- 贾晓红;王人成;Winson Lee;
This study accesses the effects of shoe heel heights on loading, muscle activity, and plantar foot pressure of trans-tibial amputees during standing. Five male subjects with unilateral trans-tibial amputation volunteered to participate in this study. Three pairs of shoes with zero, 20 mm, and 40 mm heel heights were used. The loading line of the prosthetic side, the plantar foot pressure, and the surface electromyography (EMG) of 10 muscles were simultaneously recorded. With increasing shoe heel heights during standing, the loading line of the prosthetic side shifted from the anterior to the posterior side of the knee center, the peak pressure was increased in the medial forefoot region, and the peak pressure was reduced in the heel region. The EMG of the medial and lateral gastrocnemius of the sound leg almost doubled and that of the rectus fomris, vastus lateralis, and vastus medialis of the prosthetic side increased to different extents with in- creasing heel heights from zero to 40 mm. These results show a high correlation with human physical behavior. Changing of the heel heights for trans-tibial amputees during standing actually had similar effects to altering the prosthetic sagittal alignment. The results suggest that an alignment change is necessary to accommodate heel height changes and that prosthesis users should be cautious when choosing shoes.
2009年03期 v.14 281-286页 [查看摘要][在线阅读][下载 676K] [下载次数:80 ] |[网刊下载次数:0 ] |[引用频次:3 ] |[阅读次数:26 ] <正>Former British Prime Minister Tony Blair delivered a speech to Tsinghua students entitled "A Global Deal in Climate Change: the Role of Business and New Technologies" on March 24, 2009 in the Lecture Hall of the School
2009年03期 v.14 286页 [查看摘要][在线阅读][下载 192K] [下载次数:29 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:24 ] - 王春洪;胡金春;朱煜;尹文生;
An optimal synchronous trajectory tracking controller was developed for multi-axis systems. The position synchronization error on each axis was defined as the position difference between this axis and the following axes. The following error of each axis, the synchronization error, and its derivative were considered in the cost function. A Riccati equation was deduced from the Hamilton-Pontryagin equation. The optimal control law was set up from the Riccati equation solution. Simulations of a two-axis system show that the synchronization error can be significantly reduced and the synchronization performance can be adjusted based on the parameters in the cost function.
2009年03期 v.14 287-292页 [查看摘要][在线阅读][下载 514K] [下载次数:72 ] |[网刊下载次数:0 ] |[引用频次:8 ] |[阅读次数:31 ] - 王国庆;吴爱萍;邹贵生;赵玥;陈强;任家烈;
Ti-23Al-17Nb alloy is an important high temperature structural material used in the space and aerospace fields. Welding of this alloy is an indispensable processing method, so the microstructures and mechanical properties of these welded joints must be studied to improve the welds. Longitudinal three-point bending tests were conducted to measure the bending ductility of laser beam welded joints. The crack distribution and fracture surface were investigated to further analyze the fracture behavior. The results indicate that the bending ductility decreases as the heat input by the laser beam welding increases. The crack inducing strain reaches 4.24%, while the fracturing strain exceeds 5% when the heat input is below 316 J/cm. If the columnar crystal grain of the weld metal exhibits a uniform orientation, the bending ductility is worse. The fractography analysis shows that the cracking propagates transgranularly and the fracture surface has a cleavage mode.
2009年03期 v.14 293-299页 [查看摘要][在线阅读][下载 6524K] [下载次数:149 ] |[网刊下载次数:0 ] |[引用频次:13 ] |[阅读次数:31 ] - 宋海霞;吴运新;唐传安;袁帅;巩前明;梁吉;
FeAl intermetallics were prepared by mechanical alloying and vacuum hot-pressing. The Fe-48 at.% Al powder was ball-milled for 3-12 h, producing a solid solution structure of Fe (Al) with trace Al (Fe). Subsequent vacuum annealing or hot-pressing introduced phase transformations into the FeAl (B2) intermetallics and Al2O3 inclusions. The hot-pressed FeAl intermetallics possess a high flexural strength of 831 MPa and a fairly good strain at break of 3.2%. The results show that the addition of 0.5 at.% B reduces the peak temperature for hot-pressing from 1180℃ to 1100℃, and increases the density of the compacts from 95% to 96.3%, but results in no significant improvement in the mechanical properties.
2009年03期 v.14 300-306页 [查看摘要][在线阅读][下载 1879K] [下载次数:67 ] |[网刊下载次数:0 ] |[引用频次:3 ] |[阅读次数:50 ] - 霍亮;李斌;石玉峰;许庆彦;韩志强;柳百成;
A two-dimensional modified cellular automaton model was developed to simulate the solidification process of magnesium alloy. The stochastic nucleation, solute redistribution, and growth anisotropy effects were taken into account in the present model. The model was used to simulate the grain size of magnesium alloy AZ91D for various cooling rates during the solidification process. To quantitatively validate the current model, metallographic experiments were carried out on specimens obtained from sand mold AZ91D step castings. The metallographic results agree well with the prediction results. The current model can be used to accurately predict the grain sizes of cast AZ91D magnesium alloy.
2009年03期 v.14 307-312页 [查看摘要][在线阅读][下载 1014K] [下载次数:121 ] |[网刊下载次数:0 ] |[引用频次:4 ] |[阅读次数:41 ] - 刘圆圆;陈志强;赵自然;张丽;王振天;
Cosmic ray muon radiography which has good penetration ability and is sensitive to high-Z materials, is an effective method to detect shielded nuclear materials. This paper summarizes methods developed to process muon radiography in Tsinghua University. The methods include detector data correction, recon-struction algorithms (maximum likelihood scattering, MLS, and the maximum likelihood scattering and displacement, MLSD) acceleration, and the modification of the normalized mean absolute distance measure (NMADM) into a picture comparison binarization method (PCBM) which is more suitable for cosmic ray muon radiographs. Simulations demonstrate that all these methods give excellent results, so that cosmic muon radiography can become more widely used.
2009年03期 v.14 313-321页 [查看摘要][在线阅读][下载 979K] [下载次数:100 ] |[网刊下载次数:0 ] |[引用频次:6 ] |[阅读次数:29 ] <正>Tsinghua University unveiled its Center for Engineering Education (CEE), a cross-disciplinary research center for engineering education studies, at an inauguration ceremony in the Main Building on January 12, 2009.
2009年03期 v.14 321页 [查看摘要][在线阅读][下载 217K] [下载次数:22 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:23 ] - 莫怡华;欧丽;钟宏志;
The vibrations of beams on a nonlinear elastic foundation were analyzed considering the effects of transverse shear deformation and the rotational inertia of beams. A weak form quadrature element method (QEM) is used for the vibration analysis. The fundamental frequencies of beams are presented for various slenderness ratios and nonlinear foundation parameters for both slender and short beams. The results for slender beams compare well with finite element results. The analysis shows that the transverse shear deformation and the nonlinear foundation parameter significantly affect the fundamental frequency of the beams.
2009年03期 v.14 322-326页 [查看摘要][在线阅读][下载 381K] [下载次数:112 ] |[网刊下载次数:0 ] |[引用频次:8 ] |[阅读次数:72 ] - Dirk Donath;Danny Lobos;
This paper analyzes the problem of the design of envelopes for high-rise isolated residential buildings. The phenomenon of envelope creation appears in the early stages of the architectural design. Variables that influence the final shape and size of such envelopes are then identified. This paper presents the state-of-the-art tools for the current solutions at the commercial and academic/scientific level. The variables identified in this research are the client's needs, the urban code and architectural practice, and their specific components for the final creation of a new decision support system tool based on the building information modeling (BIM) software platform, to facilitate the work in the project development and drawing production stages. This tool generates several options for building envelopes according to the parameters required by the city Zoning Planning Commission. These options then lead to deliver reliable data and a geometry that can be analyzed in a timely fashion by the engineers, builders, architects, government, and clients in the early stages of the building's design. The results show that use of specific information and communication technologies (ICT) tools in the early stages of a building design helps reduce the working time, increases confidence in the generated solution, and contributes to the exploration of alternatives in a short period of time.
2009年03期 v.14 327-332页 [查看摘要][在线阅读][下载 1517K] [下载次数:393 ] |[网刊下载次数:0 ] |[引用频次:3 ] |[阅读次数:212 ] - 陈晓明;邵春福;李妲;董春娇;
The reliability of capacity of signalized intersections in mixed traffic conditions involving vehicles, bicycles, and pedestrians was investigated to complete the conventional, deterministic capacity calculations. Simulations using VISSIM provided estimates of capacity distributions, and demonstrated the effects of the analysis intervals on the distributions. With the random vehicle arrivals taken into account, a capacity reliability assessment method was given as a function. Assessments were also performed regarding the effects of the conflicting pedestrian and bicycle volumes on capacity reliability. The simulation indicates that the pedestrians and bicycles result in greater random fluctuations of exclusive turning lane capacities, but have less effect on the variability of shared lane capacities. Normal distributions can be used to model the capacities for intervals not less than 10 min. At higher vehicular volumes, the capacity reliability is more sensitive to the mean and standard deviation of the pedestrian and bicycle volumes.
2009年03期 v.14 333-340页 [查看摘要][在线阅读][下载 663K] [下载次数:153 ] |[网刊下载次数:0 ] |[引用频次:9 ] |[阅读次数:28 ] - Anthony Chen;Juyoung Kim;Seungjae Lee;Jaisung Choi;
The network design problem (NDP) is one of the most difficult and challenging problems in trans-portation. Traditional NDP models are often posed as a deterministic bilevel program assuming that all relevant inputs are known with certainty. This paper presents three stochastic models for designing transporta-tion networks with demand uncertainty. These three stochastic NDP models were formulated as the expected value model, chance-constrained model, and dependent-chance model in a bilevel programming framework using different criteria to hedge against demand uncertainty. Solution procedures based on the traffic assignment algorithm, genetic algorithm, and Monte-Carlo simulations were developed to solve these stochastic NDP models. The nonlinear and nonconvex nature of the bilevel program was handled by the genetic algorithm and traffic assignment algorithm, whereas the stochastic nature was addressed through simulations. Numerical experiments were conducted to evaluate the applicability of the stochastic NDP models and the solution procedure. Results from the three experiments show that the solution procedures are quite robust to different parameter settings.
2009年03期 v.14 341-351页 [查看摘要][在线阅读][下载 344K] [下载次数:92 ] |[网刊下载次数:0 ] |[引用频次:11 ] |[阅读次数:23 ] - 张青松;赵国敏;刘金兰;
Performance-based design is more holistic and flexible than prescriptive design for providing safety in large complex buildings. Here, a multi-agent method to model the egress patterns of evacuees is combined with a microscopic pedestrian simulation model used to analyze the forces between individuals in a densely populated enclosed space in a crowd crushing and trampling analysis (CroC&Ts). The system is used to model egress patterns in a typical crowd evacuation simulation. The simulations indicate that some individuals will die from crushing in 2 m and 4 m wide exits in emergencies. The simulations also show that the fatality probability increases when barriers obstacled the path and when the egress distances were larger. The simulations validate the conclusions of the stranded crowd model (SCM) and provide quantitative predictions of the crowd crushing and trampling risk. Therefore, the CroC&Ts can provide performancebased egress designs for large pubic buildings and improve crowd safety management and emergency planning.
2009年03期 v.14 352-359页 [查看摘要][在线阅读][下载 601K] [下载次数:96 ] |[网刊下载次数:0 ] |[引用频次:8 ] |[阅读次数:29 ] <正>A new kind of flexible, stretchable, and transparent loudspeaker made with carbon nanotube (CNT) film was in-vented by a team led by Professor Fan Shoufan and Associate Professor Jiang Kaili from Tsinghua’s Department of
2009年03期 v.14 359页 [查看摘要][在线阅读][下载 217K] [下载次数:30 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:22 ] - 龙奋杰;郭明;郑思齐;
The housing market, an important component of the urban economy, is closely integrated with urban development. Urban development attracts labor inflows which then increase the housing demand in the cities. Urban dwellers' willingness-to-pay (WTP) for housing, as part of their living costs, depends on their incomes they can earn in the cities and the quality of life (QOL) they want to enjoy. Urban wage growth and quality of life improvements are always accompanied by increased demand and increased WTP. This paper uses the average wages of fully-employed employees and various city indicators to reflect the urban QOL to explain the relationships among people's WTP for housing, their urban wages and their urban QOL across 35 metropolitan cities in China. The empirical results illustrate that the urban QOL represented by city indicators and the average wage level accounts for approximately 70% of the housing price variation. Although wages still have significant impact on the WTP, the QOL in Chinese metropolitan cities tends to contribute more to the residents' WTP for housing, indicating that social and natural environments are valued by urban residents more and more.
2009年03期 v.14 360-366页 [查看摘要][在线阅读][下载 410K] [下载次数:237 ] |[网刊下载次数:0 ] |[引用频次:3 ] |[阅读次数:38 ] - 方神光;魏加华;吴保生;尚毅梓;
The unsteady flow in the Middle Route South-to-North Water Transfer Channel was simulated numerically using an implicit solution procedure for the Saint Venant equations. An equivalent roughness was used to simulate the effect of many transfer structures on the water levels in the main channel. Various gate operating and control methods were analyzed to study the response to disturbances produced by varying the flow rates through the Tianjin outlet. The results show that when the inflow at the head changes in the same way as the sum of the flow rates through all the outlets, the transition time and the fluctuation of the water levels using the timed gate operation method are less than when using the simultaneous gate operation method, but the variations of the gate openings and flow rates through each control gate are much larger. The flow disturbances produced by the Tianjin outlet can be rectified within several channel sections and the transition time can be greatly shortened by allowing the water levels immediately upstream of the control gates to vary within proscribed ranges, rather than being held constant.
2009年03期 v.14 367-377页 [查看摘要][在线阅读][下载 1338K] [下载次数:144 ] |[网刊下载次数:0 ] |[引用频次:8 ] |[阅读次数:70 ] - 王昊;王洪涛;陆文静;赵岩;
The digestibility of sorted municipal solid waste (MSW) is often limited by the high content of structured green waste. The objectives of this study are to investigate the effect of alkaline hydrothermal pretreatment on the anaerobic digestion of sorted waste and to analyze the biogas production of different parts of the waste. The waste was hydrothermally pretreated in a dilute alkali solution. The hydrolysis product was then incubated in a 500 mL saline bottle to determine the biochemical methane potential (BMP) under mesophilic anaerobic conditions. The optimum hydrothermal condition was 170℃ at 4 g NaOH/ 100 g solid for one hour. The concentration of chemical oxygen demand (COD) was 13 936 mg/L and the methane yield was 164 mL/g volatile solid (VS) for 6 days incubation at the optimum conditions. The biogas production was increased more than 50% over the control, with the methane conversion ratio on a carbon basis enhanced to 30.6%. The organic part of the sorted waste was mainly kitchen garbage and leaves. Model kitchen garbage completely liquified at 130℃ for one hour had a methane yield of 276 mL/g VS. The alkali addition slightly enhanced the hydrolyzation rate and methane yield. The biogas potential of leaves was improved by pre-treatment at above 150℃ under alkaline conditions.
2009年03期 v.14 378-382页 [查看摘要][在线阅读][下载 406K] [下载次数:62 ] |[网刊下载次数:0 ] |[引用频次:1 ] |[阅读次数:28 ] - 朱岩;李岩;Mathieu Leboulanger;
With the Internet connecting every part of the world, electronic marketplaces provide a world-wide transaction platform for global businesses. However, consumers from different countries demonstrate distinct purchasing behaviors online. This study uses eBay as the consumer-to-consumer electronic marketplace to investigate the eBay mechanisms and purchasing habits of consumers from the United States, France, and China to evaluate the effects of cultural and national differences on purchasing habits. This study uses data on national transactions and reputation profiles of sellers gathered from eBay.com, eBay.fr, and eBay.com.cn, including both cross-sectional data and panel data to provide a comprehensive comparison of cultural behavior in the trading process as well as the influence of eBay's reputation management system on online pricing in these three countries. Significant differences confirm that purchasing behavior differs in various national consumer-to-consumer electronic marketplaces and the importance of cultural behavioral patterns.
2009年03期 v.14 383-389页 [查看摘要][在线阅读][下载 299K] [下载次数:624 ] |[网刊下载次数:0 ] |[引用频次:8 ] |[阅读次数:314 ] - 叶冬艳;
The life insurance industry is very interested in how a person's lifetime is related to his wealth with financial advisors interested in how even a person's portfolio choice affects his lifetime. This paper presents a statistical analysis combined with intuitive relationships between lifetime and wealth. Key properties of this relationship are given and then various copulas are analyzed to see whether they have these properties. Other advantages and disadvantages of these copulas for describing the dependence are stated. The results show that some copulas are not appropriate for relating lifetime and wealth, including the Gaussian family.
2009年03期 v.14 390-396页 [查看摘要][在线阅读][下载 459K] [下载次数:36 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:22 ] <正>U.S. Secretary of State Hillary Clinton talked with Tsinghua students and teachers during her visit to the Taiyang-gong Thermal Power Plant on February 21, 2009.
2009年03期 v.14 396页 [查看摘要][在线阅读][下载 192K] [下载次数:35 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:24 ] - 汪侠;张捷;顾朝林;甄峰;
While the importance of tourist satisfaction has been recognized by academic researchers for at least four decades, adequate tourist satisfaction models have not been developed or validated. This study presents a tourist satisfaction model for a destination and explores the antecedents (tourist expectations, destination image, perceived quality, and perceived value) and the consequences (tourist complaints and tourist loyalty) of tourist satisfaction using Guilin for the case study. Structural equation modeling results support the tourist satisfaction model of tourist expectations, destination image, perceived quality, and perceived value as four key antecedents of tourist satisfaction, with tourist satisfaction having a negative effect on tourist complaints and a positive effect on tourist loyalty. Managerial implications are drawn from the study findings and suggestions are given for future work.
2009年03期 v.14 397-406页 [查看摘要][在线阅读][下载 530K] [下载次数:591 ] |[网刊下载次数:0 ] |[引用频次:14 ] |[阅读次数:298 ] - 但正刚;蔡临宁;郑力;
The vehicle routing problem with time windows (VRPTW) involves assigning a fleet of limited capacity vehicles to serve a set of customers without violating the capacity and time constraints. This paper presents a multi-agent model system for the VRPTW based on the internal behavior of agents and coordination among the agents. The system presents a formal view of coordination using the traditional contract-net protocol (CNP) that relies on the basic loop of agent behavior for order receiving, order announcement, bid calculation, and order scheduling followed by order execution. An improved CNP method based on a vehicle selection strategy is used to reduce the number of negotiations and the negotiation time. The model is validated using Solomon's benchmarks, with the results showing that the improved CNP uses only 30% as many negotiations and only 70% of the negotiation time of the traditional CNP.
2009年03期 v.14 407-412页 [查看摘要][在线阅读][下载 605K] [下载次数:152 ] |[网刊下载次数:0 ] |[引用频次:11 ] |[阅读次数:23 ] -
<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
2009年03期 v.14 413页 [查看摘要][在线阅读][下载 565K] [下载次数:6 ] |[网刊下载次数:0 ] |[引用频次:0 ] |[阅读次数:16 ] 下载本期数据