Numerical simulation of point-to-plane corona discharge using a Monte Carlo method

Monte Carlo simulation techniques are used to study the dynamical properties of charged particles in point-to-plane corona discharge. The numerical model includes the release of electron-ion pairs by photoionization and secondary electron emission from cathode as well as the first Townsend ionization. The simulation results of negative corona discharge in nitrogen show that electron avalanche takes place in the region of high electric field near pin electrode and the photoionization is the essential mechanism to sustain the discharge as well as electron impact ionization.     

Reliability analysis of composite wing subjected to gust loads

An aerospace vehicle in atmospheric flight can be exposed to random air turbulence which may cause critical structural failure. Especially, for high aspect ratio wing, the effect of gust becomes more significant and its response to random gust need to be analyzed precisely. In this paper, the reliability analysis is conducted for composite wing subject to gust loads. For this, the probability distribution function of bending moment from random gust is calculated by power spectral analysis and the material properties of composite skin are assumed to be normal random variables to consider uncertainty. With these distributions of random variables, the probability of failure of the wing structure is calculated by Monte Carlo simulation.     

地铁负荷计算及采用冰蓄冷系统的负荷分析

介绍了一种地铁空调计算比较简单，行之有效的计算方法。地铁环境控制中的制冷系统采用冰蓄冷节约运营费用。本方法用于冰蓄冷制冷系统的空调负荷计算，需根据其负荷的特点，计算逐时负荷。     

Numerical simulation of micro-nozzle and micro-nozzle-array flowfield characteristics

Preliminary numerical simulation using a Direct Simulation Monte Carlo (DSMC) method was conducted to elucidate the internal flowfield and external plume characteristics of micro-single-nozzles and micro-nozzle-arrays, since these small-sized nozzles generally undergo a severe viscous loss due to the low Reynolds numbers. This study also contains the investigation on optimization of the geometry and configuration of the micro-nozzles and micro-nozzle-arrays to achieve the improved propulsive performance. Typical sizes of each rectangular nozzle element were 0.1 mm in throat height, 0.36 mm in exit height, and 0.35 mm in length of the divergent part. For the micro-single-nozzles, calculated specific impulses were fairly in good agreement with our previous experimental data, showing a poor nozzle efficiency due to the viscous loss of low Reynolds number. Also, mechanisms of exhaust jet interaction of multi-nozzle-array jets, bringing a significant improvement in thrust performance, were investigated. As a result, it was shown that pressure and temperature increased at the exit and jet boundaries, and then the exhaust multi-jets were not expanded after the exit, or rather being confined, showing possibilities to realize the higher propulsive performance due to the augmented effect of the pressure thrust.     

Numerical modelling of ice ridge keel action on subsea structures

The present paper describes a numerical simulation of the keel-structure interaction where the keel geometry, ice thickness and interaction speed present a scaling ratio of 1:20. The rubble is represented by a Drucker-Prager material with cohesive softening and no dilatation. The partial differential equations are solved with the non-linear Eulerian finite element method of Abaqus Explicit V6.8.2. The results are compared to physical experiments conducted in the ice basin of the Hamburg ship model basin (HSVA). Two identical cubic subsea structures were impacted into the unconsolidated keel portion of two ice ridges with different thermal properties. The results indicate that only slight dilatation occurs, and the models are able to estimate the rubble action and deformed shape. A progressive failure of the rubble occurs. The rubble action is influenced more by the friction angle than it is in punch tests, due to higher confinement at the failure surface.     

Production rate of synchronous transfer lines using Monte Carlo simulation

In this paper, we consider unpaced synchronous transfer lines producing a single product. The transfer line stations are arranged in a series con?guration, have no buffers, and are subject to operation-dependent failures. Throughput is an important performance measure for transfer lines, and we have adopted that measure. Analytical methods for determining capacity of such transfer lines are available only for the simplest systems, but we show Monte Carlo simulation to be a fast, flexible, easy, and accurate method of estimating throughput in lines of any length and having a wide range of operating characteristics.     

磁控过程的计算机模拟

本文基于蒙特卡罗方法,并结合SRIM软件,编制程序跟踪模拟了磁控溅射各物理过程的粒子状态.以铝靶材为例,得到了粒子在磁控溅射各物理过程的状态分布,讨论了工作参数对薄膜沉积过程的影响.模拟结果表明:溅射原子的能量主要分布在20 eV以下,当原子沉积到基片表面时,其能量主要分布在15 eV以下,但有两个分布峰值,两个分布峰值对应着快慢两种不同形式的沉积过程.原子沉积到基片 表面的位置大致服从正态分布,气压p和靶基距离d影响正态分布的方差,也即影响沉积原子在基片表 面分布的均匀性.功率与沉积速度呈良好的线性关系,在工作气压为1 Pa,靶基距离为60mm的条件下,当入射粒子的能量为250 eV时,模拟得到的功率效率最大.     

Monte Carlo simulation of transport process of the sputtered species in the facing target sputtering technique

In facing target sputtering (FTS) technique two targets facing each other are sputtered simultaneously and the particles are collected in the off axis position. When these two targets constitute two different materials having different sputtering yields, the deposited films show a gradation in composition along the axis parallel to the targets. The process parameters involved, increase the complexity of understanding the composition profile of the deposited films. Here an attempt has been made to simulate the transport behavior of the sputtered species, which leads to the theoretical realization of the variation in composition. The simulation has been performed for SmCo system using the Monte Carlo approach based on the empirical formula proposed by Sigmund and Thompson. The theoretical results have been compared with the experimental results obtained and are explained in detail.     

Monte Carlo simulation of polycrystalline microstructures and finite element stress analysis

A two-dimensional numerical model of microstructural effects is presented, with an aim to understand the mechanical performance in polycrystalline materials. The microstructural calculations are firstly carried out on a square lattice by means of a 2-D Monte Carlo (MC) simulation for grain growth, then the conventional finite element method is applied to perform stress analysis of a plane strain problem. The mean grain size and the average stress are calculated during the MC evolution. The simulation result shows that the mean grain size increases with the simulation time, which is about 3.2 at 100 Monte Carlo step (MCS), and about 13.5 at 5000 MCS. The stress distributions are heterogeneous in materials because of the existence of grains. The mechanical property of grain boundary significantly affects the average stress. As the grains grow, the average stress without grain boundary effect slightly decreases as the simulation time, while the one with strengthening effect significantly decreases, and the one with weakening effect increases. The average stress and the grain size agree well with the Hall–Petch relationship.     

Triple test under high vacuum conditions to control the reliability of thin ice film accretion and desorption for astrophysical applications

One of the aims of astrophysics and astrochemistry is to understand the behavior and implications of the presence of ices such as carbon dioxide, nitrogen, methane, etc., in environments of astrophysical interest. In the laboratory, studies of the characterization of these ices are widely performed. During these experiments it is necessary to control three crucial points: the presence of contaminants, the rate of deposition of molecules during the growth of the ice film and the rate of desorption. In our laboratory we have implemented a methodology to control these parameters with three different and complementary techniques. This allows us not only to control but also to determine several parameters of ice analogs such as density, porosity, energy of desorption, etc. The strict control of these crucial points simultaneously allows us to ensure the reliability of our system when determining relevant parameters of ice films for astrophysical applications.     

Tensile cracks in polycrystalline ice under transient creep: Part I — Numerical modeling

The interaction between creep deformations and a stationary or growing crack is a fundamental problem in ice mechanics. Knowledge concerning the physical mechanisms governing this interaction is necessary: (1) to establish the conditions under which linear elastic fracture mechanics can be applied in problems ranging from ice-structure interaction to fracture toughness testing; and (2) to predict the ductile-to-brittle transition in the mechanical behavior of ice and, especially, the stability and growth of cracks subjected to crack-tip blunting by creep deformations. This requires a quantitative estimate of the creep zone surrounding a crack-tip, i.e., the zone within which creep strains are greater than the elastic strains.

The prediction of the creep zone in previous ice mechanics studies is based on the theory developed by Riedel and Rice (1980) for tensile cracks in creeping solids. This theory is valid for a stationary crack embedded in an isotropic material obeying an elastic, power-law creep model of deformation and for a suddenly applied uniform far-field tension load that is held constant with time. The deformation of ice at strain-rates ahead of a crack (i.e., 10⁻⁶ to 10⁻² s⁻¹) is dominated, however, by transient (not steady power-law) creep and the loading, in general, is not instantaneous and constant.

A numerical model is developed in this paper to investigate the role of transient creep and related physical mechanisms in predicting the size, shape and time evolution of the creep zone surrounding the tip of a static crack in polycrystalline ice. The model is based on the fully consistent tangent formulation derived in closed form (Shyam Sunder et al., 1993) and used in the solution of the physically-based constitutive theory developed by Shyam Sunder and Wu (1989a, b) for the multiaxial behavior of ice undergoing transient creep. The boundary value problem involving incompressible deformations ahead of a stationary, traction-free mode I crack in a semi-infinite medium is modeled and solved by a finite element analysis using the boundary layer approach of Rice (1968). This model is verified by comparing its predictions with (i) the known theoretical solutions for the elastic and HRR asymptotic stress and strain fields in an elastic-plastic material of the Ramberg-Osgood type, and (ii) the creep zone size for an isotropic material obeying the elastic power-law creep model of deformation.     

Evaluating small failure probabilities of multiple limit states by parallel subset simulation

A novel subset simulation algorithm, called the parallel subset simulation, is proposed to estimate small failure probabilities of multiple limit states with only a single subset simulation analysis. As well known, crude Monte Carlo simulation is inefficient in estimating small probabilities but is applicable to multiple limit states, while the ordinary subset simulation is efficient in estimating small probabilities but can only handle a single limit state. The proposed novel stochastic simulation approach combines the advantages of the two simulation methods: it is not only efficient in estimating small probabilities but also applicable to multiple limit states. The key idea is to introduce a “principal variable” which is correlated with all performance functions. The failure probabilities of all limit states therefore could be evaluated simultaneously when subset simulation algorithm generates the principal variable samples. The statistical properties of the failure probability estimators are also derived. Two examples are presented to demonstrate the effectiveness of the new approach and to compare with crude Monte Carlo and ordinary subset simulation methods.     

Probabilistic physics-of-failure models for component reliabilities using Monte Carlo simulation and Weibull analysis: a parametric study

In reliability engineering, component failures are generally classified in one of three ways: (1) early life failures; (2) failures having random onset times; and (3) late life or ‘wear out’ failures. When the time-distribution of failures of a population of components is analysed in terms of a Weibull distribution, these failure types may be associated with shape parameters β having values <1, 1, and >1 respectively. Early life failures are frequently attributed to poor design (e.g. poor materials selection) or problems associated with manufacturing or assembly processes.

We describe a methodology for the implementation of physics-of-failure models of component lifetimes in the presence of parameter and model uncertainties. This treats uncertain parameters as random variables described by some appropriate statistical distribution, which may be sampled using Monte Carlo methods. The number of simulations required depends upon the desired accuracy of the predicted lifetime. Provided that the number of sampled variables is relatively small, an accuracy of 1–2% can be obtained using typically 1000 simulations.

The resulting collection of times-to-failure are then sorted into ascending order and fitted to a Weibull distribution to obtain a shape factor β and a characteristic life-time η.

Examples are given of the results obtained using three different models: (1) the Eyring–Peck (EP) model for corrosion of printed circuit boards; (2) a power-law corrosion growth (PCG) model which represents the progressive deterioration of oil and gas pipelines; and (3) a random shock-loading model of mechanical failure. It is shown that for any specific model the values of the Weibull shape parameters obtained may be strongly dependent on the degree of uncertainty of the underlying input parameters. Both the EP and PCG models can yield a wide range of values of β, from β>1, characteristic of wear-out behaviour, to β<1, characteristic of early-life failure, depending on the degree of dispersion of the uncertain parameters. If there is no uncertainty, a single, sharp value of the component lifetime is predicted, corresponding to the limit β=∞. In contrast, the shock-loading model is inherently random, and its predictions correspond closely to those of a constant hazard rate model, characterized by a value of β close to 1 for all finite degrees of parameter uncertainty.

The results are discussed in the context of traditional methods for reliability analysis and conventional views on the nature of early-life failures.     

SiC电子输运的蒙特卡罗模拟

采用了Monte Carlo方法研究了2H-、4H-和6H-SiC的电子输运特性.在模拟中,采用动量弛豫率近似的方法确定散射角,显著压缩散射次数,并用高效的新查表法确定自由飞行时间,相对于阶梯值的自散射方法,完全消除了自散射,大量节省cpu时间.     

Monte Carlo simulation of nucleation and growth of thin films

We study thin film growth using a lattice-gas, solid-on-solid model employing the Monte Carlo technique. The model is applied to chemical vapour deposition (CVD) by including the rate of arrival of the precursor molecules and their dissociation. We include several types of migration energies including the edge migration energy which allows the diffusive movement of the monomer along the interface of the growing film, as well as a migration energy which allows for motion transverse to the interface. Several well-known features of thin film growth are mimicked by this model, including some features of thin copper films growth by CVD. Other features reproduced are—compact clusters, fractal-like clusters, Frank-van der Merwe layer-by-layer growth and Volmer-Weber island growth. This method is applicable to film growth both by CVD and by physical vapour deposition (PVD).     

二维晶粒演变过程的模拟及分形研究

以蒙特卡罗随机模拟方法为基础,通过对原有算法进行改进,对二维晶粒的演变过程进行了计算机模拟和分形研究,获得了统计等效组织模型．分析表明,模拟的晶粒在演变过程中具有分形特性,即晶粒形态具有与时间无关的相似性,与晶粒形核和生长的物理机制相一致,证明了模拟过程的合理性．从而为后续热变形显微组织演变的模拟中晶核的空间分布和生长提供较为精确的显微组织基础．     

导弹折叠展开机构工作可靠性的Monte Carlo模拟计算方法研究

针对导弹折叠展开机构工作可靠性计算精度较差的问题,应用力矩状态函数和功状态函数相结合的可靠性分析方法,以导弹翼面上的力矩和功作为可靠性特征变量,建立导弹折叠展开机构工作可靠性计算模型,提出了机构可靠性的Monte Carlo模拟计算方法,并对某型导弹的翼面折叠展开机构工作可靠性进行了分析计算,所得可靠度比仅仅基于力矩的计算结果提高24.34%,比仅仅基于功的计算结果提高11.74%．计算结果表明,采用所提出的方法计算得到的机构可靠度,与仅仅基于力矩或功的计算结果相比较,更接近于工程实际．     

Modeling of uncertainty for flood wave propagation induced by variations in initial and boundary conditions using expectation operator on explicit numerical solutions

This study proposes a framework for uncertainty analysis by incorporating explicit numerical solutions of governing equations for flood wave propagation with the expectation operator. It aims at effectively evaluating the effect of variations in initial and boundary conditions on the estimation of flood waves. Spatiotemporal semivariogram models are employed to quantify the correlation of the variables in time and space. The 1D nonlinear kinematic wave equation for the overland flow (named EVO_NS_KWE) is applied in the model development. Model validation is made by comparison with the Monte Carlo simulation model in the calculation of statistical properties of model outputs (ie, flow depths), that is, the mean, standard deviation, and coefficient of variation. The results from the model validation show that the EVO_NS_KWE model can produce excellent approximations of the mean and less satisfactory approximations of the standard deviation and coefficient of variation compared with those obtained by using the Monte Carlo simulation model. It concludes that the uncertainties of flow depths in the domain are significantly affected by variations in the boundary condition. Future application of the EVO_NS_KWE model enables the evaluation of uncertainty in model outputs induced by the initial and boundary condition subject to uncertainty and will also provide corresponding probabilistic information for risk quantification method.     

家用制冰机结冰过程的理论与实验研究

根据家用制冰机的工作原理和结构利用焓法对其制冰过程建立数学物理模型，采用全隐式格式开展有限差分数值模拟研究。在一台家用制冰样机上进行实验测试，通过改变环境温度观测冰层厚度与制冰时间的关系，并将实验结果与数值解进行对比。结果表明，数值解与实测值之间相对误差的绝对值小于3％。升高环境温度，在同样的时间内成品冰厚度减小，质量下降。     

Numerical simulations of blast loads and structural deformation from near-field explosions in air

Numerical simulations of air blast loading in the near-field acting on deformable steel plates have been performed and compared to experiments. Two types of air blast setups have been used, cylindrical explosive placed either in free air or in a steel pot. A numerical finite element convergence study of the discretisation sensitivity for the gas dynamics has been performed, with use of mapping results from 2D to 3D in an Eulerian reference frame. The result from the convergence study served as a foundation for development of the simulation models. Considering both air blast setups, the numerical results under predicted the measured plate deformations with 9.4-11.1%. Regarding the impulse transfer, the corresponding under prediction was only 1.0-1.6%. An influence of the friction can be shown, both in experiments and the simulations, although other uncertainties are involved as well. A simplified blast model based on empirical blast loading data representing spherical and hemispherical explosive shapes has been tested as an alternative to the Eulerian model. The result for the simplified blast model deviates largely compared to the experiments and the Eulerian model. The CPU time for the simplified blast model is however considerably shorter, and may still be useful in time consuming concept studies.All together, reasonable numerical results using reasonable model sizes can be achieved from near-field explosions in air.