Features of flow past square cylinder with a perforated plate

A numerical investigation was performed on the reduction of the fluid forces acting on the square cylinder in the laminar flow regime with a perforated plate. The effects of geometric parameters such as the distance between the square cylinder and the perforated plate on the wake of the square cylinder were discussed. Furthermore, the flow characteristics such as the drag coefficient, lift coefficient, Strouhal number and flow pattern were obtained. It can be concluded that the drag force of the square cylinder reduces to some extent due to the addition of the perforated plate. The flow structure varies when the perforated plate is located behind the square cylinder. Moreover, the recirculation zone augments with the increase of L/D, and the vortex trace on the upper and lower surface of the square cylinder moves gradually backwards until a stable recirculation zone formed between the square cylinder and the perforated plate.     

提高穿孔板吸声性能的实验研究

在穿孔板结构设计中，蜂窝芯夹在两层金属穿孔面板之间，主要增加了板的刚性，同时一定程度上提高了其低频吸声系数；板后吸声薄层增加了穿孔吸声板的声阻，拓宽其吸声频带宽度，提高了吸声效果。本研究通过测试穿孔板结构的吸声系数，研究了不同结构参数（主要是穿孔板的厚度、穿孔率等）及板后空腔深度对其性能的影响。实验表明，吸声薄层的加入及板后空腔的设计。使蜂窝穿孔金属吸声板具有较宽的吸声频带、理想的共振频率、较高的吸声系数，特别是对船艇机舱噪声辐射峰值具有良好的吸声效果。     

Numerical study on the aerodynamic performance and safe running of high-speed trains in sandstorms

The influence of sandstorms on train aerodynamic performance and safe running was studied in response to the frequent occurrence of sandstorm weather in north China. An Eulerian two-phase model in the computational fluid dynamic (CFD) software FLUENT, validated with published data, was used to solve the gas-solid multiphase flow of a sandstorm around a train. The train aerodynamic performance under different sandstorm levels and no sand conditions was then simulated. Results showed that in sandstorm weather, the drag, lift, side forces and overturning moment increase by variable degrees. Based on a numerical analysis of aerodynamic characteristics, an equation of train stability was also derived using the theory of moment balance from the view of dynamics. A recommended speed limit of a train under different sandstorm levels was calculated based on the stability analysis.     

翅片结构对金属-聚合物散热器散热性能影响的数值模拟研究

针对芯片的冷却问题,基于热量传递平衡原理,提出了1种新型的金属-聚合物复合式散热器,该散热器利用金属基板优良的导热性能和聚合物微结构翅片优良的散热性能实现热量的高效导出和高效散出。采用FloEFD热分析软件对翅片的高度、厚度、间距等翅片结构对复合式散热器散热性能的影响进行了研究。结果表明当翅片高度为50mm时,热源温度最低;当翅片厚度和间距分别为1mm和2.5mm时,对流换热系数存在最大值,分别为32.54和31.93 W/(m~2·K),为高效、低成本的金属-聚合物散热器的推广和应用提供依据。     

气泡对多电极电磁流量计电流密度影响的数值仿真

针对气水两相流动,研究了气泡对多电极电磁流量计电流密度分布的影响。使用量化指标表示电流密度分布均匀程度以及气泡对电流密度分布影响程度,采用有限元方法,对多电极电磁流量计的电流密度进行数值仿真,分析比较气泡大小、形状不同时电流密度的分布情况。仿真结果为研究电磁流量计应用于气水两相流提供了参考。     

横向磁场中磁性液体表观密度研究

利用磁天平和电子天平测量了不同电流和高度下磁性液体的表观密度。使用霍尔传感器测量不同电流下轴线上的磁场强度,通过origin软件对磁场强度拟合并求偏导,得到不同电流下轴线上的磁场梯度分布曲线。根据磁性液体的Bernoulli方程推导了磁性液体表观密度的表达式,磁性液体的表观密度由磁性液体的固有密度、磁化强度以及外加磁场及磁场梯度决定。无磁场作用时,磁性液体密度各处相等,说明磁性液体中的纳米磁性颗粒均匀分散在载液中;有外磁场作用时,随着外加磁场增强,磁场梯度也逐渐增强,导致磁性液体中悬浮的磁性颗粒移动到磁场及磁场梯度增强的位置,使得该处磁性液体的表观密度增加,理论结果与实验结果一致。     

Experimental study on bonding properties between steel strand and concrete at cryogenic temperatures

To study the bonding properties between steel strand and concrete at room and cryogenic temperatures, a series of center pullout experiments were conducted on 96 bonding anchorage specimens at the lowest temperature of-165 ℃. The impacts on the bonding property of such parameters as the temperature, concrete strength, the relative concrete cover thickness, and the relative anchorage length were analyzed. The test results indicate that the changes in temperature have a clear effect on the bonding property between steel strand and concrete. As the temperature decreases, the bond stress, which corresponds to a 1 mm slip of steel strand in relation to concrete, and the ultimate bond strength initially increase and subsequently decrease at the inflection point of-80 ℃. The impact of the concrete strength on the bonding property, as shown by the tensile strength and the moisture content interaction, indicates that the bond stress vs concrete strength curve initially increases and later decreases with a decrease in temperature; the bond stress vs concrete cover thickness curve linearly increases, but the bond stress vs anchorage length curve linearly decreases at first and finally levels off.     

Optimization study of a PEM fuel cell performance using 3D multi-phase computational fluid dynamics model

INTRODUCTION Water management is one of the critical opera-tion issues in proton exchange membrane (PEM) fuelcells. Spatially varying concentrations of water inboth vapour and liquid form are expected throughoutthe cell because of varying rates of production andtransport (Sui and Djilali, 2005). Devising betterwater management is therefore a key issue in PEMFCdesign, and this requires improved understanding ofthe parameters affecting water transport in the mem-brane. Proper thermal m…     

The impact of size and specialisation on universities’ department performance: A DEA analysis applied to Austrian universities

This paper explores the performance efficiency of natural and technical science departments at Austrian universities using Data Envelopment Analysis (DEA). We present DEA as an alternative tool for benchmarking and ranking the assignment of decision-making units (organisations and organisational units). The method applies a multiple input and output variables approach, which is a clear advantage to other approaches using simple performance ratios. To deliver reasonable results, suitable input and output variables have been determined in a previous step using correlation analyses and OLS regression. The results validate the methods applied, and reveal performance differences and scale effects. The use of multiple output variables enables the revealing of detailed improvement or reduction amounts of each input and output of the evaluated units and furthermore for identifying the specialisation of teaching, research, and industrial cooperation. We find significant evidence that the size of a department influences its overall and specialisation performance; both small and large departments perform above average, which proves that simple linear scale effects do not exist.     

对我国优秀短道速滑运动员常规训练手段的生理学研究

[目的]通过我国优秀短道速滑运动员在年训练周期中血乳酸的监控,探讨其训练手段的合理性 [方法]采用检测运动员运动后即刻的血乳酸并以此反映运动强度的现场测试方法 [结果]运动员在冰期速度耐力训练后即刻的血乳酸浓度偏高,非冰期速度耐力训练后的血乳酸浓度偏低;冰期一般耐力(有氧耐力)训练后的血乳酸偏高,非冰期有氧耐力训练后的血乳酸偏低 [结论]旨在提高速度耐力的训练手段中,运动强度安排的不合理,导致了运动员的速度耐力不足,因而在剧烈比赛中运动员表现出后程无力的现象 调整训练强度是今后训练方法中的重点     

再生碎砖混凝土应用于农田土渠硬化的可行性研究

针对配套落后、渗漏严重的农田土渠进行硬化的需求,对不同粉煤灰取代率的再生碎砖混凝土进行强度、和易性、抗渗性研究。对掺入不同附加水的再生碎砖混凝土进行强度与和易性试验,得到按照粗骨料80%~90%的吸水率掺入附加水的再生碎砖混凝土强度与和易性满足要求。试验结果表明:强度等级为C10的再生碎砖混凝土抗渗性随着粉煤灰取代率的增加先升高再降低,抗渗等级均可满足农田土渠硬化的要求,且具有较高的强度,和易性良好。最后通过经济性分析得出再生碎砖混凝土比普通混凝土的成本低10%左右,再生碎砖混凝土在农田土渠硬化工程中具有实际应用价值。     

Dominance hierarchies: Observational techniques applied to the study of children at play

The seven major observational techniques generally used in experimental or naturalistic settings are discussed within the context of research on dominance relationships in young children. In particular, findings related to sex differerces and developmental changes in children's social interactions are presented. The observational techniques used in these studies are described in terms of the type of data collected (frequency counts versus measures of duration of the behaviors), and the research questions answerable through use of each technique.Illustrations of other types of data (e.g., from questionnaires) are presented to show how the results relate to the findings from observational data. The use of more than one technique in many of the dominance relationship studies shows how different techniques can be used to examine various facets of complex social interactions.The strengths and weaknesses of each method are discussed within the framework of the illustrative studies. Problems associated with the use of One-zero or Time-Sampling techniques are discussed in detail.     

分布式应用系统结构的一种性能模型

本文给出分布式应用系统结构的一种性能模型，并给出了其形式化描述。     

From training to performance: A case study

Faced with the prospect of engaging nearly two thousand employees before the situation became urgent, this case study shares why the best interests of employees, customers, and shareholders are neither disparate nor a zero‐sum game. It also shows how it takes passionate leadership coupled with proven HPT principles to unlock synergies. Its hallmarks are high employee engagement, quality customer interaction, and improved profitability.     

高性能混凝土工作性及耐久性试验研究

结合福建当地材料,配置C50等级高性能混凝土,研究不同矿物质及掺量对拌合物工作性、劈裂强度及耐久性等影响。试验得到粉煤灰、矿粉在改善拌合物工作性、抗渗性及劈裂强度等指标的较优掺量及影响规律。双掺性能更优,体现了其在粒形级配改善,工作性提高及火山灰效应的复合优势。提出可综合考虑质量变化率、相对动弹性模量及强度等的混凝土损伤程度指标,对混凝土在快速冻融、盐水腐蚀等因素共同作用下的损伤情况进行半定量评估。结果表明,素混凝土损伤程度最大,而矿粉掺量20%和粉煤灰掺量15%的损伤指标明显降低,表明该掺量下混凝土的耐久性能明显提高。     

流体机械性能测试及模拟系统设计

基于LabVIEW软件技术及NI-DAQmx数据采集模块,构建了流体机械工作性能测试及模拟系统。该系统主要实现了流体机械性能参数的数据采集、处理、显示及存储功能,并能够模拟流体机械的串并联工作特性。整个系统操作方便、界面直观,有助于学生自主设计实验,为丰富流体机械的测控系统提供了一种新的思路和切实可行的方案,具有较高的教学实用性。     

Influence of graphite particle size and its shape on performance of carbon composite bipolar plate

INTRODUCTION The proton exchange membrane fuel cell(PEMFC)is a very promising power source for resi-dential and mobile applications due to its attractive features such as high power density,relatively low operating temperature,convenient fuel supply,long lifetime,etc.(Chalk et al.,2000;Lee and Mukergee,1998).Despite these advantages,commercialization of PEMFC is delayed mainly due to the high fabrica-tion cost.Bipolar plates,most commonly used among the PEMFC components,are the gra…     

鲨鱼皮盾鳞结构减阻性能数值模拟

海洋中快速游动的鲨鱼因其皮肤表面具有减少摩擦阻力的肋条状盾鳞结构,从而受到研究学者的广泛关注。为了指导减阻表面的设计与制造,选用数值模拟的方法对鲨鱼皮盾鳞结构的减阻机理进行了研究。通过扫描电子显微镜(scanning electron microscope,SEM)观察,获取鲨鱼盾鳞的微观结构,构建出简化的几何模型单元,并设计了不同分布规则和排布的单元阵列,形成了大面积非光滑仿生结构表面。利用Fluent软件对建立的计算域模型进行流体动力学仿真,同时与传统的矩形沟槽非光滑表面进行比较。数值模拟结果表明:仿生非光滑表面均减少了摩擦阻力,并且相较于矩形沟槽非光滑表面,简化盾鳞结构表面的减阻率明显增加;当入口速度为5 m/s时,对齐排布的简化盾鳞结构表面的减阻率可达到17.86%。最后,从壁面附近区域的流体流速和湍流动能2个方面对非光滑表面的减阻机理进行了分析。研究结果可望对设计和制备具有减阻功能的海洋装备仿生表面起到指导作用。     

学习绩效的内涵及其评价研究

学习绩效评价在整个学习活动中起反馈调节的作用,是每个学习者获取学习信息、改善学习决策、提高学习效率的关键因素。通过对学生学习绩效概念的明确界定,详细论述学生学习绩效所包含的三个维度的内涵与逻辑关系,它为绩效评价的设计与实施提供了新的思路,其价值取向更有利于学生面对真实情境下的问题解决和未来职业生涯规划。