蒙特卡罗方法中分子作用模型的设计及应用

利用Lennard-Jones(6-12)作用势,设计GSS-3分子碰撞模型,通过与已有的分子模型及实验数值对比,该模型能够给出正确的气体输运性质.将GSS-3模型应用到非结构网格DSMC中,模拟二维超声速平板绕流和圆柱绕流.数值计算结果初步验证该分子模型的合理性与有效性.     

电子束激发原子对金属蒸发的影响

在原子蒸气激光同位素分离工程(AVLIS)中,金属受电子束的加热而熔化,并向真空蒸发,蒸气原子通过电子束的过程中,可能与电子发生非弹性碰撞,被激发到高能级.在膨胀的过程中,高能级的原子通过与原子的非弹性碰撞消激发,将原子内的电子能量转换为蒸气的平动能.为了分析电子束与原子作用对蒸气的密度、速度和温度等物理特性的影响,采样直接模拟蒙特卡罗法(DSMC)模拟钆原子蒸发过程.模拟结果表明,电子束与原子的作用使得原子蒸气的速度增加,密度减小,温度升高 关键词： 金属蒸发 AVLIS DSMC 电子枪     

循环床内气固两相流中稠密颗粒间碰撞的数值模拟

循环床内部的流动属于复杂不均匀的稠密气固两相流，稠密颗粒间的相互作用是影响颗粒运动和浓度分布的不可忽视的重要因素。本文采用直接模拟Monte－Carlo算法（DSMC算法）来模拟颗粒间的相互碰撞过程，并与随机轨道模型结合起来综合考虑湍流和颗粒碰撞对颗粒运动和浓度分布的影响，模拟结果预报了床内分层流动结构和颗粒在稀相区的不均匀分布，计算结果与实验定性符合。     

LES-DSMC方法研究超细颗粒气固两相流动过程

采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流,采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞。单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定,考虑超细颗粒间的van der Waals作用力。数值模拟垂直管内超细颗粒气固两相流动,对颗粒相速度、浓度以及团聚物流动过程进行分析。     

超快速反应动力学

十年前,《今日物理》发表了一期有关激光化学方面的文章,其中一篇强调了时间尺度在化学反应中的重要性以及用超短脉冲引发化学反应的可能性.十年来新的激光技术的出现以及气相分子束实验,揭示了基本化学反应各步骤.这些实验所揭示出的化学动力学过程的细节正是本篇文章的主题. 一般地从反应物到生成物的化学过程为 (1),(2)两式指出,有两种获得过渡态的办法:(1)过渡态产生于两个反应物间的碰撞,或称双分子反应2(2)过渡态是当一个稳定的分子获得了足够的能量,在碰撞的后半部分形成的生成物,因而也叫半碰撞反应或单分子反应.为了更好地了解化学…     

含振动能激发Boltzmann模型方程气体动理论统一算法验证与分析

为模拟研究高温高马赫数下多原子气体内能激发对跨流域非平衡流动的影响,将转动能、振动能分别作为气体分子速度分布函数的自变量,把转动能和振动能处理为连续分布的能量模式,将Boltzmann方程的碰撞项分解成弹性碰撞项和非弹性碰撞项,同时将非弹性碰撞按一定松弛速率分解为平动-转动能松弛过程和平动-转动-振动能松弛过程,构造了一类考虑振动能激发的Boltzmann模型方程,并证明了其守恒性和H定理.基于内部能量变量对分布函数无穷积分,引入三个约化速度分布函数,得到一组考虑振动能激发的约化速度分布函数控制方程组,使用离散速度坐标法,基于LU-SGS隐式格式和有限体积法求解离散速度分布函数,建立含振动能激发的气体动理论统一算法.通过开展高稀薄流到连续流圆柱绕流问题统一算法与直接模拟蒙特卡罗法模拟结果对比分析,特别是过渡流区平动、转动、振动非平衡效应对绕流流场与物面力热特性的影响机制,证实了所建立的含振动能激发的Boltzmann模型方程及气体动理论统一算法的准确可靠性.     

氮等离子体枪中电子输运行为的蒙特卡罗模拟

采用蒙特卡罗模拟,对氮等离子体枪的两个环形电极间的氮气辉光放电过程中电子的输运过程进行研究.计算在不同平均电场与粒子数密度的比值(E/N)下,电子与氮分子发生不同碰撞的概率、出射电子的平均能量、方向角分布和电子的能量分布.结果表明,电子能量近似服从玻尔兹曼分布.随着E/N的升高,电子平均能量升高,发生激发、离化、电离和离化电离碰撞的概率增大;非均匀分布的电场使分子获得更高的离化率,同时显著增强出射电子的能量.模拟结果为等离子体应用设计提供了参考依据.     

循环流化床内颗粒团聚物流动特性的DSMC-LES模拟

采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞,采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流.单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定。数值模拟垂直管内气固两相上升流动,对管内气相速度和颗粒相速度、浓度以及聚团的流动特性进行分析.研究平均单个颗粒团聚物的存在时间、颗粒团聚物的时间份额和颗粒团聚物的生成频率分布特性,模拟结果与Manyele等(2002)和Sharma等(2000)试验结果吻合.     

跨流域高超声速绕流环境Boltzmann模型方程统一算法研究

如何准确可靠地模拟从外层空间高稀薄流到近地面连续流的航天器高超声速绕流环境与复杂流动变化机理是流体物理的前沿基础科学问题. 基于对Boltzmann方程碰撞积分的物理分析与可计算建模, 确立了可描述自由分子流到连续流区各流域不同马赫数复杂流动输运现象统一的Boltzmann模型速度分布函数方程, 发展了适于高、低不同马赫数绕流问题的离散速度坐标法和直接求解分子速度分布函数演化更新的气体动理论数值格式, 建立了模拟复杂飞行器跨流域高超声速飞行热环境绕流问题的气体动理论统一算法. 对稀薄流到连续流不同Knudsen数0.002 ≤Kn_∞ ≤1.618、不同马赫数下可重复使用卫星体再入过程(110–70 km)中高超声速绕流问题进行算法验证分析, 计算结果与典型文献的Monte Carlo直接模拟值及相关理论分析符合得较好. 研究揭示了飞行器跨流域不同高度高超声速复杂流动机理、绕流现象与气动力/热变化规律, 提出了一个通过数值求解介观Boltzmann模型方程, 可靠模拟高稀薄自由分子流到连续流跨流域高超声速气动力/热绕流特性统一算法.     

C60分子与石墨(0001)、硅(111)面的碰撞

用分子动力学模拟的方法和Tersoff多体势函数对以一定能量入射的C 60在石墨(0001)表面以及硅(111)表面碰撞的过程进行模拟研究.结果发现:碰撞过程是高度非弹性的,在弹回过程中,C60分子质心的运动可被看作是在准谐势下的运动 .C60以240 eV初始能量入射到石墨表面时,C60分子有严重的扭曲,最终将平铺在石墨表面形成薄膜;C60分子以30 eV初始动能入射到石墨表面时,将保持完好球形沉积在石墨表面;C60分子以60 eV的初始动能碰撞硅(111)表面时,C60分子最终沉积在硅表面,碰撞过程中C60分子有形变.     

Simulation of unsteady flows by the DSMC macroscopic chemistry method

In the Direct Simulation Monte-Carlo (DSMC) method, a combination of statistical and deterministic procedures applied to a finite number of ‘simulator’ particles are used to model rarefied gas-kinetic processes. In the macroscopic chemistry method (MCM) for DSMC, chemical reactions are decoupled from the specific particle pairs selected for collisions. Information from all of the particles within a cell, not just those selected for collisions, is used to determine a reaction rate coefficient for that cell. Unlike collision-based methods, MCM can be used with any viscosity or non-reacting collision models and any non-reacting energy exchange models. It can be used to implement any reaction rate formulations, whether these be from experimental or theoretical studies. MCM has been previously validated for steady flow DSMC simulations. Here we show how MCM can be used to model chemical kinetics in DSMC simulations of unsteady flow. Results are compared with a collision-based chemistry procedure for two binary reactions in a 1-D unsteady shock-expansion tube simulation. Close agreement is demonstrated between the two methods for instantaneous, ensemble-averaged profiles of temperature, density and species mole fractions, as well as for the accumulated number of net reactions per cell.     

Atomistic hybrid DSMC/NEMD method for nonequilibrium multiscale simulations

A multiscale hybrid method for coupling the direct simulation Monte Carlo (DSMC) method to the nonequilibrium molecular dynamics (NEMD) method is introduced. The method addresses Knudsen layer type gas flows within a few mean free paths of an interface or about an object with dimensions of the order of a few mean free paths. It employs the NEMD method to resolve nanoscale phenomena closest to the interface along with coupled DSMC simulation of the remainder of the Knudsen layer. The hybrid DSMC/NEMD method is a particle based algorithm without a buffer zone. It incorporates a new, modified generalized soft sphere (MGSS) molecular collision model to improve the poor computational efficiency of the traditional generalized soft sphere GSS model and to achieve DSMC compatibility with Lennard-Jones NEMD molecular interactions. An equilibrium gas, a Fourier thermal flow, and an oscillatory Couette flow, are simulated to validate the method. The method shows good agreement with Maxwell–Boltzmann theory for the equilibrium system, Chapman–Enskog theory for Fourier flow, and pure DSMC simulations for oscillatory Couette flow. Speedup in CPU time of the hybrid solver is benchmarked against a pure NEMD solver baseline for different system sizes and solver domain partitions. Finally, the hybrid method is applied to investigate interaction of argon gas with solid surface molecules in a parametric study of the influence of wetting effects and solid molecular mass on energy transfer and thermal accommodation coefficients. It is determined that wetting effect strength and solid molecular mass have a significant impact on the energy transfer between gas and solid phases and thermal accommodation coefficient.     

Direct Monte Carlo simulation of high-temperature chemical reactions in air

A novel approach to modeling high-temperature nonequilibrium dissociation in air at a level of molecular collisions is proposed. Information on the energy dependence of the specific reaction cross sections, which is necessary for such modeling, is determined numerically from available macroscopic information on the dependence of the reaction rate constant on translational and vibrational temperatures. The results of Direct Simulation Monte Carlo (DSMC) computations show that the proposed model yields a correct reaction rate in vibrational-translational nonequilibrium. The use of the new model in DSMC computations of high-altitude aerothermodynamics results in obtaining a noticeably different flow structure and a higher heat flux, as compared to that predicted by standard DSMC models (such as the total collision energy model).     

Beyond direct simulation Monte Carlo (DSMC) modelling of collision environments

ABSTRACT

Direct simulation Monte Carlo (DSMC) models have been successfully adopted and adapted to describe gas flows in a wide range of environments since the method was first introduced by Bird in the 1960s. We propose a new approach to modelling collisions between gas-phase particles in this work – operating in a similar way to the DSMC model, but with one key difference. Particles move in a mean field, generated by all previously propagated particles, which removes the requirement that all particles be propagated simultaneously. This yields a significant reduction in computation effort and lends itself to applications for which DSMC becomes intractable, such as when a species of interest is only a minor component of a large gas mixture.     

A hybrid particle approach for continuum and rarefied flow simulation

A hybrid particle scheme is presented for the simulation of compressible gas flows involving both continuum regions and rarefied regions with strong translational nonequilibrium. The direct simulation Monte Carlo (DSMC) method is applied in rarefied regions, while remaining portions of the flowfield are simulated using a DSMC-based low diffusion particle method for inviscid flow simulation. The hybrid scheme is suitable for either steady state or unsteady flow problems, and can simulate gas mixtures comprising an arbitrary number of species. Numerical procedures are described for strongly coupled two-way information transfer between continuum and rarefied regions, and additional procedures are outlined for the determination of continuum breakdown. The hybrid scheme is evaluated through a comparison with DSMC simulation results for a Mach 6 flow of N₂ over a cylinder, and good overall agreement is observed. Large potential efficiency gains (over three orders of magnitude) are estimated for the hybrid algorithm relative to DSMC in a simple example involving a rarefied expansion flow through a small nozzle into a vacuum chamber.     

Convergence behavior of a new DSMC algorithm

The convergence rate of a new direct simulation Monte Carlo (DSMC) method, termed “sophisticated DSMC”, is investigated for one-dimensional Fourier flow. An argon-like hard-sphere gas at 273.15 K and 266.644 Pa is confined between two parallel, fully accommodating walls 1 mm apart that have unequal temperatures. The simulations are performed using a one-dimensional implementation of the sophisticated DSMC algorithm. In harmony with previous work, the primary convergence metric studied is the ratio of the DSMC-calculated thermal conductivity to its corresponding infinite-approximation Chapman–Enskog theoretical value. As discretization errors are reduced, the sophisticated DSMC algorithm is shown to approach the theoretical values to high precision. The convergence behavior of sophisticated DSMC is compared to that of original DSMC. The convergence of the new algorithm in a three-dimensional implementation is also characterized. Implementations using transient adaptive sub-cells and virtual sub-cells are compared. The new algorithm is shown to significantly reduce the computational resources required for a DSMC simulation to achieve a particular level of accuracy, thus improving the efficiency of the method by a factor of 2.     

Multiple temperature model for the information preservation method and its application to nonequilibrium gas flows

The information preservation (IP) method has been successfully applied to various nonequilibrium gas flows. Comparing with the direct simulation Monte Carlo (DSMC) method, the IP method dramatically reduces the statistical scatter by preserving collective information of simulation molecules. In this paper, a multiple temperature model is proposed to extend the IP method to strongly translational nonequilibrium gas flows. The governing equations for the IP quantities have been derived from the Boltzmann equation based on an assumption that each simulation molecule represents a Gaussian distribution function with a second-order temperature tensor. According to the governing equations, the implementation of IP method is divided into three steps: molecular movement, molecular collision, and update step. With a reasonable multiple temperature collision model and the flux splitting method in the update step, the transport of IP quantities can be accurately modeled. We apply the IP method with the multiple temperature model to shear-driven Couette flow, external force-driven Poiseuille flow and thermal creep flow, respectively. In the former two cases, the separation of different temperature components is clearly observed in the transition regime, and the velocity, temperature and pressure distributions are also well captured. The thermal creep flow, resulting from the presence of temperature gradients along boundary walls, is properly simulated. All of the IP results compare well with the corresponding DSMC results, whereas the IP method uses much smaller sampling sizes than the DSMC method. This paper shows that the IP method with the multiple temperature model is an accurate and efficient tool to simulate strongly translational nonequilibrium gas flows.     

微通道内气体流动的DSMC方法

基于压力边界条件开展了微尺度低速流动DSMC方法的研究, 定义了两个无量纲参数作为微尺度DSMC方法下网格尺寸与时间步长的约束条件, 通过微尺度Poiseuille流进行了方法的验证与比较, 获得了网格尺寸与时间步长的一般原则。在此基础上, 对变截面的单孔和双孔模型的微通道气体流动进行DSMC模拟, 结果表明, 通道几何形状对微尺度气体流动具有显著影响, 孔口后由于通道收缩, 产生压降, 导致气流加速, 并在孔口下游拐角处发生分离; 双孔口模型的流动结构与单孔口模型相似, 且在相同压差情况下, 经双孔口后的气体流速低于经单孔口后的气体流速; 随着入口压力的增加, 经过孔口压缩后的速度越大, 分离区尺寸也越大。