A comparison of time marching schemes for the transient heat conduction equation

This paper investigates the phenomenon of ‘noise’ which is common in most time-dependent problems. The emphasis is on the achievable accuracy that is obtained with various time-stepping algorithms and how this can be improved if noise is artificially damped to an acceptable level. A series of experiments are made where the space domain is discretized using the finite element method and the variation with time is approximated by several finite difference methods. The conclusion is reached that the Crank–Nicolson scheme with a simple averaging process is superior to the other methods investigated.     

A cost comparison of two static condensation-stress recovery algorithms

We consider finite elements that have internal degrees-of-freedom. These d.o.f. are condensed before elements are assembled into the structure, and must be recovered later for use in element stress computation. References 1 and 2 describe two different ways to do the calculations. This paper compares the computational costs of the two methods. Test runs were made to assess costs. Relative costs depend on several factors. The factors are discussed, and suggestions are made as to the better method in various circumstances.     

天然气液化流程技术方案的选择与比较

通过对MRC循环制冷工艺和氮膨胀循环制冷这两种天然气液化流程的制冷原理的阐述,并对计算机优化模拟结果进行对比,分析天然气液化流程的设备组成及功耗差异,借鉴LNG工厂的实际经验,说明不同液化流程的优劣性。     

A comparison of two algorithms for solving closed crack problems

Interface (or joint) elements introduce errors in the computation of the Stress Intensity Factor, and uncertainties in the definition of the ‘slipping’ and ‘stuck’ regions in some fracture problems. The PATH algorithm is free of these disadvantages and displays the maximum error in the results; in a test case it required about 30% more machine time than the fast algorithm of Elvin and Leung.     

A comparison of T 2 control charts with variable sampling schemes as opposed to MEWMA chart

Variable sampling plans have been widely employed to increase the power of control charts, mainly for the univariate case. In this paper, a T 2 control chart with variable sample size and variable sampling frequency is developed. Near optimum schemes are obtained to detect faster shifts in the mean vector. A comparison with a MEWMA chart is made.     

A comparison of two methods of analysing staircase fatigue test data

When the fatigue strength of components or material is tested by the staircase or up‐and‐down method, the data can be analysed in either the fatigue strength domain, which is the most popular approach, or the life domain, based on life regression models. In this paper we exploit a simple relationship between families of models in the two domains to obtain an estimate of the distribution of fatigue strength directly from a fitted life regression model. We use simulation to compare the two approaches, concluding that for estimating important features of the fatigue strength distribution the method of maximum likelihood is more effective if it is applied in the life domain. In particular, it is more robust to the choice of x_h, the step height of the staircase, provided x_h is in the range σ_X–2σ_X, where σ_X is the standard deviation of the log‐strengths. Most of our results are insensitive to the value of the initial stress amplitude. Copyright © 2007 John Wiley & Sons, Ltd.     

空分设备压缩机两种自动加载方案的比较

目前空分设备压缩机加载方案主要有自动加载和手动加载两种,近年来自动加载由于具有方便性和平稳性而越来越受到用户的肯定。文章从控制原理和控制方案的具体步骤两方面,对采用入口导叶和放空阀同时调节以及采用入口导叶和回流阀分段动作两种自动加载方案进行了比较。     

Size dependent buckling analysis of nano sandwich beams by two schemes

Abstract

Within the framework of Timoshenko beam theory, the buckling of nano sandwich beams is developed. The material properties are assumed to vary arbitrarily in both axial and thickness directions. These types of beams are referred to as bi-directional functionally graded (BDFG) beams. Two types of nano sandwich beams with different material distribution patterns and immovable supports are considered. Since the size effects play a significant role in mechanical behavior of nanostructures, the small-scale effects are captured by Eringen’s nonlocal theory of elasticity. The governing equations are derived using the variational formulation. Symmetric smoothed particle hydrodynamics (SSPH) and the Galerkin method are adopted as numerical solution approaches. As a truly meshless method, the convergence of the SSPH technique mainly depends on the smoothing length value and distribution of particles in the compact support domain of the kernel function. The Revised Super Gauss Function is used as the kernel function and an optimum value for the smoothing length that bears the fastest convergence rate is obtained. The solution methods are verified through benchmark problems found in the literature. Numerical and illustrative results show that various parameters, including the aspect ratio, nonlocal parameter, gradient indexes, and cross-sectional types have significant effects on the buckling responses of BDFG nano sandwich beams.     

A comparison of the factor loading matrices of two driver behavior investigations

The article reports on two analyses of driver behavior conducted separately, one carried out in Vienna (Austria) and the other in Tübingen (Germany). The data on which the analysis was based were in both cases subjective judgements of 40 behavior characteristics of drivers. In each case two observers accompanying the subject in his vehicle noted the degree in which these characteristics were evident on a nine-point rating scale. Differences between the Vienna investigation and the Tübingen study lay in the team of observers, the test route, the type of vehicle and the size of the samples. The programs for measuring similarities between factor matrices have only recently been developed. The comparison deals primarily with interpretation of the contents of the resulting factor loading matrix and additionally with the interpretation of the measures of similarity for each variable. The experiments are described only as far as is desirable for an understanding of the two basic matrices. The aim of the project was to determine components of driver behavior in terms of reliable, meaningful measures which allow generalization.     

Inventive output of academic research: A comparison of two science systems

This paper compares the inventive output of two science systems in small European countries. More specifically, we examine patented inventions of Finnish and Flemish university researchers. The comparison includes inventive output as such and its concentration on organizations, inventors, and corporate owners as well as foreign assignations and the degree to which individual inventors have retained the ownership of the patents. While there are commonalities between the Finnish and Flemish systems in terms of patent concentration on key institutions and corporate assignees, there are also pronounced differences with respect to the ownership structure of academic patents, which was expected in light of the different intellectual property regulations. Our observations seem to suggest that the total inventive output of a research system is not a function of the prevailing intellectual property system but rather in correspondence to overall national inventiveness thereby pointing to more general (national, cultural) drivers of academic inventive activity. From a methodological viewpoint, this research illustrates that tracing university-owned patents alone would leave considerable technological contributions of academics unidentified - also in countries where universities own the rights to their researchers patents. Another finding with potential methodological implications is that patents are highly concentrated on institutions. If such a distribution law applies to large countries as well, analysts could cover most of the national academic patent output by an intelligent selection of universities.     

A comparison of two bibliometric methods for mapping of the research front

Summary This paper builds on previous research concerned with the classification and specialty mapping of research fields. Two methods are put to test in order to decide if significant differences as to mapping results of the research front of a science field occur when compared. The first method was based on document co-citation analysis where papers citing co-citation clusters were assumed to reflect the research front. The second method was bibliographic coupling where likewise citing papers were assumed to reflect the research front. The application of these methods resulted in two different types of aggregations of papers: (1) groups of papers citing clusters of co-cited works and (2) clusters of bibliographically coupled papers. The comparision of the two methods as to mapping results was pursued by matching word profiles of groups of papers citing a particular co-citation cluster with word profiles of clusters of bibliographically coupled papers. Findings suggested that the research front was portrayed in two considerably different ways by the methods applied. It was concluded that the results in this study would support a further comparative study of these methods on a more detailed and qualitative ground. The original data set encompassed 73,379 articles from the fifty most cited environmental science journals listed in Journal Citation Report, science edition downloaded from the Science Citation Index on CD-ROM.     

Statistical comparison of two fracture criteria

Two fracture criteria are compared within the stochastic model of brittle fracture of a component with the random ensemble of crack-like defects.

The component comprises a random array of internal defects, represented by equivalent cracks. Under assumption of stochastic independence of defects and Poisson distribution of defects population the probability of fracture is determined. This probability is dependent on local fracture criterion applied to an arbitrary crack. The maximum normal stress and energy release rate criteria were applied to check as to whether they may yield significantly different prediction of above probability.

Computer calculations show that with the increase of defects density the difference in predictions of above two criteria became unsignificant and for both limiting fracture-stress curves converges to maximum stress theory. For low densities energy release rate criterion yields more conservative predictions in almost all range of λ. The limiting fracture-stress curves deviate in this case from both the maximum stress and von Mises theories.     

两个宽带噪声矢量场模型的空间相关特性

针对宽带噪声矢量场的空间相关特性问题,文章讨论了各向同性噪声模型和表面噪声模型。在高频近似条件下,根据射线声学理论和简谐平面行波满足的欧拉公式,采用时域建模方法分别建立了宽带各向同性噪声和表面噪声矢量场模型。给出了噪声场相关函数的积分表达式和竖直方向空间相关系数的解析表达式。数值分析结果表明,相较于窄带噪声矢量场,宽带噪声矢量场各分量空间相关系数振荡幅度更小,随空间两点间距的增大更快地趋近于零;噪声源指向性对表面噪声场竖直方向上的空间相关特性的影响显著。相较于已有的宽带噪声空间相关系数计算方法,文中模型无需使用傅里叶合成方法,可通过时域建模直接获得宽带噪声空间相关函数。     

A comparison of two SGBEM solutions of an unsteady heat conduction interface problem

A comparison of two approaches for solving time dependence in an unsteady heat conduction problem with a material or other interface, based on a boundary element technique is presented. A quadrature algorithm is used in the first approach for calculating convolutional integrals, which leads directly to the time-domain solution of the problem; the other approach uses the Laplace transform and a numerical inversion of the Laplace domain solution. The new numerical techniques lead together with the Symmetric Galerkin Boundary Element Method applied for space variables and a weak formulation of interface conditions to an interface problem with generally curved interfaces and to independent meshing of each side of the interface. The discussion on numerical results is focused on the non-matching discretization of the interface. The obtained data are also compared with known analytical solutions, if available, and discussed in the cases of different material properties pertinent to substructures on both sides of the interface.     

A comparison of two Nextel 440 Fibre reinforced aluminium composites using acoustic emission

The acoustic emission (AE) response and mechanical behaviour of two Nextel 440 fibre reinforced aluminium composites were compared. The total cumulative AE events were found to occur in two regions. The first region occurred at smaller strains and had a large exponential rise in events for the reinforced 6061 composite and few events for the high purity aluminium matrix composite. The difference was attributed to the alloy constituents of the 6061 aluminium matrix. The events were attributed to a dislocation release mechanism that occurred prior to yielding of the matrix. In the second region of events in the reinforced 6061 composite the event rate was constant and continued to failure. The second region of events in the high purity aluminium (HPAL) composite had a steady increase in the event rate until a constant rate was reached prior to failure. The events in the reinforced HPAL composite were attributed to the fracturing of fibres and the associated plastic deformation of the matrix that accompanies fibre failure. Thus failure in the materials occurs due to the propagation of fracturing fibres. The propagation was rapid in the reinforced 6061 aluminium composite. The reinforced HPAL composite had a slower propagation due to the high ductility of the HPAL.     

低声能辐射下的两种水声跳频通信方法对比研究

水声信道是一个极其复杂的时变-空变-频变信道,因而进行水声通信,必须克服时变多途干扰等困难。将抗干扰能力强的跳频通信技术应用于水声通信领域,对于隐蔽性、保密性要求高的系统来说,该体制有着卓越的特性。在仿真环境下,对比研究了m序列和基于二维logistic映射的混沌序列的相关性、平衡性、功率谱特性、多址性等性能。构建了一种在低声能条件下的水声跳频通信系统,给出了系统编解码技术的实现方法。发射的帧信号由激活信号、同步信号、时延信号和三组(15,11)汉明码循环3次组成,采用了时间与频率的双分集技术进行纠错解码,从而增强了系统的保密性。通过外场试验,验证了利用m序列和混沌序列进行水声跳频通信的可行性,并对试验结果进行了对比研究。由于混沌序列的数量巨大且性能优异,非常适用于水声组网通信和保密性要求高的水声通信系统。因此,混沌序列在今后的水声跳频通信领域应有更广阔的应用前景。