Simple Efficient Smart Finite Elements for the Analysis of Smart Composite Beams

This paper is concerned with the development of new simple 4-noded locking-alleviated smart finite elements for modeling the smart composite beams. The exact solutions for the static responses of the overall smart composite beams are also derived for authenticating the new smart finite elements. The overall smart composite beam is composed of a laminated substrate conventional composite beam, and a piezoelectric layer attached at the top surface of the substrate beam. The piezoelectric layer acts as the actuator layer of the smart beam. Alternate finite element models of the beams, based on an “equivalent single layer high order shear deformation theory”, and a “layer-wise high order shear deformation theory”, are also derived for the purpose of investigating the required number of elements across the thickness of the overall smart composite beams. Several cross-ply substrate beams are considered for presenting the results. The responses computed by the present new “smart finite element model” excellently match with those obtained by the exact solutions. The new smart finite elements developed here reveal that the development of finite element models of smart composite beams does not require the use of conventional first order or high order or layer-wise shear deformation theories of beams. Instead, the use of the presently developed locking-free 4-node elements based on conventional linear piezo-elasticity is sufficient.     

A new approach to the definition of power components in three-phasesystems under nonsinusoidal conditions

A method has been proposed for the definition of active and nonactive power components in three-phase systems under nonsinusoidal conditions. The method is more attractive than others since it is not a mere extension of methods employed in single-phase systems, but comes from the application of a quite powerful and synthetic mathematical tool specifically studied for the representation of three-wire three-phase systems in any possible condition: the Park transformation and the Park vectors. It is proven that the application of this method leads to the definition of two quantities, the real and the imaginary power, that are measurable in a quite simpler way than those proposed by other theories. The two satisfy all properties typical of the electrical power and are directly related, under sinusoidal and balanced conditions, to the active and reactive powers. It is shown how this method fits with other proposed methods that can be regarded in terms of this more general theory     

On the meaning of nonsinusoidal active currents

L.S. Czarnecki's (1985, 1987, 1988) orthogonal decomposition of nonsinusoidal currents is briefly discussed. The minimum property of Czarnecki's active currents in single-phase and three-phase nonsinusoidal circuits is discussed, using a Lagrangian multiplier approach. Czarnecki divided current, i, into three components: the active current i_a, the scattered current i _s, and the reactive current i_r. For three-phase four-line systems, several meanings of Czarnecki's active current i_a are given     

行为理论——设计活动的通用方法

 对工程设计的实证研究指出了提高解决方案质量的步骤特征:机会策略;用脑思考和用手思考(画草图)相结合;对自己的成果进行提问式反思。这些步骤特征符合工作活动的思维调控理论的预测(“行为理论”)。     

Time domain analysis by the point-matched finite element method

The solution of Maxwell's equations using the point-matched finite-element time-domain approach is presented. The solution scheme is explicit and does not require the solution of any matrix equations. The solution is carried out over a finite domain terminated using appropriate absorbing boundary conditions. Far-field quantities are obtained from the near-field solution by first computing the frequency-domain solutions from the time-domain solution using the fast Fourier transform and then using a near-field to far-field transformation algorithm.<>     

Performance evaluation of a multi-product system under CONWIP control

Analytical models of multi-product manufacturing systems operating under CONWIP control are composed of closed queuing networks with synchronization stations. Under general assumptions, these queuing networks are hard to analyze exactly and therefore approximation methods must be used for performance evaluation. This research proposes a new approach based on parametric decomposition. Two-moment approximations are used to estimate the performance measures at individual stations. Subsequently, the traffic process parameters at the different stations are linked using stochastic transformation equations. The resulting set of non-linear equations is solved using an iterative algorithm to obtain estimates of key performance measures such as throughput, and mean queue lengths. Numerical studies indicate that the proposed method is computationally efficient and yields fairly accurate results when compared to simulation.     

Time-domain calculation of spectral centroid from backscattered ultrasound signals

Spectral centroid estimation from backscattered ultrasound RF signals is the preliminary step for quantitative ultrasound analysis in many medical applications. The traditional approach of estimating the spectral centroid in the frequency domain takes a long time because discrete Fourier transform (DFT) processing for each RF segment is required. To avoid this, we propose time-domain methods to estimate the spectral centroid in this paper. First, we derive the continuous-time-domain equations for the spectral centroid estimation using Parseval's theorem and Hilbert transform theory. Then, we extend the method to the discrete-time domain to ease the implementation while maintaining the same accuracy as the calculation in the frequency domain. From the result, we observe that it is not practical to apply the discrete-time equations directly, because a high sampling rate is needed to approximate the time derivative in the discrete-time domain. Therefore, we also derive the feasible version of the discrete-time equations using a circular autocorrelation function, which has no constraints on the sampling rate for real RF signals acquired from pulse-echo ultrasound systems. Simulation results using numerical phantoms show that the time-domain calculation is approximately 4.4 times faster on average than the frequency-domain method when the software's built-in functions were used. The average estimation error compared with that of the frequency-domain method using DFT is less than 0.2% for the entire propagation depths. The proposed time-domain approach to estimate the spectral centroid can be easily implemented in real-time ultrasound systems.     

Domain decomposition of stochastic PDEs: Theoretical formulations

We present a novel theoretical framework for the domain decomposition of uncertain systems defined by stochastic partial differential equations. The methodology involves a domain decomposition method in the geometric space and a functional decomposition in the probabilistic space. The probabilistic decomposition is based on a version of stochastic finite elements based on orthogonal decompositions and projections of stochastic processes. The spatial decomposition is achieved through a Schur‐complement‐based domain decomposition. The methodology aims to exploit the full potential of high‐performance computing platforms by reducing discretization errors with high‐resolution numerical model in conjunction to giving due regards to uncertainty in the system. The mathematical formulation is numerically validated with an example of waves in random media. Copyright © 2008 John Wiley & Sons, Ltd.     

复阻尼结构动力方程的高斯精细时程积分法

复阻尼振动系统的瞬态响应通常只能在频域内求解。该文根据复阻尼理论应遵循对偶复化的原则,结合精细积分指数矩阵和高斯积分的运算方法,通过理论推导,给出了复阻尼结构动力方程的高斯精细时程积分法。分别采用Newmark-法和高斯精细时程积分法计算了复阻尼多自由度系统地震时程响应,并与频域法（视为精确解）的计算结果进行比较。结果...     

考虑桩土耦合作用时桩基扭转振动特性研究

假设桩周土层为均质线性粘弹性体,土材料阻尼为粘性阻尼,桩为竖直弹性均匀圆形截面桩,从三维轴对称角度,对考虑桩土耦合作用时桩基扭转振动桩顶频域振动特性进行了理论研究。利用拉普拉斯变换,首先通过对土层进行求解得到其振动扭转角位移形式解,然后利用该解并以小应变条件下桩土接触界面位移连续来考虑桩土的耦合作用,来分析基桩的动力反应,研究得到了考虑桩土耦合作用时桩顶扭转频域响应函数的解析解。基于所得解对桩土系统的扭转振动特性进行了对比和分析,并重点探讨了桩侧土粘性阻尼对桩顶响应的特殊影响,校验了基于平面应变假定桩基扭转振动理论解的精确性。     

Iterative domain decomposition meshless method modeling of incompressible viscous flows and conjugate heat transfer

We develop an effective domain decomposition meshless methodology for conjugate heat transfer problems modeled by convecting fully viscous incompressible fluid interacting with conducting solids. The meshless formulation for fluid flow modeling is based on a radial basis function interpolation using Hardy inverse Multiquadrics and a time-progression decoupling of the equations using a Helmholtz potential. The domain decomposition approach effectively reduces the conditioning numbers of the resulting algebraic systems, arising from convective and conduction modeling, while increasing efficiency of the solution process and decreasing memory requirements. Moreover, the domain decomposition approach is ideally suited for parallel computation. Numerical examples are presented to validate the approach by comparing the meshless solutions to finite volume method (FVM) solutions provided by a commercial CFD solver.     

Pulse propagation along multi wire electric fences

Mathematical modelling of electric fence structures for high-voltage (HV) pulse propagation is complex in nature and, as such, electric fence energisers are currently manufactured using empirical rules together with a trial-and-error design approach. With no insight into the behaviour of HV pulse propagation along the fence line, the trend has merely been to develop electric fence energisers that could generate pulses with the highest voltage and the longest pulse duration allowed by the safety standards. Consequently, most current electric fence energisers are over rated and not optimally designed to produce an effective electric shock. An accurate insight into HV pulse propagation is an essential requirement for electric fence design and a semi-analytical technique is proposed to predict the propagation characteristics of HV transient pulses along multi-wire electric fences. The technique models the multi-line fence in frequency-domain to facilitate an analytical solution for the propagation of HV pulses along the fence. A modal transformation is used to decouple the frequency-domain solution, which is then transformed into time domain through a numerical Laplace inversion algorithm to determine the propagation characteristics of the fence at a given location and time. For various line conditions, the propagation of HV pulses is investigated, and the results are presented with comparisons to simulations by power systems computer-aided design to show the validity of theoretical analysis. The technique provides an insight into the propagation characteristics of HV pulses along multi-wire fence lines and thus serves as a tool at the design phase of electric fence energisers.     

A survey of quantitative and qualitative methods of sensitivity analysis for stochastic dynamic systems

In this paper a brief literature survey concerning sensitivity analysis of stochastic dynamic systems described by Ito equations is presented. In the first part of the paper we review the quantitative methods in the time and frequency domain. We quote the definitions of output and moment sensitivity measures and we present applications for linear systems and nonlinear oscillators with stochastic coefficients under stochastic excitations. The cases of white and coloured noise are considered. This is followed by a discussion of the application of the output sensitivity process to response approximation of nonlinear oscillators. We discuss also the application of sensitivity methods to the approximation of characteristics of the solution of stochastic differential equations. We present another approach to the study of response sensitivity of stochastic systems in the frequency-domain with applications to secondary structural systems. In the second part of the paper we review the qualitative methods. The concept of exponential and practical insensitivity for linear and a class of nonlinear stochastic dynamic systems is presented. We quote the definitions and criteria of the exponential and practical insensitivity which are very close to the definitions of stability concerning selected response variables.     

Considerations on the Reactive Power in Nonsinusoidal Situations

This paper discusses the strong and weak points of the time-domain and frequency-domain approaches to the energy transmission investigations in nonsinusoidal systems. The comprehension of possibilities relevant to these approaches is fundamental for the metrological tasks in nonsinusoidal systems formulation. It was shown, that despite a more complex instrumentation, the frequency-domain approach provides a theoretical basis not only for a conspicuous physical elucidation of the energy transmission decreased efficiency, but also for power-factor improvement in situations where the time-domain approach proves ineffective.     

Extended Bennia-Riad criterion for iterative frequency-domaindeconvolution

In this paper, we present a new, fast, robust and accurate iterative frequency-domain deconvolution technique. The deconvolution problem is mathematically classified as an ill-posed problem. We propose a new generalized deconvolution filter, and we use the so-called extended “Bennia-Riad” criterion to determine the optimal filter parameters     

不同约束体系曲线梁桥震害调查及损伤模式分析

为明确地震作用下曲线梁桥震害成因、结构传力机制及薄弱部位,对采用不同约束体系的曲线梁桥进行震害调查及损伤模式分析。归纳总结了汶川地震中采用不同约束体系的曲线梁桥震害;采用非线性时程方法,对比分析了不同水平地震动作用下不同约束体系曲线梁桥构件损伤状态及损伤顺序,探讨了不同约束体曲线梁桥传力机制及耗能机制。研究结果表明:曲线梁桥典型震害主要表现为主梁刚体移位,并伴有转动,主梁移位易导致其与桥台、挡块等发生碰撞,抗震分析中应考虑主梁双向碰撞效应的影响;约束体系为桥梁系统关键薄弱部位,在地震作用下易发生损伤,可将其设计为"保险丝式"单元;不同约束体系曲线梁桥的震害表现及传力机制差异明显,主要取决于约束体系的力学特性,对于采用橡胶支座的桥梁应设置合理限位装置,设置固定墩的桥梁应加强桥墩塑性铰区设计。     

基于结构动力学方法的气动弹性分析

摘 要：对于气动与弹性的耦合模型,给出气动力的频域公式,并表示成有理多项式,通过等价的变换,能够推导出与结构动力学方程完全相似的、关于求解变量的二阶常微分方程组。于是,可以在计算结构动力学框架下实现气动弹性问题的分析和计算。此方法将通用计算结构动力学程序的功能与试验或数值分析得到的气动力模型相结合,简化了气动弹性的耦合分析、提高计算效率。     

一种用于跨域轴承故障诊断的深度自适应网络EI北大核心CSCD

针对轴承在不同工况条件下的振动数据存在分布差异,导致诊断精度低的问题,提出一种新的深度自适应网络用于跨域条件下的轴承故障诊断。采用傅里叶变换将原始时域振动信号转换为频域信号并通过深度特征提取器提取其分类特征;利用最大均值差异(maximize mean discrepancy,MMD)来进行深度特征的边缘分布对齐;利用Wasserstein度量网络将源域中有标签数据的类别结构与目标域中无标签数据的类别结构进行匹配,即对齐不同域的类别条件分布,使得故障数据在不同域的分布能够更好的对齐,从而提高模型在目标域未标签数据集上的分类准确率。试验利用凯斯西储大学公开的故障轴承数据集进行了两种跨域条件的模型迁移,验证了该网络在不同迁移场景中都具有较高的准确率,且优于其他深度自适应网络。     

基于信号时频分解的模态参数识别

给出了基于响应信号Gabor展开与重构的模态参数辨识的时频分析方法。通过响应信号的展开与重构，单频特征振动可从复杂的响应信号中分离出来，由它些特征振动信号可进一步提取系统物模态振动参数。论述了频率、阻尼和特征振型的估计方法以及估计方法对系统响应信号的特殊要求。此方法可适用于平衡、非平衡的响应信号，且无需输入信号，属于环境激励下的一种参数辨识方法。仿真结果说明明频展开与重构方法是模态参数辩识的有效手段之一。     

Balancing reduction of linear systems having multiple delays

Abstract

In this paper we apply the balancing reduction method to derive reduced‐order models for linear systems having multiple delays. The time‐domain balanced realization is achieved through computing the controllability and observability gramians in the frequency domain. With the variable transformation s = i tan(θ/2), the gramians of linear multi‐delay systems can be accurately evaluated by solving first‐order differential equations over a finite domain. The proposed approach is computationally superior to that of using the two‐dimensional realization of delay differential systems.