Density functional study of the pressure tensor for inhomogeneous Lennard—Jones fluids

Based on classical density functional theory,an expression of the pressure tensor for inhomogeneous fluids is presented.This takes into account greater correlation between particles,especially for systems that are geometrically confined or involve an interface.The density and pressure components of Lennard-Jones fluids confined in hard and softened nano-cavities are calculated.A comparison between the results of this work and IK expression suggests that the agreement depends on temperature.The interfacial tension for hard sphere fluids agrees well with the Monte Carlo result when the bulk density is not too large.The results of the solid-fluid interfacial tension for Lennard-Jones fluids demonstrate that different types of external potentials modulate the interfacial tension in different manners.     

Order and fluctuations in nonequilibrium molecular dynamics simulations of two-dimensional fluids

Finite systems of hard disks placed in a temperature gradient and in an external constant field have been studied, simulating a fluid heated from below. We used the methods of nonequilibrium molecular dynamics. The goal was to observe the onset of convection in the fluid. Systems of more than 5000 particles have been considered and the choice of parameters has been made in order to have a Rayleigh number larger than the critical one calculated from the hydrodynamic equations. The appearance of rolls and the large fluctuations in the velocity field are the main features of these simulations.     

The microscopic stress tensor field in particle systems with many-body interactions

It is argued that the up to now only existing expression for the microscopic stress tensor in the presence of many-body interactions, while being formally acceptable, displays some physical shortcomings. These unpleasant features are remedied by explicitly constructing and symmetrizing a new stress tensor field. With the help of this construction, some recent results on the appearance of extremely long-ranged correlations involving the stress tensor field in systems with spontaneously broken symmetries are generalized.     

Effect of tensor force on dissipation dynamics in time-dependent Hartree-Fock theory

The role of tensor force on the collision dynamics of ¹⁶O+¹⁶O is investigated in the framework of a fully three-dimensional time-dependent Hartree-Fock theory. The calculations are performed with modern Skyrme energy functional plus tensor terms. Particular attention is given on the analysis of dissipation dynamics in heavy-ion collisions. The energy dissipation is found to decrease as an initial bombarding energy increases in deep-inelastic collisions for all the Skyrme parameter sets studied here because of the competition between the collective motion and the single-particle degrees of freedom. We reveal that the tensor forces may either enhance or reduce the energy dissipation depending on the different parameter sets. The fusion cross section without tensor force overestimates the experimental value by about 25%, while the calculation with tensor force T11 has good agreement with experimental cross section.     

Molecular dynamics study of two-and three-dimensional classical fluids using double Yukawa potential

We have carried out a molecular dynamics simulation of two- and three-dimensional double Yukawa fluids near the triple point. We have compared some of the static and dynamic correlation functions with those of Lennard—Jones, when parameters occurring in double Yukawa potential are chosen to fit Lennard-Jones potential. The results are in good agreement. However, when repulsive and attractive parameters occurring in double Yukawa potential are varied, we found distinct differences in static and dynamic correlation functions. We have also compared the two-dimensional correlation functions with those of three-dimensional to study the effect of dimensionality, near the triple point region.     

The role of the dynamic pressure in stationary heat conduction of a rarefied polyatomic gas

The effect of the dynamic pressure (non-equilibrium pressure) on stationary heat conduction in a rarefied polyatomic gas at rest is elucidated by the theory of extended thermodynamics. It is shown that this effect is observable in a non-polytropic gas. Numerical studies are presented for a para-hydrogen gas as a typical example.     

Long-time behavior of the angular velocity autocorrelation function for a fluid of rough spheres

According to hydrodynamical and mode-coupling theories, the angular velocity autocorrelation function decays at long times as ₀(t/10^–14 sec)^–5/2. For rough spheres under the conditions reported here, the quantity ₀ is predicted to be 262. The molecular dynamics studies presented here yield a long-time tail of the form 230(t/10^–14 sec)^–2.38. The disagreement between theory and computer results probably arises from statistical error intrinsic to the computations.The authors are indebted to the National Science Foundation and the Computer Center of the University of Minnesota for financial support of the research reported here.     

电磁场角动量守恒定律的简明推导

对电磁场角动量守恒定律作了一种简明的推导。     

The effect of pressure on the elastic constants of Cu, Ag and Au: a molecular dynamics study

This paper describes the effect of pressure on some the mechanical properties of transition metals Cu, Ag, and Au, such as elastic constants and bulk modulus. Using molecular dynamics (MD) simulation, the present study was carried out using the modified many-body Morse potential function expression in the framework of the Embedded Atom Method (EAM). The effect of pressure on equilibrium volume, elastic constants, and bulk modulus were determined, and found to be in agreement with other theoretical calculations and experimental data.     

Direct numerical simulation study of the interaction between the polymer effect and velocity gradient tensor in decaying homogeneous isotropic turbulence

Direct numerical simulation of decaying homogeneous isotropic turbulence (DHIT) of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we directly investigate the influence of polymers on velocity gradient tensor including vorticity and strain. By visualizing vortex tubes and sheets, we observe a remarkable inhibition of vortex structures in an intermediate-scale field and a small-scale field but not for a large scale field in DHIT with polymers. The geometric study indicates a strong relevance among the vorticity vector, rate-of-strain tensor, and polymer conformation tensor. Joint probability density functions show that the polymer effect can increase "strain generation resistance" and "vorticity generation resistance", i.e., inhibit the generation of vortex sheets and tubes, ultimately leading to turbulence inhibition effects.     

激光等离子体中高能电子各向异性压强的粒子模拟

直接驱动惯性约束聚变(ICF)的实现需要对靶丸进行严格的对称压缩,以达到自持热核反应(点火)所需的条件.快点火方案的应用降低了对靶丸压缩对称性以及驱动能量的要求,但压缩及核反应过程中良好的靶丸对称性无疑有助于核反应增益的提高.本文研究了快点火方案中高能电子注入高密等离子体后导致的各向异性电子的压强张量.这一现象存在于ICF快点火方案中的高能电子束"点火"及核反应阶段.鉴于高能电子加热离子过程以及靶丸核反应自持燃烧过程的时间较长,高密靶核会由于超高的各向异性压强的作用破坏高密靶丸的对称性,降低核燃料密度,进而降低了核燃料燃烧效率以及核反应增益.     

二维静态时空中Dirac场的重正化能动张量和Casimir效应

在一般渐近平直的二维静态黑洞时空中,利用重正化能动张量的一般性质, 对位于两“平行板”间满足Dirichlet条件的无质量Dirac场的重正化能动张量的真空期待值进行了分析和计算, 得到了一般表达式.利用该表达式可以给出各种具体渐近平直二维静态黑洞时空中的相应Casimir力.对于重正化能动张量及Casimir力与真空态定义(包括Boulware 真空态、Hartle-Hawking真空态和Unrum真空态三种情况)、Hawking辐射和反常迹的关系分别进行了讨论，给出了相应的表达式和计算结果. 关键词： 能动张量 Casimir 效应 黑洞 真空态     

壳层模型分子动力学在CaO研究中的应用

利用壳层分子动力学方法结合有效的对势,研究了高压条件下CaO的熔化曲线。研究表明,分子动力学模拟结果精确地再现了广泛压强范围内CaO的状态方程。研究中考虑了分子动力学模拟熔化存在的过热现象,通过晶体的现代熔化理论,对CaO的分子动力学模拟熔化温度进行了修正,获得了高温高压下CaO正确的熔化温度。因此,常压下引入壳层模型的分子动力学为研究物质熔化提供了一个很好的方法,这种方法可进一步推广到其它物质的高压熔化研究中。     

Computer simulation of the vacancy defects interaction with shuffle dislocation in silicon

The interactions between the 60^° shuffle dislocation and two different types of vacancy defects in silicon are separately studied via the molecular dynamics simulation method. The Stillinger–Weber potential is used to describe the atomic interactions. The results show that the dislocation slip velocity will decrease due to the interaction with the vacancy cluster (V ₆). The simulation also reveals that the divacancy will be absorbed by the dislocation. Meanwhile, a climbing of the dislocation occurs during their interactions. However, the divacancy has little effect on the dislocation slip velocity. Based on the above results, the decrease in threading dislocation density in SiGe/Si heterostructures with the use of low-temperature Si buffer layer may be explained.     

The fractal nature of molecular trajectories in fluids

The scaled lengths of molecular trajectories obtained by molecular dynamics simulation of a hard-sphere fluid are shown to have the same fractal dimensionD=2 as the random walk. Self-similarity first appears on length scales typically a factor of 25 greater than the mean free-path length, whereas for the simple random walk with constant step size the onset occurs after only six steps; the reason for the slow convergence is shown to be the near exponential distribution of intercollision path lengths of the fluid molecules. The influence of density on the scaled path lengths is also discussed.     

The effects of powder properties on in-flight particle velocity and deposition process during low pressure cold spray process

In cold spray process, impacting velocity and critical velocity of particles dominate the deposition process and coating properties for given materials. The impacting velocity and critical velocity of particles depend on the powder properties and cold spray conditions. In the present study, the in-flight particle velocity of copper powder in low pressure cold spraying was measured using an imaging technique. The effects of particle size and particle morphology on in-flight particle velocity and deposition efficiency were investigated. The critical velocity of copper powder was estimated by combining the in-flight particle velocity and deposition efficiency. The effect of annealing of feedstock powder on deposition and critical velocity was also investigated. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 μm. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390 °C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study.     

高压下固相硝基甲烷分解的分子动力学计算

基于ReaxFF, 采用NVT系综和Berendsen方法对0–7 GPa时和2500 K时固相硝基甲烷的 分解过程进行分子动力学计算, 通过分析硝基甲烷发生分解反应生成的碎片数量随时间的变化, 对不同压强下硝基甲烷的分解机理进行研究. 计算结果表明在0–3 GPa时, 初始分解路径为C–N键断裂和硝基甲烷的异构化; 在4–7 GPa 时, 初始分解路径为分子间质子转移和C–N, N–O键的断裂; 在硝基甲烷的第二阶段反应中存在H₂O, NO, NO₂, HONO, 硝基甲烷分子自身的催化反应. 硝基甲烷在高温高压下发生热分解反应生成碳团簇, 且团簇中碳原子的数量和碳团簇的空间构型随着压强的变化而变化. 关键词： ReaxFF 分子动力学 热分解 压强效应 碳团簇     

The influence of pressure on freezing in molecular liquids

We present the results of experimental investigation of some liquid alkanes PVT behaviour. Their molecular structure was investigated by MoK X-ray scattering method. The obtained data allow suggesting the equation of state and determination of the relation of these equation parameters to molecular characteristics such as molecular mass and exponents of the potential of molecular interaction.     

Molecular dynamics study of helium bubble pressure in titanium

In this paper,the pressure state of the helium bubble in titanium is simulated by a molecular dynamics(MD) method.First,the possible helium/vacancy ratio is determined according to therelation between the bubble pressure and helium/vacancy ratio;then the dependences of the helium bubble pressure on the bubble radius at different temperatures are studied.It is shown that the product of the bubble pressure and the radius is approximately a constant,a result justifying the pressure-radius relation predicted by thermodynamics-based theory for gas bubble.Furthermore,a state equation of the helium bubble is established based on the MD calculations.Comparison between the results obtained by the state equation and corresponding experimental data shows that the state equation can describe reasonably the state of helium bubble and thus could be used for Monte Carlo simulations of the evolution of helium bubble in metals.