结构随机分析的Monte Carlo加权残值法

本文提出一种结构随机分析的Monte Carlo加权残值法。文中建立了这种方法的基本列式,并通过静力挠度、固有频率和屈曲荷载等算例,表明本文方法理论简捷,计算工程量少,精度较高,是随机结构数值分析的有效方法。     

Monte Carlo Simulations of Observation Time-Dependent Self-Diffusion in Porous Media Models

Observation time-dependent self-diffusion coefficients can be used to obtain microstructural information of porous media. This paper presents two different kinds of Monte Carlo simulations of the self diffusion process of fluids like water in porous systems, a lattice-free method and a lattice-based method. The results for simple porous media model geometries agree well with each other and with published analytical as well as semi-analytical equations. The use of these equations, which are important for the interpretation of Pulsed Field Gradient-Nuclear Magnetic Resonance (PFG-NMR) time-dependent diffusion data with respect to properties of porous media, is discussed.     

Monte Carlo simulations on the thermodynamic properties of high‐density gases

The thermodynamic properties of Ar, H₂ and CH₄ at high‐density conditions are studied using Monte Carlo simulations. The isotherms of Ar at 500K, H₂ at 1000K and CH₄ at 500K are obtained respectively. To validate the accuracy of the simulation results, the thermodynamic properties of these gases are also studied with van der Waals equation and compared with the reference data. The agreement with reference shows that Monte Carlo method can produce reliable thermodynamic properties of high‐density gases based on the accurate intermolecular potential model. Therefore, the accuracy of the simulations depends primarily on the accuracy of the potential model, and this dependence is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Effective Diffusivity Tensors of Point-Like Molecules in Anisotropic Porous Media by Monte Carlo Simulation

Monte Carlo simulations of random walks in anisotropic structured media are performed to determine the dependence of effective diffusivities on geometrical properties. The anisotropic media used in this study are periodic systems, which are generated by extending primitive, face-centered, and body-centered unit cells indefinitely in all axial directions. Results of simulations compare well with published experimental data and the calculations by the volume averaging method. In addition, these results suggest that if the 2D media with percolation thresholds subtantially differ from those of 3D, 2D approximations of 3D media are not satisfactory. When percolation thresholds are the same, the effective diffusivity tensors depend solely on the porosity. This fact has been suggested for isotropic media and it seems to hold for anisotropic media.     

Spectral simulation and shock absorber identification

In vehicle dynamics shock absorbers are used for the optimization of driving comfort and driving safety. Therefore, it is necessary to identify characteristics of shock absorbers under real conditions. This paper introduces the use of hardware-in-the-loop simulations for the identification of shock absorbers involving stochastic models of the road roughness. For this purpose a dynamic hydraulic test stand is used replacing the classical mechanical test stands which allow only sinusoidal excitation. For the Monte Carlo simulation with a real shock absorber in the loop, the random excitation of ground roughness is generated using a modified spectral representation method based on the famous contributions of Shinozuka. Motion and force of the shock absorber are measured and fed back to the Monte Carlo simulation of a car model in real time. The characteristic of the shock absorber is identified using the classical least squares method and a correlation-based method. A piecewise linear model for the characteristic relating the damping force and the velocity of the piston is applied for the shock absorber identification.     

Ionization and nonequilibrium radiation of air behind strong shock waves

It is shown that at high velocities of shock waves (V 9.5 km/sec) an important factor influencing the rate of ionization is the depletion of the number of excited states of the atoms through de-excitation. In the case of low pressures (p 1 torr) and for a bounded and optically transparent region of gas heated by the shock wave (for example, for the motion of gas in a shock tube or in a shock layer near a blunt body), the effective ionization rate k_f depends on the pressure [1], which leads to violation of the law of binary similarity which holds under these conditions without allowance for de-excitation. On leaving the relaxation zone, the gas arrives at a stationary state with constant parameters differing from those in thermodynamic equilibrium. The electron concentration and also the radiation intensity in the continuum and the lines are lower than the values for thermodynamic equilibrium. These considerations explain the results of known experiments and some new experiments on ionization and radiation of air behind a travelling shock wave.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 105–112, January–February, 1980.     

Nonideal effects behind reflected shock waves in a high-pressure shock tube

Abstract. Shock tubes often experience temperature and pressure nonuniformities behind the reflected shock wave that cannot be neglected in chemical kinetics experiments. Because of increased viscous effects, smaller tube diameters, and nonideal shock formation, the reflected-shock nonidealities tend to be greater in higher-pressure shock tubes. Since the increase in test temperature () is the most significant parameter for chemical kinetics, experiments were performed to characterize in the Stanford High Pressure Shock Tube using infrared emission from a known amount of CO in argon. From the measured change in vibrationally equilibrated CO emission with time, the corresponding ddt (or for a known time interval) of the mixture was inferred assuming an isentropic relationship between post-shock temperature and pressure changes. For a range of representative conditions in argon (24–530 atm, 1275–1900 K), the test temperature 2 cm from the endwall increased 3–8 K after 100 s and 15–40 K after 500 s, depending on the initial conditions. Separate pressure measurements using a shielded piezoelectric transducer confirmed the isentropic assumption. An analytical model of the reflected-shock gas dynamics was also developed, and the calculated 's agree well with those obtained from experiment. The analytical model was used to estimate the effects of temperature and pressure nonuniformities on typical chemical kinetics measurements. When the kinetics are fast (s), the temperature increase is typically negligible, although some correction is suggested for kinetics experiments lasting longer than 500 s. The temperature increase, however, has a negligible impact on the measured laser absorption profiles of OH (306 nm) and CH (216 nm), validating the use of a constant absorption coefficient. Infrared emission experiments are more sensitive to temperature and density changes, so nonuniformities should be taken into account when interpreting ir-emission data. Received 25 April 2000 / Accepted 8 September 2000     

Monte Carlo simulation of self-avoiding lattice chains subject to oscillatory shear flow in polymer solution

 This paper has introduced a pseudo-potential in bond-fluctuation model to simulate oscillatory shear flow of multiple self-avoiding chains in three dimensions following our previous work under simple shear flow. The oscillatory flow field was reasonably reproduced by lattice Monte Carlo simulation using this pseudo-potential neglecting hydrodynamic interaction. By sampling the configuration distribution functions, the macroscopic viscoelasticity of semi-concentrated polymer solution was determined. Both Newtonian and non-Newtonian regimes were studied. The complex modulus and dynamic viscosity exhibit a reasonable power relation with oscillatory frequency, which is consistent with present theories and experiments. Consequently, lattice Monte Carlo simulation has been extended to model free-draining self-avoiding multi chains subject to oscillatory shear flow and to investigate associated viscoelasticity on the molecular level. Received: 1 October 1999 Accepted: 19 October 1999     

Possibility of acceleration of the threshold processes for multi-component gas in the front of a shock wave

Studies of translational nonequilibrium in the front of a shock wave propagating in a three-component gas were performed by the Monte Carlo simulation method. Simulations were performed for mixtures of components with molecular mass ratios , and shock Mach number . The distribution of relative velocities for pairs of molecules of heavy low-concentration additives 2 and 3 substantially exceeded, in the front, its equilibrium values behind the wave at high values of . The maximum value of this superequilibrium was about for the numerical density ratio: 1000:1:1 and . Calculations showed that high values of the effect of superequilibrium take place up to a ratio of densities 200:1:1. Simulations performed for and a mixture of He, molecular oxygen and Xe with the numerical density ratio 200:1:1 showed also the high value of the superequilibrium effect at corresponding to dissociation threshold of oxygen. Thus, dissociation of oxygen by collisions with Xe in the front of a wave may have a considerably higher rate than total dissociation behind the wave. Received 4 August 1995 / Accepted 25 April 1996     

Monte Carlo simulation of self-avoiding lattice chains under slit flow

This paper proposes a pseudo-potential describing slit flow in the formalism of lattice Monte Carlo simulation with the bond fluctuation algorithm as the unique basic micro-relaxation mode. The main characteristics of slit flow, such as the parabolic velocity profile and the pressure-flux relationship, are successfully reproduced in a three-dimensional self-avoiding multi-chain system. Both Newtonian and non-Newtonian regimes are revealed. The chain conformation and nonlinear rheological behavior are investigated. The simulation results agree with experimental measurements. This method can be used to investigate the viscoelastic properties at different layers as well as the global properties. Some peculiar phenomena in inhomogeneous flow are found to be consistent with previous theoretical predictions by others. Received: 20 April 1999/Accepted: 22 July 1999     

Monte Carlo simulation in the stochastic analysis of non-linear systems under external stationary Poisson white noise input

A method for the evaluation of the probability density function (p.d.f.) of the response process of non-linear systems under external stationary Poisson white noise excitation is presented. The method takes advantage of the great accuracy of the Monte Carlo simulation (MCS) in evaluating the first two moments of the response process by considering just few samples. The quasi-moment neglect closure is used to close the infinite hierarchy of the moment differential equations of the response process. Moreover, in order to determine the higher order statistical moments of the response, the second-order probabilistic information given by MCS in conjunction with the quasi-moment neglect closure leads to a set of linear differential equations. The quasi-moments up to a given order are used as partial probabilistic information on the response process in order to find the p.d.f. by means of the C-type Gram-Charlier series expansion.     

Monte Carlo simulation of stage separation dynamics of a multistage launch vehicle

This paper provides the formulation used for studing the cold and hot separating stages of a multistage launch vehicle. Monte Carlo simulation is employed to account for the off nominal design parameters of the bodies undergoing separation to evaluate the risk of failure for the separation event. All disturbances, effect of dynamic unbalance, residual thrust, separation disturbance caused by the separation mechanism and misalignment in cold and hot separation are analyzed to find out nonoccurrence of collision between the separation bodies. The results indicate that the current design satisfies the separation requirements.     

On the dynamics of grafted branched polymers—a Monte Carlo simulation study

A simplified model of grafted branched polymers was designed and investigated. The model consisted of star-branched chains constructed on a simple cubic lattice. The star polymers were built of three arms of equal lengths. The chains were attached to an impenetrable flat surface with one arm’s end. The arm attached to the surface (a stem) was built of segments different from those in two remaining arms (branches). During the Monte Carlo simulation of the system, the conformation of each chain was modified according to the metropolis sampling algorithm with local changes of chain conformations. The simulations were performed for different chain lengths and the temperature of the system (solvent conditions). The structure of a polymer film formed on the grafting surfaces depended strongly on the temperature and the low temperature films consisted of two separate layers with the insoluble layer located near the grafting surface. The short-time relaxation of the branches and stems of chains was also investigated. The analysis of the dynamics of the model system shows the influence of the structure of the system on relaxation times of various parameters. Paper presented at the AERC 2005 held on April 21–23, 2005 in Grenoble, France.     

Acceleration of chemical reactions within a shock wave front

The aim of the present numerical study was to illustrate the possible influence of translational nonequilibrium in the front of a shock wave on the rate of the threshold chemical reaction. The Monte Carlo method of nonstationary statistical simulation with variable weighting factors was used. Gas mixtures which contained, ahead of the front, two chemically interacting small additives , and an inert light main component were considered. A chemical reaction of the additives started in the front of a shock wave and led to formation of two new low-concentration components and . It was shown that for the ratio of molecular number densities of the additives , and an inert component of 1:10:200 and for the molecular mass ratio of components , , , , of 34.5:8:38.5:4:1 the value of the direct reaction rate obtained in the front exceeds its equilibrium value behind the wave by more than 100 times. As a result, the reaction occurs more intensively in the zone of translational nonequilibrium. It was also shown that for the cases of an exothermic reaction and a weak endothermic reaction, a small amount of the light reaction product has the velocity of the shock wave and is carried by the front. Received 13 June 1997/ Accepted 13 July 1998     

Tortuosity based on Anisotropic Diffusion Process in Structured Plate-like Obstacles by Monte Carlo Simulation

The diffusion of fluids in porous media, composed of regularly aligned plate-like obstacles, was studied by Monte Carlo simulation. The diffusion coefficients and all diagonal components of the diffusion tensor were estimated for these media. The calculated tortuosities were modeled as a function of porosity by using the Koponen’s equation related to percolation threshold. These results indicated that a media with a homogeneous porosity has a heterogeneous tortuosity, is affected by the alignments of the plate-like obstacles. Furthermore, the calculation results were compared with the experimental results for fixed perpendicular plates of Comiti and Renaud as a function of porosity. The results for tortuosity compared well for porosity larger than 0.86.     

Direct Monte Carlo Simulation of Transverse Supersonic Rarefied Gas Flow around a Cylinder

Supersonic flow around a cylinder is investigated using the direct simulation Monte Carlo method over a wide rarefaction range: from the Knudsen number Kn = 0.1 to free-molecular flow. The effect of the cylinder temperature on the region of sharp nonequilibrium near the cylinder and the heat flux is studied.     

利用蒙特卡洛方法计算陶瓷刀具磨损可靠性

在连续切削条件下,陶瓷刀具的失效形式是以磨粒磨损为主的摩损失效,其磨损寿命由材料的断裂韧性、硬度和切削过程的参数决定,利用蒙特卡洛方法分别随机生成断裂韧性与硬度的样本值,利用连续车削试验确定切削过程参数,将得到的样本值与切削参数相结合可计算刀具在相应切削条件下的磨损寿命及其可靠性,计算结果表明：陶瓷刀具的断裂韧对磨损寿命的可靠性影响较为明显,因而可以利用断裂韧性来估算磨损可靠性。     

等壁温加热微通道内气体流动换热特性的直接蒙特卡罗模拟

用直接模拟蒙特卡罗方法对压力边界条件下气体在微通道内的流动换热特性进行了研究,给出了壁面与来流存在温差时的沿程速度分布特点,以及在可压缩性与换热条件综合作用下的温度分布特点。研究结果表明:微通道内气体可压缩性作用显著,温度分布由可压缩性和换热强度的相对强弱综合决定;壁面与来流存在温差时气体沿程速度分布型线在入口段内上凸;壁温高于来流温度时,气流速度与等温流动工况下的速度的相对大小与气体稀薄性有关。     

An energy localization principle and its application to fast kinetic Monte Carlo simulation of heteroepitaxial growth

Simulation of heteroepitaxial growth using kinetic Monte Carlo (KMC) is often based on rates determined by differences in elastic energy between two configurations. This is computationally challenging due to the long range nature of elastic interactions. A new method is introduced in which the elastic field is updated using a local approximation technique. This involves an iterative method that is applied in a sequence of nested domains until a convergence criteria is satisfied. These localized calculations yield energy differences that are highly accurate despite the fact that the energies themselves are far less accurate: an effect referred to as the principle of energy localization. This is explained using the continuum analogue of the discrete model and error estimates are found. In addition, a rejection algorithm that relies on a computationally inexpensive estimate of hopping rates is used to avoid a substantial fraction of the elastic updates. These techniques are applied to 1+1-dimensional KMC simulations in physically interesting regimes.