- T. T. Nguyen;K. Y. Lam;
The paper examines three selective schemes for the smoothed finite element method (SFEM) which was formulated by incorporating a cell-wise strain smoothing operation into the standard compatible finite element method (FEM). These selective SFEM schemes were formulated based on three selective integration FEM schemes with similar properties found between the number of smoothing cells in the SFEM and the number of Gaussian integration points in the FEM. Both scheme 1 and scheme 2 are free of nearly incompressible locking, but scheme 2 is more general and gives better results than scheme 1. In addition, scheme 2 can be applied to anisotropic and nonlinear situations, while scheme 1 can only be applied to isotropic and linear situations. Scheme 3 is free of shear locking. This scheme can be applied to plate and shell problems. Results of the numerical study show that the selective SFEM schemes give more accurate results than the FEM schemes.
2007年05期 497-508页 [查看摘要][在线阅读][下载 248K] [下载次数：62 ] |[网刊下载次数：0 ] |[引用频次：18 ] |[阅读次数：0 ] <正>2007 English Summer Camp OpensTsinghua launched its fourth English Summer Camp on July 9,2007 for nearly 3200 students who have just completed their freshman year. The English Summer Camp features a flexible learning style,the students can choose from four
2007年05期 508+526+539+545+553+571+584页 [查看摘要][在线阅读][下载 170K] [下载次数：10 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ] - 张见明;Masa Tanaka;姚振汉;
The hybrid boundary node method (HdBNM) combines a modified function with the moving least squares approximation to form a boundary-only truly meshless method. This paper describes two implementations of the HdBNM, the singular hybrid boundary node method (ShBNM) and the regular hybrid boundary node method (RhBNM). The ShBNM and RhBNM were compared with each other, and the parameters that influence their performance were studied in detail. The convergence rates and their applicability to thin structures were also investigated. The ShBNM and RhBNM are found to be very easy to implement and to efficiently obtain numerical solutions to computational mechanics problems.
2007年05期 509-519页 [查看摘要][在线阅读][下载 249K] [下载次数：35 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - S. Hirose;Y. Ushida;
This paper describes a time-domain boundary element method developed to analyze the interactions of acoustic and elastic waves near the interfaces between water and an anisotropic elastic solid. Two models are analyzed with one being the interface between two half spaces of fluid and solid and the other being a fluid region sandwiched by half space domains of anisotropic elastic solids. Both monopole and dipole point sources are used to generate an initial pressure wave in the fluid. Some snapshots of the transient wave behavior near the fluid-solid interfaces are given. The effect of the anisotropy in the solid on the pressure waveforms in the fluid is discussed. The numerical results allow detailed arrival identification and interpretation of acoustic and elastic waves propagating along the fluid-solid interfaces.
2007年05期 520-526页 [查看摘要][在线阅读][下载 469K] [下载次数：26 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - 陈永强;孙树立;刘奕;
A general numerical approach was developed to simulate the mechanical properties and the failure of heterogeneous elasto-plastic materials using statistical distributions of the material properties. An appropriate elastic-plastic constitutive relation is used to describe the material behavior and failure in each element, with a two-parameter Weibull distribution used to produce the initial heterogeneous material property variations. An adaptive incremental load-step is applied so that only one or a few elements (or integra- tion points) change their status (i.e., from elastic to plastic, or from plastic to strain failure) within one load step. A failed element is then assigned a very small modulus to simulate the failure rather than removing it from the model, which keeps the continuity of the geometric mesh. The numerical results show that the model is suitable for simulating the effective mechanical properties and failure of heterogeneous materials with local elasto-plastic constitutive relations.
2007年05期 527-532页 [查看摘要][在线阅读][下载 196K] [下载次数：62 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ] - 孙红灵;张鲲;陈海波;张培强;
The control force, feedback gain, and actuator stroke of several active vibration isolation systems were analyzed based on a single-layer active vibration isolation system. The analysis shows that the feedback gain and actuator stroke cannot be selected independently and the active isolation system design must make a compromise between the feedback gain and actuator stroke. The performance of active isolation systems can be improved by the joint vibration reduction using an active vibration isolation system with an adaptive dynamic vibration absorber. The results show that the joint vibration reduction method can successfully avoid the compromise between the feedback gain and actuator stroke. The control force and the object vibration amplitude are also greatly reduced.
2007年05期 533-539页 [查看摘要][在线阅读][下载 135K] [下载次数：176 ] |[网刊下载次数：0 ] |[引用频次：12 ] |[阅读次数：0 ] - 葛洪海;许金余;
An analytical solution for the natural frequencies of a beam containing a cavity on an elastic foundation is presented. Based on the analytical solution, a numerical method for identifying cavities in the foundation is developed. The position and size of the cavities are identified by minimizing an objective function, which is formulated according to the difference between the computed and measured natural frequencies of the system. The conjugate gradient algorithm is adopted for minimizing the objective function. Some numerical examples are presented to demonstrate the applicability of the presented cavity determination method. The results show that the presented method can be used to identify the cavity position and size conveniently and efficiently.
2007年05期 540-545页 [查看摘要][在线阅读][下载 112K] [下载次数：74 ] |[网刊下载次数：0 ] |[引用频次：13 ] |[阅读次数：0 ] - 周马生;李毅谦;向志海;G. Swoboda;岑章志;
This paper presents a modified extended Bayesian method for parameter estimation. In this method the mean value of the a priori estimation is taken from the values of the estimated parameters in the previous iteration step. In this way, the parameter covariance matrix can be automatically updated during the estimation procedure, thereby avoiding the selection of an empirical parameter. Because the extended Bayesian method can be regarded as a Tikhonov regularization, this new method is more stable than both the least-squares method and the maximum likelihood method. The validity of the proposed method is illustrated by two examples: one based on simulated data and one based on real engineering data.
2007年05期 546-553页 [查看摘要][在线阅读][下载 305K] [下载次数：37 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - 王海涛;姚振汉;
Fiber-reinforced composites are commonly used in various engineering applications. The mechanical properties of such composites depend strongly on micro-structural parameters. This paper presents a new boundary element method (BEM) for numerical analysis of the mechanical properties of 3-D fiber-reinforced composites. Acceleration of the BEM is achieved by means of a fast multipole method (FMM), in allowing large scale simulations of a finite elastic domain containing up to 100 elastic fibers to be performed on one personal computer. The maximum number of degrees of freedom can reach a value of over 250 000. The effects of several key micro-structural parameters on the local stress fields and on the effective elastic moduli of fiber-reinforced composites are evaluated. The numerical results are compared with analytical predictions and good agreement is observed. The results show that the fast multipole BEM could be a prom- ising tool for further understanding of the mechanical behavior of such composites.
2007年05期 554-561页 [查看摘要][在线阅读][下载 423K] [下载次数：60 ] |[网刊下载次数：0 ] |[引用频次：4 ] |[阅读次数：0 ] - 王朋波;姚振汉;危银涛;
A fast multipole boundary element method (FM-BEM) was applied for the analysis of microcracked solids. Both the computational complexity and memory requirement are reduced to O(N), where N is the number of degrees of freedom. The effective elastic moduli of a 2-D solid containing thousands of randomly distributed microcracks were evaluated using the FM-BEM. The results prove that both the differential method and the method proposed by Feng and Yu provide satisfactory estimates to such problems. The effect of a non-uniform distribution of microcracks has been studied using a novel model. The numerical results show that the non-uniform distribution induces a small increase in the global stiffness.
2007年05期 562-566页 [查看摘要][在线阅读][下载 381K] [下载次数：48 ] |[网刊下载次数：0 ] |[引用频次：1 ] |[阅读次数：0 ] - 郑小平;黄拳章;王彬;
A boundary collocation method based on the least-square technique and a corresponding adaptive computation process have been developed for the plate bending problem. The trial functions are constructed using a series of the biharmonic polynomials, and the local error indicators are given by the residu- als of the energy density on the boundary. In comparison with the conventional collocation methods, the solution accuracy in the present method can be improved in an economical and efficient way. In order to dem- onstrate the efficiency and advantages of the adaptive boundary collocation method proposed in this paper, two numerical examples are presented for circular plates subjected to uniform loads and restrained by mixed boundary conditions. The numerical results for the examples show good agreement with ones presented in the literature.
2007年05期 567-571页 [查看摘要][在线阅读][下载 130K] [下载次数：41 ] |[网刊下载次数：0 ] |[引用频次：5 ] |[阅读次数：0 ] - 王永岗;胥掌世;
Large deflection theory of thin anisotropic circular plates was used to analyze the bifurcation behavior and chaotic phenomena of a corrugated thin circular plate with combined transverse periodic excitation and an in-plane static boundary load. The nonlinear dynamic equation for the corrugated plate was derived by employing Galerkin's technique. The critical conditions for occurrence of the homoclinic and subharmonic bifurcations as well as chaos were studied theoretically using the Melnikov function method. The chaotic motion was also simulated numerically using Maple, with the Poincaré map and phase curve used to evaluate when chaotic motion appears. The results indicate some chaotic motion in the corrugated plate. The method is directly applicable to chaotic analysis of an isotropic circular plate.
2007年05期 572-576页 [查看摘要][在线阅读][下载 170K] [下载次数：33 ] |[网刊下载次数：0 ] |[引用频次：2 ] |[阅读次数：0 ] - 汤哲;孙增圻;周长久;胡凌云;
Humanoid walking planning is a complicated task because of the high number of degrees of freedom (DOFs) and the variable mechanical structure during walking. In this paper, a planning method for 3-dimensional (3-D) walking movements was developed based on a model of a typical humanoid robot with 12 DOFs on the lower body. The planning process includes trajectory generation for the hip, ankle, and knee joints in the Cartesian space. The balance of the robot was ensured by adjusting the hip motion. The angles for each DOF were obtained from 3-D kinematics calculation. The calculation gave reference trajectories of all the DOFs on the humanoid robot which were used to control the real robot. The simulation results show that the method is effective.
2007年05期 577-584页 [查看摘要][在线阅读][下载 361K] [下载次数：94 ] |[网刊下载次数：0 ] |[引用频次：5 ] |[阅读次数：0 ] - 余建明;薛一波;李军;
As the core algorithm and the most time consuming part of almost every modern network intrusion management system (NIMS), string matching is essential for the inspection of network flows at the line speed. This paper presents a memory and time efficient string matching algorithm specifically designed for NIMS on commodity processors. Modifications of the Aho-Corasick (AC) algorithm based on the distribution characteristics of NIMS patterns drastically reduce the memory usage without sacrificing speed in software implementations. In tests on the Snort pattern set and traces that represent typical NIMS workloads, the Snort performance was enhanced 1.48%-20% compared to other well-known alternatives with an automaton size reduction of 4.86-6.11 compared to the standard AC implementation. The results show that special characteristics of the NIMS can be used into a very effective method to optimize the algorithm design.
2007年05期 585-593页 [查看摘要][在线阅读][下载 119K] [下载次数：81 ] |[网刊下载次数：0 ] |[引用频次：16 ] |[阅读次数：0 ] - 陈兵旗;王志强;
The use of computer vision technology to collect and analyze statistics during badminton matches or training sessions can be expected to provide valuable information to help coaches to determine which tactics should be used by a player in a given game or to improve the player's tactical training. A method based on 2-D seriate images by which statistical data of a badminton match can be obtained is presented. Image capture and analysis were performed synchronously using a multithreading technique. The regions of movement in the images were detected using a temporal difference method, and the trajectories of the movement regions were analyzed using seriate images. The shuttlecock trajectory was extracted from all detected trajectories using various characteristic parameters. The stroke type was determined by comparing the shuttlecock trajectory data with a set of stroke definition data. The algorithm was tested at a training center, and the results were compared with baseline data obtained by expert visual inspection using four video samples, which included approximately 10 000 frames. The shuttlecock trajectory and stroke type were detected correctly in almost 100% of the analyzed video sequences. The average speed of the automated analysis was approximately 40 frames/s, indicating that the method can be used for real-time analysis during a badminton match. The system is convenient for use by a sports coach.
2007年05期 594-601页 [查看摘要][在线阅读][下载 336K] [下载次数：170 ] |[网刊下载次数：0 ] |[引用频次：15 ] |[阅读次数：0 ] - 周一鸣;张超;张曾科;
As fractal image encoding algorithms can yield high-resolution reconstructed images at very high compression ratio, and therefore, have a great potential for improving the efficiency of image storage and image transmission. However, the baseline fractal encoding algorithm requires a great deal of time to complete the best matching search between the range and domain blocks, which greatly limits practical applications of the algorithm. In order to solve this problem, a necessary condition of the best matching search based on an image feature is proposed in this paper. The proposed method can reduce the search space significantly and excludes the most inappropriate domain blocks for each range block before carrying out the best matching search. Experimental results show that the proposed algorithm can produce good quality reconstructed images and requires much less time than the baseline encoding algorithm. Specifically, the new algorithm can speed up encoding by about 85 times with a loss of just 3 dB in the peak signal to noise ratio (PSNR), and yields compression ratios close to 34.
2007年05期 602-606页 [查看摘要][在线阅读][下载 278K] [下载次数：31 ] |[网刊下载次数：0 ] |[引用频次：4 ] |[阅读次数：0 ] - 罗凌智;张宇;孙增圻;
This paper deals with a flexible macro-micro manipulator system, which includes a long flexible manipulator and a relatively short rigid manipulator attached to the tip of the macro manipulator. A flexible macro manipulator possesses the advantages of wide operating range, high speed, and low energy consumption, but the disadvantage of a low tracking precision. The macro-micro manipulator system improves tracking performance by compensating for the endpoint tracking error while maintaining the advantages of the flexible macro manipulator. A trajectory planning scheme was built utilizing the task space division method. The division point is chosen to optimize the error compensation and energy consumption for the whole system. Then movements of the macro-micro manipulator can be determined using separate inverse kinematic models. Simulation results for a planar 4-DOF macro-micro manipulator system are presented to show the effectiveness of the control system.
2007年05期 607-613页 [查看摘要][在线阅读][下载 357K] [下载次数：153 ] |[网刊下载次数：0 ] |[引用频次：3 ] |[阅读次数：0 ] - 张颖超;赵争鸣;郭伟;孙晓瑛;
A position control system was developed using a synchronous reluctance permanent magnetic (SR-PM) motor with the field oriented control (FOC). To get satisfactory control performance, a fuzzy adaptive proportion integration (PI) controller was used rather than a conventional PI controller for the position control. The controller tunes the PI parameters online with fuzzy logic, based on the error and the change of the error between the feedback and reference position. The superiority of the fuzzy PI controller compared to conventional controller is illustrated by simulations and experiments which confirm that the fuzzy PI controller effectively restrains overshoot of the position response. As a result, the servo system gives satisfac- tory steady state and dynamic performance.
2007年05期 614-619页 [查看摘要][在线阅读][下载 326K] [下载次数：67 ] |[网刊下载次数：0 ] |[引用频次：9 ] |[阅读次数：0 ] - 潘赟;葛宁;董在望;
Many communication systems use the cyclic redundancy code (CRC) technique for protecting key data fields from transmission errors by enabling both single-bit error correction and multi-bit error detection. The look-up table design is very important for the error-correction implementation. This paper presents a CRC look-up table optimization method for single-bit error correction. The optimization method minimizes the address length of the pre-designed look-up table while satisfying certain restrictions. The circuit implementation is also presented to show the feasibility of the method in the application specific integrated circuit design. An application of the optimization method in the generic framing procedure protocol is implemented using field programmable gate arrays. The result shows that the memory address length has been minimized, while keeping a very simple circuit implementation.
2007年05期 620-623页 [查看摘要][在线阅读][下载 105K] [下载次数：74 ] |[网刊下载次数：0 ] |[引用频次：9 ] |[阅读次数：0 ] - H. Yurtseven;T. ünsal;
The pressure dependence of the molar volume was at constant temperatures close to the melting point in benzene. The molar volume of benzene was calculated using experimental data for the thermal expansivity for constant temperatures of 25℃, 28.5℃, 40℃, and 51℃ at various pressures for both the solid and liquid phases. The predictions are in good agreement with the observed volumes in both the solid and liquid phases of benzene. The predicted values of the molar volume for a constant temperature of 28.5℃ in the liquid phase of benzene agree well with experimental data in the literature.
2007年05期 624-628页 [查看摘要][在线阅读][下载 279K] [下载次数：15 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ] -
<正>Tsinghua Science and Technology (Tsinghua Sci Technol),a comprehensive academic journal sponsored by Tsinghua University,is published bimonthly. This journal aims at presenting the up-to-date scientific achievements with high creativity
2007年05期 629页 [查看摘要][在线阅读][下载 793K] [下载次数：10 ] |[网刊下载次数：0 ] |[引用频次：0 ] |[阅读次数：0 ] 下载本期数据