Bungee jump model with increased stretch-prediction accuracy

This bungee jumping model improves the stretch prediction accuracy of prior models by including the effects of: rubber viscoelasticity, stiffness nonlinearities, jumper air drag, and jumper horizontal push-off. Bungee (bungy) cords are made from rubber, a viscoelastic material whose stiffness is a function of the number of cycles (cycle effect), the time interval since the last cycle (interval effect), strain rate, and temperature. Stiffness was measured, on an MTS machine, over 100 cycles at four intervals (1, 5, 60, 1440 min), three strain rates (0.17, 1.5, 5.0 s⁻¹) and constant temperature (20.5°C). At a constant cycle interval and strain rate, rubber stiffness decreased 9.8% from the 10th to the 100th cycle. This decrease was linear with the logarithm of the cycle number. Stiffness recovered 6.5% after 24 hours of non-use. Neglecting viscoelastic effects causes significant stretch-prediction errors of -11.0% (cycle effect), -3.2% (interval effect), and +1.6% (strain rate). Less significantly, neglecting jumper air drag, jumper push-off, and stress-strain nonlinearities cause prediction errors of +1.9%, +0.7%, and +0.2%, respectively. These errors are based on a typical cord, of unstretched length 7.5 m, that elongates 250%. For accurate stretch prediction, with typical sized bungee cords, viscoelastic effects are both more important and easier to implement than air drag and stress-strain non-linearity.     

Three Routes to Chaos in Electrical Power Systems

Study on power systemchaotic phenomena is oneimportant aspect in power system stability stu-dies[1—3]. Early studies, such as Lee[4], Chiang[5],Wang[6]and Tan[7], mainly focused on interpretingthe behavior of chaotic oscillations. Later researches,such as Wu[8], Rajesh[9], Jia[10,11], Srivastava[12]and Abed[13]began to concern the following topics,such as interaction of chaotic motion and system dy-namics, the relationship of power system stability re-gion and chaos, and the methods to eli…     

语言顺应论与英语习得及教学

Vershueren认为,语言使用实际上是人们不断地对语言做出选择的过程,语言选择有很多特点并且发生在语言的各个层面。顺应性使人们通过协商,最终达到理想的交际目的。因此无论英语习得及教学都要不可避免的遵循这些选择,更重要的是要顺应。     

Constraints on dynamic stability during forward,backward and lateral locomotion in skilled football players

The aim of this study was to investigate effects of speed and plane of motion on stability during locomotion in skilled football players. Ten male national-level football players participated in this study to run forward, backward and in lateral directions on a treadmill at 80%, 100% and 120% of their preferred running speeds. The coordinate data of passive reflective markers attached to body segments were recorded using motion capture systems. Time series data obtained from the ankle marker were used for further analyses. The largest finite-time Lyapunov exponent and maximum Floquet multiplier were adopted to quantify local and orbital dynamic stabilities, respectively. Results showed that speed did not significantly change local and orbital dynamic stabilities in any of running patterns. However, both local and orbital dynamic stability were significantly higher in the secondary plane of progression. Data revealed that in running, unlike walking, stability in the direction perpendicular to the direction of running is significantly higher, implying that less active control is required in the secondary plane of progression. The results of this study could be useful in sports training and rehabilitation programmes where development of fundamental exercise programmes that challenge both speed and the ability to maintain stability might produce a tangible enhancement of athletic skill level.     

全过程课题式教学与考核方法的理论与实践

基于《多孔介质流体动力学》研究生课程,提出了全过程课题式教学方法。在课堂教学中,首先建立基础知识的概念框架,然后让同学们自行寻找应用领域和科学问题,撰写并提交课题申请,完成课题工作,提交研究报告,通过演讲对课题进行总结和汇报,最终采用多指标体系对成绩进行评定。该方法教学效果良好,可为类似课程教改提供参考。     

Mechanical energy generation and transfer in the racket arm during table tennis topspin backhands

The ability to generate a high racket speed and a large amount of racket kinetic energy on impact is important for table tennis players. The purpose of this study was to understand how mechanical energy is generated and transferred in the racket arm during table tennis backhands. Ten male advanced right-handed table tennis players hit topspin backhands against pre-impact topspin and backspin balls. The joint kinetics at the shoulder, elbow and wrist of the racket arm was determined using inverse dynamics. A majority of the mechanical energy of the racket arm acquired during forward swing (65 and 77% against topspin and backspin, respectively) was due to energy transfer from the trunk. Energy transfer by the shoulder joint force in the vertical direction was the largest contributor to the mechanical energy of the racket arm against both spins and was greater against backspin than against topspin (34 and 28%, respectively). The shoulder joint force directed to the right, which peaked just before impact, transferred additional energy to the racket. Our results suggest that the upward thrust of the shoulder and the late timing of the axial rotation of the upper trunk are important for an effective topspin backhand.     

从动态顺应看《李尔王》的三种译本

Jef Verschueren的语用综论观下的顺应论的动态性认为，语言选择过程的动态顺应可以从三个方面得到体现：时间顺应、语境的顺应和语言线性结构的灵活变化。以该理论对《李尔王》三个不同时期的译本进行分析，指出翻译标准应谊被不断地重新阐释以顺应不同时间、不同语境和不同交际目的。     

Molecular dynamics simulation study on behaviors of liquid 1,2-dichloroethane under external electric fields

Molecular dynamics simulation was carried out to study the behavior of liquid 1,2-dichloroethane molecules under external electric fields including direct current field, alternating current field and positive-half-period cosin field. The maximum applied field strength was 10⁸ V/m, the maximum frequency of the alternating current field and that of the positive-half-period cosine field was 10¹² Hz. The simulation revealed that the field type and field strength act on the population of the molecular configuration. In the strong direct current field, all trans forms converted completely into gauche forms. Order parameter and the correlation of the system torsion angle were also investigated. The results suggested that these two dynamical parameters depended also on the field type and the field strength. The maximum of order parameter was found to be at 0.6 in the strong direct current field.     

群体动力学方法在不同引证网络中探测新趋势的效能差异

[目的/意义]针对采用不同引证网络探测新兴趋势的问题,比较群体动力学方法用于直接引证网络和文献耦合网络上的效能差异。[方法/过程]首先构建并分析直接引证网络、文献耦合网络和同被引网络的特征,然后基于群体动力学方法对文献耦合网络进行实证研究。[结果/结论]对比以往的研究结果发现：群体动力学方法作用于直接引证网络进行新兴趋势的预测结果较基线方法为好,而在文献耦合网络中预测的效果并不比基线方法更佳。     

中国城镇化的生态足迹影响——基于系统动力学模型的模拟仿真分析

众多研究表明生态足迹与城镇化进程密切相关。本研究通过构建系统动力学模型,进行综合和单一情景分析,就不同的城镇化路径选择进行仿真模拟。结果显示：未来 15 年中国的生态足迹变化将经历两个不同的阶段。2015-2025 年,产业结构仍然为资源消耗型,生态足迹仍在逐步增加,碳足迹占比达到峰值,但逐渐接近生态足迹的峰值。2026-2030 年,城镇化转向以集约的方式发展,使得人均生态足迹的增值效应逐渐减弱,在达到峰值后开始下降。中国的人均生态足迹将在 2029 年达到峰值。情景对比分析表明,走绿色发展的城镇化道路对生态足迹的降低有重要影响。最后对我国未来城镇化的发展从生态足迹角度提出了相应的建议。