Comparisons of Sidewall Correction of Bed Shear Stress in Open-Channel Flows

When investigating sediment transport in laboratory open-channel flows, it is often necessary to remove sidewall effects for computing effective bed shear stress. Previous sidewall correction methods are subject to some assumptions that have not been completely verified, and different values of the bed shear stress may be obtained depending on the approach used in making sidewall corrections. This study provides a quantitative assessment of the existing correction procedures by comparing them to a new sidewall correction model proposed in this study. The latter was derived based on the shear stress function and equivalent roughness size for both rigid and mobile bed conditions, which were obtained directly from experimental measurements. The comparisons show that the Einstein correction formula and the Vanoni and Brooks method generally predict relatively lower and higher bed shear stresses, respectively, while the Williams’ empirical function leads to more scatter. This study also demonstrates that the widely used Vanoni and Brooks approach can be well approximated by a simple formula derived based on the Blasius resistance function. The sidewall effects, when removed in the different ways, would consequently affect the presentation of the bedload function. Experimental results of bedload transport, when plotted as the dimensionless transport rate against the dimensionless shear stress with the latter being corrected using the present model, exhibit less scatter than those associated with the previous procedures.     

Multiobjective design and control optimization for minimum thermal postbuckling dynamic response and maximum buckling temperature of composite laminates

Design and control optimization is presented to minimize the thermal postbuckling dynamic response and to maximize the buckling temperature level of composite laminated plates subjected to thermal distribution varying linearly through the thickness and arbitrarily with respect to the in-plane coordinates. The total elastic energy of the laminates is taken as a measure of the dynamic response. The optimization control problem is solved under constraints on the laminate thickness and the control energy produced by a transverse dynamic load distributed over the upper surface of the laminate. The constrained control objective is expressed as the sum of the total elastic energy and penalty term involving the control force, which may be considered as a measure of the control energy. The thickness of layers and the fibers orientation angles are taken as optimization design variables. The design and control objectives are formulated based on shear deformation theory accounting for the von-Karman nonlinearity. The displacements are chosen as the sum of time-independent displacements due to the static thermal load and time-dependent displacements due to the initial disturbances and the applied control force. Liapunov–Bellman theory is used to obtain the optimal control force, buckled deflections and controlled elastic energy. Numerical examples are presented for angle-ply antisymmetric laminates with simply supported edges. Graphical studies are carried out to show the advantages of the present design and control procedures.     

DMPP农业应用研究进展

本文对新型硝化抑制剂3,4-二甲基吡唑磷酸盐(简称DMPP)的特性作了介绍,对DMPP提高作物产量,改善品质,提高氮肥利用率。降低氮温室气体的释放,减少NO3-淋失,促进环境保护方面的作用及其影响因素作了评述。     

飞剪的定位控制

针对飞剪剪切完后返回时的定位控制,提出分段直线控制与抛物线控制两种方法,使飞剪以最小的机械冲击力且不产生超调地返回原位。实践证明,用这两种方法进行定位控制,飞剪返回距原位不大于±15°,剪切棒材端面光滑,误差不大于2cm,剪切棒材的成材率大大提高。     

填料对环氧胶粘涂层耐蚀及耐磨性影响

在环氧肢粘涂层中加入不同填料，通过试验研究其对环氧肢粘涂层拉伸剪切强度、耐腐蚀及耐冲蚀磨损性能的影响，由此推荐粉煤灰作为冷敷环氧肢粘涂层填料的最佳含量。     

Sn-8Zn无铅钎料性能的研究

研究了不同微量合金元素(Bi、Ag)对Sn-8Zn无铅钎料高温抗氧化性能及接头剪切强度的影响,采用氧化质量增加△m值的方法,在高温下观察钎料表面氧化膜形状和颜色的变化并对氧化膜进行X射线衍射分析,探讨了钎料的高温抗氧化性能的机理,通过对钎料的金相显微组织观察和对热处理后钎焊接头的剪切强度试验,分析了提高接头剪切强度的原因.试验结果表明:在Sn-8Zn钎料中加入适量的合金元素(Bi、Ag)均可以改善和提高钎料的高温抗氧化性能和接头的剪切强度.     

在线测试技术的现状和发展

以电气接触为手段,在线测试设备除了能够测试制造过程的缺陷和故障以外,还能够测试 PCB的某些电气性能,因而得到了广泛应用。但是,在线测试仪将面临高密度PCB测试的挑战。文章扼要地介绍在线测试技术的原理及未来发展趋势。     

二维切削中切削方向毛刺与亏缺的界限转换条件

建立了二维切削中切削方向毛刺与亏的生成模型,给出了切削方向毛刺与亏缺的判别准则,并对影响切削方向毛刺与亏缺转换的主要因素进行了实验研究和相应的理论分析,提示出切削方向毛刺与亏缺的界限转换条件为：剪切应变ε≤3,为亏缺;3＜ε＜4,为过渡区;ε＞4,为毛刺。并且,切削方向毛刺与亏缺的界限转换条件因切削条件变化而变化。为进一步深入研究切削毛刺的生成与抑制理论提供了科学的基础。     

反应挤出HDPE／PET共混物加工流变性能的研究

为改善聚对苯二甲酸乙二醇酯（ＰＥＴ）的成型加工性能,将高密度聚乙烯（ＨＤＰＥ）与ＰＥＴ进行共混。研究了不同配比的ＨＤＰＥ／ＰＥＴ共混物在不同转速下的剪切流变性能,同时,探讨了作为相容剂的ＥＶＡ、ＥＡＡ对ＨＤＰＥ／ＰＥＴ共混物加工流变性能的影响。     

钢管飞锯机同步传动机构─—浮动式齿轮传动机构的设计

分析了飞锯机同步传动机构的工作原理及结构特点、在固定齿轮齿条传动和固定齿轮浮动齿条传动机构的基础上,设计的浮动式齿轮传动机构结构简单,工作可靠,运行平稳,安装和维修方便。简介了该机构的结构及工作原理。