安装间隔棒的双分裂导线微风振动分析

为了分析安装了间隔棒的双分裂导线微风振动强度,研究了利用有限差分法(FDM)分析微风振动的关键技术。基于能量等效原理,将实测的输电线自阻尼功率等效成为粘滞阻尼系数,将输入风功率等效成为微风激励力。推导了分裂导线-间隔棒体系动力微分方程,利用四阶有限差分格式求解动力方程得到导线的最大振幅;根据振幅和曲率的关系推导了输电线动弯应变幅值沿输电线的分布规律。通过某220kV双分裂导线算例分析:未安装间隔棒时,有限差分结果与能量平衡法分析结果吻合较好;安装了间隔棒的情况下,有限差分法能够反应间隔棒刚度、阻尼对分裂导线微风振动的影响,并且可以考虑由于子导线振动差异导致的相互拉扯效应。     

Time domain modeling of a thin wire in a two-media configuration featuring a simplified reflection/transmission coefficient approach

The paper deals with a transient analysis of a straight thin wire in a presence of a two-media configuration using a simplified reflection coefficient approach. A direct time domain formulation is based on thin wire antenna theory and on the corresponding space–time integral equation for the wire above a dissipative half-space, or buried in a real ground. The effect of a half-space is taken into account via the simplified reflection/transmission coefficient arising from the modified image theory. The resulting space–time integral equation for the wire above and below ground, respectively, is handled via the time domain Galerkin–Bubnov variant of the indirect boundary element method (GB-IBEM) and some illustrative numerical results are presented.The transient response computed via the simplified reflection/transmission coefficient approach is compared, where possible, to the results obtained via the Fresnel coefficients approach.     

Transient dynamic elastoplastic analysis by the time-domain BEM formulation

This paper presents a time-domain BEM formulation applied to the solution of transient dynamic elastoplastic problems. The initial stress approach is adopted to solve the elastoplastic problem. Linear time variation is assumed for the displacements and initial stress components whereas traction components are assumed to have a constant time variation. Boundary discretization employs linear elements and the part of the domain where plastic deformation is expected to occur is discretized by employing linear triangular cells. Time integrals are computed analytically, boundary integrals are computed numerically and the domain integrals are computed by following a semi-analytical procedure. A numerical example is presented and the results are compared with another BEM formulation.     

Physical phantom of typical Korean male for radiation protection purpose

Dose distribution within a human body can be measured using physical anthropomorphic phantoms. In an effort to establish reference Korean physical model, the first Korean physical phantom of average Korean adult male was constructed using computed tomography (CT) images of a healthy volunteer. The body dimension of the subject was close to that of average Korean male. The source images were obtained using fusion positron emission tomography machine at Radiation Health Research Institute in Korea, and ported into rapid prototyping process. The physical phantom was composed of three tissue-equivalent materials: epoxy resin, urethane foam and polyurethane representing bone, lungs and soft tissues, respectively. The densities of the tissue-equivalent materials were close to those recommended by the International Commission on Radiation Units and measurements. To facilitate dose mapping, the phantom was sliced into 2 cm sections. Hole grids for thermoluminescence (TL) dosemeter chips were drilled. To verify the appropriateness of the physical phantom, organ doses of selected organs were measured for reference photon beam, and compared with those computed by tomographic model constructed from the same CT images. Absorbed doses converted from TL relative response showed good agreement within 7% with those calculated.     

Time domain boundary element method for dynamic stress intensity factor computations

This paper presents a procedure for transient dynamic stress intensity factor computations using traction singular quarter-point boundary elements in combination with the direct time domain formulation of the Boundary Element Method. The stress intensity factors are computed directly from the traction nodal values at the crack tip. Several examples of finite cracks in finite domains under mode-I and mixed mode dynamic loading conditions are presented. The computed stress intensity factors are represented versus time and compared with those obtained by other authors using different methods. The agreement is very good. The results are reliable and little mesh dependent. These facts allow for the analysis of dynamic crack problems with simple boundary discretizations. The versatile procedure presented can be easily applied to problems with complex geometry which include one or several cracks.     

Potential discontinuity simulations with the numerical Green’s function in steady and transient problems

This paper applies the numerical Green’s function (NGF) boundary element formulation (BEM) first in standard form to solve the Laplace equation and then, coupled to the operational quadrature method (OQM), to solve time domain problems (TD-BEM). Both involve the analysis of potential discontinuities in the respective scalar model simulation. The implementation of the associated Green’s function acting as the fundamental solution is advantageous since element discretization of actual discontinuity surfaces are no longer required. In the OQM the convolution integral is substituted by a quadrature formula, whose weights are computed using the fundamental solution in the Laplace domain, producing the direct solution to the problem in the time domain. Applications of the NGF to problems involving the Laplace equation and its transient counterpart are presented for two-dimensional potential flow examples, confirming that the formulation is stable and accurate.     

A meshless model for transient heat conduction in functionally graded materials

A meshless numerical model is developed for analyzing transient heat conduction in non-homogeneous functionally graded materials (FGM), which has a continuously functionally graded thermal conductivity parameter. First, the analog equation method is used to transform the original non-homogeneous problem into an equivalent homogeneous one at any given time so that a simpler fundamental solution can be employed to take the place of the one related to the original problem. Next, the approximate particular and homogeneous solutions are constructed using radial basis functions and virtual boundary collocation method, respectively. Finally, by enforcing satisfaction of the governing equation and boundary conditions at collocation points of the original problem, in which the time domain is discretized using the finite difference method, a linear algebraic system is obtained from which the unknown fictitious sources and interpolation coefficients can be determined. Further, the temperature at any point can be easily computed using the results of fictitious sources and interpolation coefficients. The accuracy of the proposed method is assessed through two numerical examples.     

单根和双分裂导线风噪声的数值模拟研究

针对单根和双分裂导线风噪声采用混合计算方法进行数值模拟。基于导线的绞线结构进行建模,获得单导线风场和声场的分布特征,并与同直径的光滑圆柱结果进行对比,并针对水平和垂直布置双分裂导线风场和声场进行研究。研究表明:导线的阻力系数和总声压级均小于光滑圆柱,而斯特劳哈尔数大于光滑圆柱;在卓越频率段导线和光滑圆柱的风噪声呈现了明显的8字形指向性,而在远离卓越频率段呈现圆形指向,说明导线和圆柱风噪声是一种偶极子声源;导线和光滑圆柱风噪声的总声压级总体呈圆形、略呈8字形分布;垂直布置双分裂圆柱因相互之间无干扰,总声压级为两根单导线风噪声声压级之和;垂直布置的双分裂导线风噪声略小于水平布置双分裂导线风噪声。     

Dielectric spectroscopy — an analytical tool for monitoring the deterioration due to high energy radiation in ethylene propylene diene terpolymer (EPDM) vulcanizates

This paper demonstrates the use of a dielectric spectroscopy technique for monitoring the high energy radiation ageing and stabilization of ethylene propylene diene terpolymer vulcanizates. The measurement of dielectric constants and dielectric losses in the frequency domain help to quantify the physico-chemical changes in the bulk due to high energy irradiation. It is found that the relaxation time calculated using an empirical approach is exponentially related to the total exposed dose. A quantitative comparison of different antirads with a control vulcanizate has been demonstrated by using parameters defined as relative efficiency indices (REI) and relative stability indices (RSI). Three types of REI parameters are defined and computed from dielectric measurements after irradiation over a period of time at a fixed frequency. These REI values are a good estimate of antirad efficiency for energy scavenging and transient charge scavenging. Two types of RSI parameters are defined and computed from the frequency scans of dielectric properties for a material exposed to various doses of γ irradiation. These RSI values provide a good estimate of the rate and extent of changes in the physico-chemical structure due to irradiation. A third type of RSI, computed from equilibrium swelling measurements, closely matches that obtained from dielectric data. It is concluded that the deterioration is predominantly created by a cross-linking reaction even after the addition of antirads. It is observed that polymeric antirads provide the highest efficiency stability. This revised version was published online in November 2006 with corrections to the Cover Date.     

Three-dimensional fundamental solutions for transient heat transfer by conduction in an unbounded medium,half-space,slab and layered media

This paper presents analytical Green's functions for the transient heat transfer phenomena by conduction, for an unbounded medium, half-space, slab and layered formation when subjected to a point heat source. The transient heat responses generated by a spherical heat source are computed as Bessel integrals, following the transformations proposed by Sommerfeld [Sommerfeld A. Mechanics of deformable bodies. New York: Academic Press; 1950; Ewing WM, Jardetzky WS, Press F. Elastic waves in layered media. New York: McGraw-Hill; 1957]. The integrals can be modelled as discrete summations, assuming a set of sources equally spaced along the vertical direction. The expressions presented here allow the heat field inside a layered formation to be computed without fully discretizing the interior domain or boundary interfaces.The final Green's functions describe the conduction phenomenon throughout the domain, for a half-space and a slab. They can be expressed as the sum of the heat source and the surface terms. The surface terms need to satisfy the boundary conditions at the surfaces, which can be of two types: null normal fluxes or null temperatures. The Green's functions for a layered formation are obtained by adding the heat source terms and a set of surface terms, generated within each solid layer and at each interface. These surface terms are defined so as to guarantee the required boundary conditions, which are: continuity of temperatures and normal heat fluxes between layers.This formulation is verified by comparing the frequency responses obtained from the proposed approach with those where a double-space Fourier transformation along the horizontal directions [Tadeu A, António J, Simões N. 2.5D Green's functions in the frequency domain for heat conduction problems in unbounded, half-space, slab and layered media. CMES: Computer Model Eng Sci 2004;6(1):43–58] is used. In addition, time domain solutions were compared with the analytical solutions that are known for the case of an unbounded medium, a half-space and a slab.     

DC performance, AC loss and transient field stability of five medium size NbTi cable-in-conduit conductors with parametric variations

Five medium size NbTi cable-in-conduit conductors with parametric variations in the cable layout were fabricated of three different strands to investigate their DC performance, AC loss behaviour and transient stability by means of the SULTAN facility. A comparison of measured strand data and the cable performances at the same background field indicates that the current carrying capacity of the conductors is limited by the total magnetic field on the high field side. Due to the self-field contribution the total magnetic field on the high field side is considerably larger than the background field at high currents. The take-off electric fields were found to decrease with increasing quench currents, resulting in sudden take-offs for large overall current densities. Effective self-field factors are used to describe the DC performance in the current range, where the critical current and the current sharing temperature are accessible. The quench behaviour of the conductors is discussed in the framework of a model based on peak-field-induced quenches. Differences in the DC performance of the conductors are related to the variation in the transverse resistance determining the ability of the conductors to redistribute the current in the case of a current imbalance originated at the joints. This aspect was studied in more detail by modifying one of the joints allowing a defined number of subcables to be disconnected. The voltage-current characteristics of strands and cable-in-conduit conductors are compared. The reasons for cable n values deviating from the strand n values are discussed. Moreover, the effect of different transverse resistances on the AC loss behaviour and the transient field stability was investigated. Ni coating of the strands was found to be sufficient to eliminate the interstrand coupling current loss. Thus, additional subcable wraps provided no significant further loss reduction.     

覆冰输电线结构及载荷对舞动的影响

为深入研究导线结构形式对输电线路系统动力学特性影响,以及导线结构形式及气动扭转载荷对覆冰输电线路舞动规律及幅值的影响,采用ANSYS参数化设计语言编写了计算导线舞动的非线性有限元分析程序,计算得出了系统气动载荷作用下的时间历程曲线。发现：在相同的气象条件下,分裂导线的振幅明显大于单根导线,且分裂数越高,导线越容易在短时间内形成稳态的舞动,应尽量采用单根导线或减少导线分裂数;扭转气动力载荷对面内外舞动的规律及幅值影响不大,对扭转角的幅值有较大影响,因此可忽略扭转气动载荷对断线和倒塔的作用。     

Computation of 3D MEMS electrostatics using a nearly exact BEM solver

The electrostatic properties of thin plate shaped structures relevant to the micro-electro-mechanical systems (MEMS) have been computed using a nearly exact boundary element method (BEM) solver. The solver uses closed form expressions for three-dimensional potential and force fields due to uniform sources/sinks distributed on finite flat surfaces. The expressions have been validated and, being analytical, have been found to be applicable throughout the physical domain. The solver has been applied to compute accurately and efficiently the charge densities on thin plate shaped conductors as used in MEMS components. We have presented results for the model problem of parallel plate capacitors and compared them with results obtained from several other BEM based solvers.     

Probe-beam diffraction in a pulsed top-hat beam thermal lens with a mode-mismatched configuration

The Fresnel diffraction integral is used directly to describe the thermal lens (TL) effect with a mode-mismatched collinear configuration. The TL amplitudes obtained with Gaussian, Airy, and top-hat beam excitations are computed and compared. Numerical results for beam geometries optimized for both near- and far-field detection schemes are presented, and the analytical results developed by Bialkowski and Chartier [Appl. Opt. 36, 6711 (1997)] for a Gaussian beam TL effect are summarized in simplified form. Both the numerical and the analytical results demonstrate that, under a beam geometry optimized for either near- or far-field detection, the Gaussian beam TL experiment has approximately the same maximum signal amplitude as does the photothermal-interference scheme. A comparison between the optimum near- and far-field detection beam geometries indicates that a practical mode-mismatched TL instrument should be based on the far-field detection geometry. The computation results further demonstrate that the optimum beam geometry and the TL amplitude depend largely on the excitation-beam profile. The top-hat beam TL experiment is approximately twice as sensitive as the Gaussian beam TL scheme.     

四分裂导线上风荷载变化规律研究

该文采用数值方法,对四分裂导线上风荷载进行了计算,分析了导线相对位置、风速、风向角和直径等因素对导向上的升力系数和拖曳阻力系数的影响。结果表明,当导线上下运动时,导线上的升力和阻力系数呈对称分布,距离越远,阻力系数越大,升力系数越小;导线水平距离变化时,水平距离增大,升力系数减小;当导线发生转动,转角为30°时,升力系数和阻力系数存在一个最小值;随着直径和风速的增大,导线的升力和阻力系数均增大。最后将计算结果与实验结果对比,验证了该文计算结果的可靠性。该研究结果可以为工程应用提供理论参考。     

Characterization of Probe Dynamic Behaviors in Critical Dimension Atomic Force Microscopy

This paper describes a detailed computational model of the interaction between an atomic force microscope probe tip and a sample surface. The model provides analyses of dynamic behaviors of the tip to estimate the probe deflections due to surface intermittent contact and the resulting dimensional biases and uncertainties. Probe tip and cantilever beam responses to intermittent contact between the probe tip and sample surface are computed using the finite element method. Intermittent contacts with a wall and a horizontal surface are computed and modeled, respectively. Using a 75 nm Critical Dimension (CD) tip as an example, the responses of the probe to interaction forces between the sample surface and the probe tip are shown in both time and frequency domains. In particular, interaction forces between the tip and both a vertical wall and a horizontal surface of a silicon sample are modeled using Lennard-Jones theory. The Snap-in and Snap-out of the probe tip in surface scanning are calculated and shown in the time domain. Based on the given tip-sample interaction force model, the calculation includes the compliance of the probe and dynamic forces generated by an excitation. Cantilever and probe tip deflections versus interaction forces in the time domain can be derived for both vertical contact with a plateau and horizontal contact with a side wall. Dynamic analysis using the finite element method and Lennard-Jones model provide a unique means to analyze the interaction of the probe and sample, including calculation of the deflection and the gap between the probe tip and the measured sample surface.     

覆冰四分裂导线舞动近似解析解分析

建立了带有刚性间隔棒的覆冰四分裂导线垂直、水平和扭转三自由度的耦合模型,并对由强非线性系统转化为弱非线性系统的方法进行了研究,运用平均法分析了非内共振、3∶1内共振和1∶1内共振时系统的振动情况,得到了风速、横向阻尼比、来流密度、线路档距等相关参数对导线舞动的影响及在不同参数影响情况下导线舞动的响应曲线,其近似解析解与数值解基本吻合。     

近场竖向地震激励下桥梁支座对竖向碰撞的影响

考虑桥梁支座的作用, 针对两跨连续梁桥的竖向碰撞问题, 建立了梁-弹簧-杆连续体模型. 采用瞬态波特征函数展开法和组合体瞬态内力法, 推导了多次竖向碰撞问题的理论解, 分析了桥梁支座对竖向碰撞的影响. 研究结果表明：桥梁支座直接影响桥面与桥墩的局部碰撞接触刚度, 显著影响竖向碰撞响应, 包括碰撞次数、竖向碰撞力和碰撞位移响应等. 桥梁支座对竖向碰撞具有显著的吸能减震效果, 且影响竖向碰撞响应的基本特征, 如次碰撞出现的频次和响应的频率. 当近场竖向地震激励周期逼近桥梁的固有振动周期时, 桥面上抛跳起, 与桥梁支座产生剧烈竖向碰撞. 由于调节桥梁支座的刚度可以改变桥梁的固有振动周期, 因此, 合理设计桥梁支座对于降低竖向碰撞的破坏作用, 提高桥梁结构抵抗竖向地震的能力, 具有重要的价值.     

Augmented Hooke's law based on alternative stress relaxation models

 Relationships are presented which are considered to be the most general, linear, constitutive stress–strain relationships for a simple, solid material at isothermal conditions. Formally, they are obtained in frequency domain by adding “anelastic”, frequency dependent terms to the elastic constants of the material. The resulting, basic, augmented Hooke's law (AHL) is proposed as a general framework for comparison of alternative linear material damping models. Contained, as special cases, are both the classical, purely mechanical theory of viscoelasticity and more recent damping models based on linear, irreversible thermodynamics. The original AHL Helmholtz free energy density function, Dovstam (1995), is generalised to materials with continuously distributed relaxation frequency spectra. It is shown that there corresponds a continuously distributed relaxation spectrum to each admissible linear damping model and how such relaxation spectra may be computed using the Stieltjes–Perron inversion formula and explicit (analytical) models of the complex, frequency dependent, parts of a corresponding AHL. Traditional relaxation time spectra (discrete or continuously distributed relaxation times) are shown to be directly related to AHL relaxation amplitude distributions (relaxation frequency spectra) derived in the paper. The relationships between traditional relaxation time spectra and frequency domain AHL relaxation amplitude distributions connect experimental time domain data in linear viscoelasticity, with corresponding AHL relaxation frequency spectra which may be used in linear, constitutive material damping modelling. The results indicate that the information supplied by relaxation spectra (in time or frequency domain) is completely equivalent to any suitable and physically realistic damping model, properly curve fitted to experimental complex material moduli. Fractional derivative models are demonstrated to simulate the mean properties of the relaxation processes in the material during vibration. In this context, fractional derivative models are completely equivalent to frequency domain, continuously distributed AHL relaxation models, with well defined and easily computed relaxation frequency spectra. Using experimentally estimated data, it is explicitly demonstrated that linear, material damping may be simulated using discrete as well as continuously distributed AHL relaxation models or corresponding fractional derivative models. Which damping model to use is a matter of convenience. Received 17 December 1999     

Accuracy and security analysis of transient flows in relatively long pipelines

Consideration is given, in this paper, to the accurate numerical solution of transient flows in relatively long pipelines caused by the water hammer phenomenon. The governing equations for such flows are a set of two coupled non-linear hyperbolic partial differential equations where the friction factor is a very important parameter. In these equations, the gravity force is introduced to take into account the pipeline inclination. The mathematical equations are solved in the time domain by the method of characteristics using linear integration. To reproduce correctly the transient flow, very small time increments were used in the constructed computer program. This program permits to get some systematic indications on the evolution and the damping of the pressure head waves due to a fast closing valve at the downstream end of a long pipeline. The study shows that results are different from those of short pipelines and small viscous fluids. To check the validity of the numerical model, computed results have been successfully compared with those found in the relevant literature. These results show that the gravity lift may have an important effect on the maximum pressures, which may become very important near the valve and cause failure of the pipeline especially in presence of defect. The safety factor, computed at equidistant sections of the line, determines the distance between the reservoir and the defect from which the failure may happen. For the considered application, results show that the pipeline is safe near the supply upstream tank.