Calculation of Neutral Particle Energy Spectra in Tokamak by Using the Monte Carlo Method

Neutral particle energy spectra in the HT-7 tokamak are calculated by using the Monte Carlo method. It can reproduce the spectra measured in experiment. Differences of neutral particle energy spectra in higher and lower electron density plasma are discussed. Results show that the ion temperature given by neutral particle energy spectra is lower than the real ion temperature, but the deviation is within 10% if the ion temperature is less than 800 eV and thecentral chord-averaged electron density does not exceed 3 ×1013 cm-3. But for ion temperature higher than 1000 eV at the central chord-averaged density limit up to 5 ×1013 cm-3, the neutral particle energy spectra can still give the ion temperature within 10% deviation.

Elbow precision machining technology by abrasive flow based on direct Monte Carlo method

The investigation was carried out on the technical problems of finishing the inner surface of elbow parts and the action mechanism of particles in elbow precision machining by abrasive flow.This work was analyzed and researched by combining theory,numerical and experimental methods.The direct simulation Monte Carlo(DSMC)method and the finite element analysis method were combined to reveal the random collision of particles during the precision machining of abrasive flow.Under different inlet velocity,volume fraction and abrasive particle size,the dynamic pressure and turbulence flow energy of abrasive flow in elbow were analyzed,and the machining mechanism of particles on the wall and the influence of different machining parameters on the precision machining quality of abrasive flow were obtained.The test results show the order of the influence of different parameters on the quality of abrasive flow precision machining and establish the optimal process parameters.The results of the surface morphology before and after the precision machining of the inner surface of the elbow are discussed,and the surface roughness Ra value is reduced from 1.125μm to 0.295μm after the precision machining of the abrasive flow.The application of DSMC method provides special insights for the development of abrasive flow technology.

Sequential Monte Carlo Method Toward Online RUL Assessment with Applications

Online assessment of remaining useful life(RUL) of a system or device has been widely studied for performance reliability, production safety, system conditional maintenance, and decision in remanufacturing engineering. However,there is no consistency framework to solve the RUL recursive estimation for the complex degenerate systems/device.In this paper, state space model(SSM) with Bayesian online estimation expounded from Markov chain Monte Carlo(MCMC) to Sequential Monte Carlo(SMC) algorithm is presented in order to derive the optimal Bayesian estimation.In the context of nonlinear & non-Gaussian dynamic systems, SMC(also named particle filter, PF) is quite capable of performing filtering and RUL assessment recursively. The underlying deterioration of a system/device is seen as a stochastic process with continuous, nonreversible degrading. The state of the deterioration tendency is filtered and predicted with updating observations through the SMC procedure. The corresponding remaining useful life of the system/device is estimated based on the state degradation and a predefined threshold of the failure with two-sided criterion. The paper presents an application on a milling machine for cutter tool RUL assessment by applying the above proposed methodology. The example shows the promising results and the effectiveness of SSM and SMC online assessment of RUL.

Direct Monte Carlo Method Simulation of the Synthesis of Carbon Particle Through Coagulation in the Detonation of Explosives

A model is constructed and used in computing the coagulation probability of free carbon during the detonation of explosives. A direct simulation Monte Carlo (DSMC) program is constructed to simulate the coagulation of free carbon particles. The evaluation of the distribution spectrum of particles in the system is obtained. The simulation result is consistent with the experimental curve.

Modeling and prediction of ultrasonic attenuations in liquid-solid dispersions containing mixed particles with Monte Carlo method

We develop a theoretical model for predicting the ultrasonic attenuation in the liquid-solid system containing mixed particles. The ultrasonic attenuation coefficient is obtained by counting the number of phonons that reach the receiver. Using the Monte Carlo method (MCM), numerical simulations were performed to predict the ultrasonic attenuations with not only a single particle type but also monodisperse and polydisperse mixed particles. The simulation results for the systems with a single particle type were compared with various standard models. The results show that they agree well at relatively low particle volume concentrations (within 10%). For systems with mixed particles, the particle volume concentrati on was found to in crease to around 10%, and the variation of the ultrasonic attenuation agai nst the mixing ratio yields a nonlinear trend. Moreover, the ultrasonic attenuation is significantly affected by particle properties. The numerical results also show that both the particle type and particle size distribution should be carefully taken into account in the dispersions with polydisperse mixed particles, where the MCM can give a more direct description of the physics of sound propagation compared with the conventional models.

Effect of Fusion Neutron Source Numerical Models on Neutron Wall Loading in a D-D Tokamak Device

Effect of various spatial and energy distributions of fusion neutron sourceon the calculation of neutron wall loading of Tokamak D-D fusion device has been investigated bymeans of the 3-D Monte Carlo code MCNP. A realistic Monte Carlo source model was developed based onthe accurate representation of the spatial distribution and energy spectrum of fusion neutrons tosolve the complicated problem of tokamak fusion neutron source modelling. The results show thatthose simplified source models will introduce significant uncertainties. For accurate estimation ofthe key nuclear responses of the tokamak design and analyses, the use of the realistic source isrecommended. In addition, the accumulation of tritium produced during D-D plasma operation should becarefully considered.

Assessment of Radiation Damage to the Structural Material of EAST Tokamak

Radiation damage to structural material of fusion facilities is of high concern for safety. The superconducting tokamak EAST will conduct D-D plasma experiments with the neutron production of 10^15 neutrons per second. To evaluate the material radiation damage a programme system has been devised with the Monte Carlo transport code MCNP-4C, the inventory code FISPACT99, a specific interface, and the fusion evaluated nuclear data library FENDL-2. The key nuclear responses, i.e. fast neutron flux, displacement per atom, and the helium and hydrogen production, are calculated for the structural material SS-316L of the first wall, and the vacuum vessel, using this programme. The results demonstrate that the radiation damage to the structural material is so little that it will not lead to any significant change of material properties according to the reference design. This indicates that there is a large potential space for EAST to test advanced operation regime from the viewpoint of structural material safety.

Neutral Particle Transport in Cylindrical Plasma Simulated by a Monte Carlo Code

A Monte Carlo code （MCHGAS） has been developed to investigate the neutral particle transport. The code can calculate the radial profile and energy spectrum of neutral particles in cylindrical plasmas. The calculation time of the code is dramatically reduced when the Splitting and Roulette schemes are applied. The plasma model of an infinite cylinder is assumed in the code, which is very convenient in simulating neutral particle transports in small and middle-sized tokamaks. The design of the multi-channel neutral particle analyser （NPA） on HL-2A can be optimized by using this code.

Monte Carlo Method for the Uncertainty Evaluation of Spatial Straightness Error Based on New Generation Geometrical Product Specification

Straightness error is an important parameter in measuring high-precision shafts. New generation geometrical product speeifieation（GPS） requires the measurement uncertainty characterizing the reliability of the results should be given together when the measurement result is given. Nowadays most researches on straightness focus on error calculation and only several research projects evaluate the measurement uncertainty based on ＂The Guide to the Expression of Uncertainty in Measurement（GUM）＂. In order to compute spatial straightness error（SSE） accurately and rapidly and overcome the limitations of GUM, a quasi particle swarm optimization（QPSO） is proposed to solve the minimum zone SSE and Monte Carlo Method（MCM） is developed to estimate the measurement uncertainty. The mathematical model of minimum zone SSE is formulated. In QPSO quasi-random sequences are applied to the generation of the initial position and velocity of particles and their velocities are modified by the constriction factor approach. The flow of measurement uncertainty evaluation based on MCM is proposed, where the heart is repeatedly sampling from the probability density function（PDF） for every input quantity and evaluating the model in each case. The minimum zone SSE of a shaft measured on a Coordinate Measuring Machine（CMM） is calculated by QPSO and the measurement uncertainty is evaluated by MCM on the basis of analyzing the uncertainty contributors. The results show that the uncertainty directly influences the product judgment result. Therefore it is scientific and reasonable to consider the influence of the uncertainty in judging whether the parts are accepted or rejected, especially for those located in the uncertainty zone. The proposed method is especially suitable when the PDF of the measurand cannot adequately be approximated by a Gaussian distribution or a scaled and shifted t-distribution and the measurement model is non-linear.

Differentially weighted direct simulation Monte Carlo method for particle collision in gas-solid flows

In gas-solid flows, particle-particle interaction （typical, particle collision） is highly significant, despite the small particles fractional volume. Widely distributed polydisperse particle population is a typical characteristic during dynamic evolution of particles （e.g., agglomeration and fragmentation） in spite of their initial monodisperse particle distribution. The conventional direct simulation Monte Carlo （DSMC） method for particle collision tracks equally weighted simulation particles, which results in high statistical noise for particle fields if there are insufficient simulation particles in less-populated regions. In this study, a new differentially weighted DSMC （DW-DSMC） method for collisions of particles with different number weight is proposed within the framework of the general Eulerian-Lagrangian models for hydrodynamics. Three schemes （mass, momentum and energy conservation） were developed to restore the numbers of simulation particle while keeping total mass, momentum or energy of the whole system unchanged respectively. A limiting case of high-inertia particle flow was numerically simulated to validate the DW-DSMC method in terms of computational precision and efficiency. The momentum conservation scheme which leads to little fluctuation around the mass and energy of the whole system performed best. Improved resolution in particle fields and dynamic behavior could be attained simultaneously using DW-DSMC, compared with the equally weighted DSMC. Meanwhile, computational cost can be largely reduced in contrast with direct numerical simulation.

考虑颗粒碰撞的多重Monte Carlo算法

从减少计算代价和改进碰撞算法出发,提出了考虑颗粒碰撞的多重MonteCarlo算法,它采用直接模拟MonteCarlo算法来考虑颗粒碰撞,并与求解颗粒拉氏Langevin方程的MonteCarlo算法耦合起来,跟踪比实际颗粒数目小得多的虚拟颗粒.提出了时间步长选定标准、虚拟碰撞伙伴所在控制容积的判断准则、颗粒碰撞发生的判断准则、虚拟碰撞伙伴的选择、基于随机碰撞角度的碰撞动力学,构成了考虑颗粒碰撞的完整多重MonteCarlo算法.对理想工况的细微颗粒流和粗重颗粒流进行了数值模拟,颗粒碰撞率的模拟结果与理论分析解和DNS结果均符合很好,颗粒场演变的细节信息,如时间平均和特定时刻的颗粒数密度,速度和颗粒湍动能等,均与DNS结果符合很好.数值模拟结果证明该算法不仅具有较低的计算代价,而且能够达到足够的计算精度.

基于特征粒子的Monte Carlo自定位方法

提出了一种基于Monte Carlo方法的多机器人自定位方法.该方法在机器人进行自定位时,对用来估计机器人位置的MCL(Monte Carlo Localization)粒子空间进行栅格划分,然后采用可变栅格法获得能代表所有粒子整体特性的特征粒子集.因为特征粒子的数量较粒子总数大大减少,该方法能避免直接将Monte Carlo方法应用于多机器人定位中产生的维数灾的问题,可以在保证精度的情况下降低运算复杂度.仿真结果表明,该方法能较好地满足多机器人自定位的要求.

气固两相流中颗粒碰撞的Monte-Carlo数值模拟

利用颗粒碰撞动力学模型和颗粒几何碰撞率模型,采用Monte-Carlo算法来模拟颗粒之间碰撞,把该算法与求解雷诺应力-概率密度函数模型的有限差分-MonteCarlo算法耦合起来,对轴对称突扩通道内的两相旋流场进行了数值模拟,模拟结果表明,由于颗粒碰撞使颗粒的动能和湍动能在三个坐标方向上进行了再分配,从而导致颗粒的动能和湍动能在三个坐标方向上趋于各向同性;另外,由于颗粒碰撞破坏了颗粒-颗粒、颗粒-流体微团之间的速度关联,从而造成颗粒湍动能及两相速度脉动关联的降低。

凝并和成核机理下颗粒尺度分布的Monte Carlo求解

颗粒的凝并和成核现象影响其尺度分布,现有的Monte Carlo方法描述颗粒尺度分布的时间演变过程存在若干困难.提出了一种新的多重Monte Carlo(MMC)算法,基于时间驱动,利用加权的虚拟颗粒的思想,在模拟过程中保持虚拟颗粒总数不变和计算区域体积不变.利用该算法对“常凝并核,一阶成核”的情况下颗粒尺度分布的时间演变过程进行了数值求解,所得结果与数值解相符,表明MMC算法具有高且稳定的计算精度.另外,MMC算法由于跟踪比实际颗粒数目少得多的虚拟颗粒而具有较低的计算代价.

用MONTE CARLO方法计算托卡马克装置的中性氢原子密度分布

简要描述了计算托卡马克中性氢密度分布的ＭｏｎｔｅＣａｒｌｏ 方法。计算了ＨＴ６Ｂ、ＨＴ６Ｍ装置的中性氢原子密度分布，与实验结果进行了比较。

模拟颗粒凝并过程的快速Monte Carlo方法

对常规异权值Monte Carlo(MC)方法进行改进,基于强核函数思想,通过颗粒群单重遍历即可求得强核函数最大值,采用接受-拒绝法随机搜寻凝并对,并利用搜寻过程中拒绝和接受的所有凝并对的信息来估计凝并事件的等待时间(时间步长),从而避免颗粒群的双重遍历,以提高MC的效率.对典型工况的模拟结果显示该快速方法计算代价仅为O(Ns),能够显著提高计算效率,同时保持足够的计算精度,较好地协调计算代价与计算精度之间的矛盾.

用差分和Monte Carlo求解相结合的方法计算有化学反应的活塞流

用差分和Monte Carlo求解相结合的方法对有化学反应的标量几率密度函数方程进行了求解,讨论了随机颗粒数和时间步长对计算结果的影响。计算结果表明,当化学反应速率是标量的线性函数时,结果与精确解很吻合;当反应速率满足Arrhenius定律时,结果也是合理的。

γ射线谱中符合相加修正系数的Monte-Carlo模拟

用Monte Carlo方法模拟放射性核素的衰变过程,得到用HPGe探测器应能测到的多道γ能谱理论谱形,同时给出各特征能峰的探测效率和各特征峰的真符合相加修正系数。采用本方法开发的I、140Co、131Cool2000软件,可模拟60Co、88Y、134Cs、133Ba、95Nb、57Co、103Ru、106Ru、110Agm、124Sb、144Ce、65Zn、58La、241Am、99Mo、166Hom、152Eu、125Sb、214Bi、212Bi(α衰变链)和208T1,共计24种核素的衰变过程。对于60Co圆柱形体源发射的两条主要γ射线进行测量和分析结果表明:符合修正系数的模拟计算值与实验值在±1%的偏差范围内相符。同时给出了一种校正同轴探测器死层厚度和冷指尺寸的方法。

气体混合物各组分化学势的Monte Carlo模拟

用正则系综测试粒子MonteCarlo(GCMC)方法模拟常温下空气(以氮气为代表)及其污染物微量有机物(以苯为例)的混合物中各组分的化学势.模拟中,氮气和苯分子采用LJ球型分子势能模型,采用Metropolis抽样及周期边界条件.通过模拟并拟合得到了在300.2K、苯的摩尔分数为0.0625 ,氮气及苯化学势与压力的关联式,以用于狭缝碳孔中该混合物体系的选择性吸附.

结构可靠度分析中Monte Carlo法的研究

结构可靠度作为一种预测结构破坏的量化形式,现已得到广泛运用。根据实际工程的需要,采用合适的可靠度计算方法非常重要。论文通过对结构可靠度的Monte Carlo法的具体方法,如直接抽样法、重要抽样法、方向抽样法和基于神经网络的Monte Carlo法进行了分析,得出了各种计算方法的特点。