基于次同步振荡局部传播机制的建模边界识别

根据次同步谐振的诱发机理,揭示了汽轮发电机组与串补线路相互作用的次同步振荡局部传播机制,阐明了电气谐振约束了次同步振荡的传播。然后,推导了次同步功率传播与次同步谐振电气负阻尼系数的关系式,根据这个关系式提出了一种确定次同步谐振实际系统建模边界的新方法。最后,应用IEEE 10机39节点和16机68节点标准系统,从时域仿真轨迹和频域模态两方面验证了此建模方法的合理性。

波导安装边界精确建模方法研究

针对大规模波导组件在力学简化建模时存在边界建模不精确,计算结果误差大的问题,对卫星载荷舱波导组件的结构特征、边界条件、振动环境等进行分析,同时结合相关部件的结构特征及安装状态,确定波导的边界条件、连接部位局部模拟方法及模型简化方法;以典型波导系统为研究对象,分别采用不同的边界处理方法建立有限元模型,通过与试验值进行对比分析,结果表明:对于连接结构较多的系统,局部连接刚度定义的偏差会对模态分析结果有较大的影响,对常见的波导连接结构,多点耦合连接(multipoint constraint)的边界建模假设会导致计算得到的结果过于乐观,采用铰接副(joint)连接进行模拟,仿真误差不超过10%;而在间隙填充物力学性能参数确定的情况下,采用薄层单元进行模拟可获得更为准确的分析结果,仿真误差不超过5%。

三维离散元法边界建模软件设计

针对目前离散元法边界建模中存在的问题,基于UG软件二次开发,结合VC++平台、MFC函数库和Access数据库,设计了三维离散元法边界建模软件,实现了与UG软件和三维离散元法分析软件的集成。以型孔轮式排种器为例,通过排种器的CAD设计、排种器工作过程的三维离散元法仿真分析以及与试验的对比,证明了边界建模软件的可行性和有效性。

基于图元的三维离散元法边界建模方法

为实现由三维计算机辅助设计模型(三维计算机辅助设计软件设计图)建立边界的三维离散元法分析模型,提出一种基于图元的三维离散元法边界建模方法及实边界图元和虚边界图元的概念。在此基础上,建立了圆形平面、圆锥面和圆柱面等二十余种三维图元与球体颗粒的接触检测、接触点求解,以及接触叠合量的计算方法,并通过几个实例,验证了该边界建模方法的正确性和有效性,为设计阶段的计算机辅助设计模型进行机械部件工作过程的仿真分析奠定了基础。

交流电网对管道持续干扰分析建模边界的确定方法

为分析大区域电网对埋地油气管道的持续干扰,本研究在分析持续干扰电压幅值随线路和管道间距变化规律的基础上,提出利用阈值确定大区域电网建模边界,以构建电网对管道持续干扰分析模型的方法,并建立了计算截断误差的概率模型。结果表明:由距离管道较远线路产生的小幅值干扰电压分量的向量和近似为零,且这些干扰电压分量向量和的模值近似服从瑞利分布;在分析线路平均间距大于200 m的220 kV电网对管道的持续干扰时,仅考虑管道两侧4.20 km内导线按ZB型杆塔排列的线路,4.10 km内导线按ZM型杆塔排列的线路,10.65 km内导线按SZ型杆塔排列的线路,11.93 km内导线按SSZ型杆塔排列的线路,11.84 km内导线按SSSZ型杆塔排列的线路,即可使截断误差有95%的概率小于2.23 V。

基于声调核参数及DNN建模的韵律边界检测研究

韵律边界对言语表达的自然度和可理解度有着重要作用。韵律建模也是语音合成、语音理解中的重要方面。该文从相邻声调的相互作用角度出发,提出基于深度神经网络(DNN)及声调核声学特征的汉语韵律边界检测方法。该方法首先采用声调核部分的声学特征来计算边界检测相关参数。然后,利用深度神经网络进行建模。作为对比,实验中采用了以整个音节的声学特征为输入特征的基线系统。结果表明,只使用调核部分声学特征的系统优于使用整个音节的系统,韵律边界检测正确率相对提高了4%,这表明该文提出的汉语韵律边界检测方法的有效性。

非规则曲面的离散元法分析模型建模软件

在采用离散元法分析机械部件与颗粒材料接触作用时,需要建立机械部件(边界)的离散元法分析模型。分析可知,机械部件中与颗粒材料接触作用的零件表面,存在不能用初等解析函数表达的非规则曲面。为此,采用推进波前法(AFT:Advancing Front Technique)进行非规则曲面网格划分,把非规则曲面离散成三角形平面片的组合,同时添加运动属性和材料特性参数,由此建立非规则曲面边界的离散元法分析模型。在对PRO/E软件进行二次开发的基础上,研制了非规则曲面边界建模软件。通过实例验证,初步证明了基于AFT边界建模方法和软件的可行性,为复杂结构机械部件工作过程的仿真分析奠定了基础。

空间薄膜天线索膜结构建模与形状优化

针对空间平面薄膜天线,提出一种索膜边界建模及形状优化方法.首先,基于索膜结构在受张力作用时边界处悬索与薄膜存在滑动的特点,提出一种非线性有限元建模方法;其次,将索膜边界视为B样条曲线,并以曲线控制点位移作为设计变量,通过优化结构实际应力与工程参考应力间的偏差,实现了对薄膜在任意初始边界下的形状优化;最后,通过数值算例与实验验证说明了方法的有效性.

基于神经网络模型的紫外极光卵边界建模

极光卵的尺度大小和太阳风、磁层、电离层以及它们间的耦合过程有密切的联系,会随着空间和地磁环境的变化而变化.建立准确的极光卵边界预测模型对空间天气的预报以及了解日地关系具有重要意义.本文利用误差反向传播(back propagation, BP)神经网络和广义回归神经网络(general regression neural network, GRNN)两种神经网络模型对极光卵边界进行建模.结果显示GRNN的极光卵边界模型具有较高的准确性,赤道向边界预测平均绝对误差在0.77～1.20磁纬度(MLAT);极向边界预测平均绝对误差在0.83～1.39 MLAT.基于GRNN的极光卵边界模型预测准确性分别在极向边界和赤道向边界的整个磁地方时(MLT)上比BP神经网络的极光卵边界模型平均提高了0.74和0.73 MLAT,比多元线性回归模型平均提高了0.82和0.82 MLAT.而在模型的外推性方面, GRNN的极光卵边界模型的外推性优于BP神经网络的极光卵边界模型,与多元线性回归模型接近.

基于半参数加性logistic的信号交叉口动态两难区驾驶行为研究

信号交叉口进口道两难区的存在增加了交叉口冲突的可能,利用间接观测计算方式提取首停车和末行车的行驶参数,进行统计分析和独立样本T检验,并通过均方根误差RMSE结合实例验证车辆行驶关键参数模型的准确性,建立最小停车距离模型与最大通过距离模型,构建了反映车辆个体差异性和符合实际道路交通环境的动态两难区边界模型。通过相关性分析、信息价值和多重共线性检验筛选影响两难区驾驶行为决策的因素,构建了考虑连续变量非线性影响的基于半参数加性logistic的动态两难区驾驶行为决策模型,经检验,该预测方法的整体精度达到89.65%。研究结论对动态两难区风险评估和有效规避以及改善信号交叉口的交通安全提供了理论依据。

基于Delaunay三角剖分的曲面求交技术

研究曲面边界建模过程中的曲面求交问题。基于两个曲面Delaunay三角剖分,首先进行包围盒判定预处理,再进行三角形相交测试,求初始交线。然后,检测初始交线存在性和连续性,提出"带权曲面边界延伸方法",解决因离散数据点采样不足导致的两曲面有相交趋势但没有初始交线和初始交线断裂两种问题。最后,将"带权曲面边界延伸"思想应用到初始交线处理中,使交线的两个端点都位于两个曲面的边界处,便于后续的限定Delaunay三角剖分、曲面裁剪等操作的进行。仿真实验结果表明算法是可行的。

The implementation of an improved NPML absorbing boundary condition in elastic wave modeling

In elastic wave forward modeling, absorbing boundary conditions （ABC） are used to mitigate undesired reflections from the model truncation boundaries. The perfectly matched layer （PML） has proved to be the best available ABC. However, the traditional splitting PML （SPML） ABC has some serious disadvantages： for example, global SPML ABCs require much more computing memory, although the implementation is easy. The implementation of local SPML ABCs also has some difficulties, since edges and corners must be considered. The traditional non-splitting perfectly matched layer （NPML） ABC has complex computation because of the convolution. In this paper, based on non-splitting perfectly matched layer （NPML） ABCs combined with the complex frequency-shifted stretching function （CFS）, we introduce a novel numerical implementation method for PML absorbing boundary conditions with simple calculation equations, small memory requirement, and easy programming.

Characteristic test of analog integrated circuit frequency

This paper presents an analog circuit built-in-test （BIT） structure based on boundary scan and realizes the BI＇I. It predigests the test process and improves the test precision by taking the rectangular pulse as stimulator and analog switch as auxiliary bridge. The experiment of uA741 shows that the design is feasible. Compared with the traditional test method, it is better regarding reliability and measurability of the analog circuit system.

Parametric Modeling of Static Three-Dimensional Problem of Buried Pipeline through the Coupling between a Finite Cylindrical Shell Element and Special Boundary Elements

This work deals with an application in parametric modeling of problems regarding pipelines buried in static system, written in C＋＋ using ObjectARX functions. The application is fully integrated with AutoCAD through saproms.dll and saproms.arx libraries where the stages of pre-processing, processing and post-processing are done in AutoCAD sotlware environment itself without having to export data to another processing environment. The analysis processing is done by functions whose algorithms are based on the solution of the coupling between the FEM （finite element method） and the BEM （boundary element method）. The representation of the homogeneous pipeline is made by the FEM using a finite element in the form of a cylindrical panel called CYS （cylindrical shell）, developed from the homogeneous isotropic model proposed initially by Djoudi Bahai, who uses the philosophy of the assumed deformation model （assumed strain based model）. The soil is taken to be a continuum infinite or semi-infinite elastic solid patterned by BEM where special boundary elements are proposed with curved surface or curved edges to represent the soil-structure interaction mainly in the areas of contact with the pipeline.

Upper mantle seismic anisotropy beneath a convergent boundary:SKS waveform modeling in central Tibet

This study used SKS waveforms from the International Deep Profiling of Tibet and the Himalayas(INDEPTH) III dataset and a new 2D method for modeling seismic waves in anisotropic media to construct an image of anisotropic structures beneath central Tibet.A preferred model revealed three-segment anisotropic structures in the upper mantle beneath the study region.Waveform modeling demonstrated that the anisotropy was mainly generated by the lithosphere but not the asthenosphere,and that an anisotropic model with a flatter axis of symmetry provides a more consistent interpretation of the observations than models having steeply dipping symmetry axes.A relatively low velocity zone may underlie or intermingle with the anisotropic structures in the northern portion of the region.Synthetic tests also indicate that variations in the elastic constants and depth extent of the anisotropy assumed by the calculations do not affect the general conclusions,although trade-offs exist among certain model parameters.The modeling results suggest that the complex seismic structures in central Tibet were associated with underthrusting of the Indian lithosphere beneath the Asian lithosphere;the inferred flat symmetry axis of the anisotropy was likely generated during this collision process.If this were not the case,the inherited anisotropy would exhibit a steeply dipping axis of symmetry,parallel to the direction of underthrusting.

Calculation of Strain Amplification Matrix for Strain Invariant Failure Theory Based on Representative Volume Element Models with Periodical Boundary Condition

Strain invariant failure theory (SIFT) is a micro-mechanics-based failure theory for multi-scale failure analysis of composite materials originally proposed by Gosse and Christensen. In this paper, the approach for obtaining strain amplification matrix which is a key step for the execution of SIFT is improved by adopting representative volume element (RVE) finite element models considering periodical boundary condition, based on which more actual deformation mode is reflected. The deformation modes and strain data at the characteristic points of the centroid cell of multi-cell RVE model are analyzed and taken as a reference. It can be concluded that more reasonable deformation mode and relationship between the micro-mechanical and macro-mechanical strain states are obtained by employing the new model. Finally, numerical examples are provided to illustrate the determination of strain amplification factors within the RVEs considering periodical boundary condition at the characteristic points. © 2017, Shanghai Jiaotong University and Springer-Verlag GmbH Germany.

An acoustic-wave theory for casing bond evaluation using viscoelastic slip boundary modeling

Slip boundary condition is commonly utilized to model elastic wave propagation through layered earth media. The same approach is used here to characterize acoustic wave propagation along a cased borehole with various cement bond conditions. By modeling the cement layer between casing and formation as a viscoelastic slip interface with complex coupling rigidity parameters, one can not only reduce the complexity in the classical elastic wave modeling of the problem, but also efficiently model various complicated wave phenomena that are difficult for the existing modeling. More specifically, the new theory can well describe the effect of the cement bond condition change and the location of the change(i.e., whether it is in the first interface between casing and cement, or the second interface between cement and formation) on the acoustic waves,demonstrating the good modeling capability and predicting power. Application of the theory to field data shows that the theory can correctly model the acoustic wave characteristics and interpret the cement bond condition, thus providing a useful fundament theory for casing bond evaluation using acoustic logging.

Modeling and behaviours of rockfill materials in three-dimensional stress space

A bounding surface model incorporating a unified nonlinear strength criterion is proposed.The proposed bounding surface model contains 9 model parameters,which can be determined from the conventional triaxial tests.The bounding surface model can reproduce such behaviours as the strain hardening,the post-peak strain softening,and the volumetric strain contraction and expansion.Based on the comparisons between the predictions and the test results,the proposed strength criterion and model can well reproduce the experimental results of the strength and stress-strain behaviours of rockfill material in three-dimensional stress space.The strength behaviour of rockfill material is summarized as:(a) the failure stress ratio decreases with the initial confining pressure on the meridian plane;(b) the failure deviatoric stress decreases with the Lode angle from 0o to 60o on the deviatoric plane.The stress ratio decreases with increasing one of such factors as the initial void ratio,the intermediate principal stress ratio and the minor principal stress at the same strain when the other factors are given.

Theoretical modeling of chemical nonequilibrium stagnation point boundary layer heat transfer under rarefied conditions

This paper theoretically studies the recombination-dominated nonequilibrium reacting flow inside the stagnation point bound- ary layer （SPBL） and the heat transfer characteristics under rarefied conditions. A general model is intuitively proposed to de- scribe the energy transfer and conversion along the stagnation streamline towards a slightly blunted nose with non-catalytic wall surface. It is found that the atoms recombination effects inside the SPBL could be equivalent to a modification on the de- gree of dissociation in the external flow. As a result, a recombination nonequilibrium criterion Dar, that is a specific DamktSh- let number, is introduced to characterize the nonequilibrium degree of the reacting flow in the SPBL, and then, based on the general model and Dar, a bridging function indicating the nonequilibrium chemical effects on the SPBL heat transfer is estab- lished. By using the explicitly analytical bridging function, the flow and heat transfer mechanisms, including the real gas flow similarity law and the nonequilibrium flow regimes classification, are discussed. In addition, the direct simulation Monte Carlo （DSMC） method has also been employed to systematically validate the analytical results.

Mathematical Modelling and Experimental Investigation of Gas Flow in Minichannels and Microchannels

The first part of this study is focused on the numerical modelling and experimental investigation of transonic flow through a 2D model of the male rotor-housing gap in a dry screw compressor.Numerical simulations of the clearance flow are performed with the help of the in-house compressible Navier-Stokes solver.Experimental measurements based on the Schlieren method in Toepler configuration are carried out.The objective of the second part of the study is to derive the analytical solution of gas microflow development in a gap between two parallel plates.The microflow is assumed to be laminar,incompressible and the velocity slip boundary conditions are considered at the walls.The constant velocity profile is prescribed at the inlet.For the mathematical description of the problem,the Oseen equation is used.The analytical results are compared with the numerical ones obtained using the developed incompressible Navier-Stokes solver including the slip flow boundary conditions.