨论.略述了采用的计算方法包括从头算法量子力学模拟、经典分子动力学和连续介质模型.多尺度和多物理模型的发展和模拟工具自然而然作为连接基础科学问题和工程应用的结果而出现,而这个主题仍然正在抓紧研究中.这里介绍了研究此主题的一些方法.注意力主要集中于研究力学性质的揭示方面,如杨氏模量、弯曲刚度、屈曲准则、拉伸和压缩强度.最后,讨论了利用这些性质的几个令人兴奋的应用例子,包括纳米绳束、填充的纳米管、纳米机电系统、纳米传感器和纳米管增强复合材料,引用了349篇参考文献. 图41参349