Dual-band reflective polarization converter based on metasurface

In this paper, a dual-band and reflective polarization converter based on metasurface is proposed. Its unit cell is composed of two layers of metal plates separated by a dielectric substrate. The simulation results show that the proposed converter is able to convert x- or y-polarized incident waves into cross-polarized waves perfectly in frequency bands of 6.75—10.59 GHz and 17.78—19.61 GHz, and the polarization conversion ratio (PCR) is nearly 100%, which can also convert linearly polarized waves into circularly polarized waves at four frequencies. It can be widely used in applications of radar satellites, antenna design and telecommunication with the function of realizing polarization conversion in two bands and achieving high PCR simultaneously.     

抽真空对独立外缸低压模块端汽封间隙的影响分析

以某独立外缸结构汽轮机低压模块为研究对象,对低压内缸及汽封体等部件在抽真空阶段的变形进行了分析和计算。为了理清抽真空阶段所受的重力、真空力、热影响等因素对低压端汽封间隙的影响,分步骤施加以上3种载荷进行有限元分析。结果表明:在重力作用下,低压内缸产生整体下沉两头上翘的变形;在真空力作用下,端汽封随汽缸进一步下沉,底部间隙变大;在投低压缸喷水情况下,高温轴封蒸汽对端汽封及低压内缸加热,端汽封随汽缸整体抬升,底部间隙减小;在真空力、热影响联合作用下,端汽封底部间隙减小0.061~0.105 mm;在投入喷水情况下,整个抽真空阶段对低压端汽封间隙的影响并不大。     

异步电机低速电制动控制

阐述了异步电机电制动的控制策略;分析实际应用中低速电制动转矩出现偏差的原因,并通过MATLAB绘制出当定向角度出现偏差时给定转矩与实际转矩在三维空间中的比例关系。为解决低速电制动时由于转子磁场定向偏差引起电机转矩不准的问题,采用基于无功功率模型的转子时间常数自适应控制进行校正。试验结果表明该控制策略可以实现校正转子磁场定向偏差的功能,具有良好的动态性能,可以满足地铁列车的性能要求。     

我国城市夜间光环境多尺度时空演变特征分析

本文以全国省、市级区域1992—2020年夜间灯光为研究对象,基于DMSP和VIRS遥感影像以及城市行政区域矢量数据,采用一元线性回归分析方法构建灯光演变趋势,从时间和空间两个维度,并结合全国、分区和城市尺度对中国夜间灯光时序变化特征进行分析。结果表明:(1)研究年份内灯光增加多集中在东部沿海区域以及中部省会中心城市,灯光减少区域多集中在山西、陕西等中西部城市;(2)在中、东、西部时空变化中,东部区域增长区和高亮度区域面积占比最高,西部最低;(3)在19个典型沿海城市中,上海等东南部城市灯光增长最快且亮度最高,灯光增速和亮度值由高到低排列顺序为由南到北。     

Cooperation and profit allocation in two-echelon logistics joint distribution network optimization

Collaborative two-echelon logistics joint distribution network can be organized through a negotiation process via logistics service providers or participants existing in the logistics system, which can effectively reduce the crisscross transportation phenomenon and improve the efficiency of the urban freight transportation system. This study establishes a linear optimization model to minimize the total cost of two-echelon logistics joint distribution network. An improved ant colony optimization algorithm integrated with genetic algorithm is presented to serve customer clustering units and resolve the model formulation by assigning logistics facilities. A two-dimensional colony encoding method is adopted to generate the initial ant colonies. Improved ant colony optimization combines the merits of ant colony optimization algorithm and genetic algorithm with both global and local search capabilities. Finally, an improved Shapley value model based on cooperative game theory and a cooperative mechanism strategy are presented to obtain the optimal profit allocation scheme and sequential coalitions respectively in two-echelon logistics joint distribution network. An empirical study in Guiyang City, China, reveals that the improved ant colony optimization algorithm is superior to the other three methods in terms of the total cost. The improved Shapley value model and monotonic path selection strategy are applied to calculate the best sequential coalition selection strategy. The proposed cooperation and profit allocation approaches provide an effective paradigm for logistics companies to share benefit, achieve win–win situations through the horizontal cooperation, and improve the negotiation power for logistics network optimization.     

改进的BP神经网络在双塔水库水质预测中的应用

疏勒河下游的瓜州绿洲水资源形成困难、水环境脆弱,属国家极端干旱荒漠自然保护区.研究利用加入动量项的BP神经网络并基于时间序列对1993年至2008年双塔水库水质指标年均值进行模拟和预测,确定模型参数为:输入节点数2,输出节点数1,隐含层数2,最小训练速率0.1,动态参数0.6,SIGMOID函数调整值0.9,允许误差0.0001,最大迭代次数10000.模型拟合相对误差值小于5％,预测检验误差小于10％.根据预测结果,水库在2009年至2013年属Ⅱ类水质,水质符合生活以及农业灌溉用水标准,但仍存在富营养化的风险.     

集中控制技术在多媒体系统中的应用

文章首先针对集中控制技术的应用背景以及功能特征展开了描述,而后对集中控制系统的构建原则以及注意事项进行了讨论,最后给出了集中控制系统的典型结构分析示意图,并对整个系统中的工作流程和数据走向做出了必要分析。     

配方组成对UPVC异型材耐候性的影响

从配方角度研究不同UPVC异型材的耐候性，探讨了不同原料和助剂（PVC、稳定剂、钛白粉、冲击改性剂、CaCO3等）对异型材老化和长期耐候性的影响，并提出优化配方的要点。     

试谈Flash动画制作中的创新技巧

随着科技的不断进步和网络技术的逐年提高,Flash动画这一信息承载媒体已经成为当前大家所公认的传输载体,它在社会的各个角度都以其独特的艺术表现形式展现了较大的影响力.Flash动画的制作是采用矢量图形和流媒体播放技术,它上手易、精通难,一旦掌握了其内在制作的精髓,其形态变化万千、充满着感染力的动画便“跃然纸上”,通过显示器表达着它独特的魅力.     

Influence of nucleation seeding on the performance of carbonated MgO formulations

The continuation of the hydration and carbonation reactions within reactive MgO cement formulations is inhibited by the formation of hydrate and carbonate phases around MgO particles, resulting in a low MgO utility and limited mechanical performance. This study introduces carbonate seeds into the pore space of MgO-based concrete mixes to enable the nucleation and growth of carbonates on the seed surfaces. The influence of seeds on the hydration and carbonation capability, mechanical performance and microstructural development was evaluated through isothermal calorimetry, water absorption and compressive strength measurements, along with TGA, XRD and SEM analyses. The introduction of ≤1% seed within the initial mix design increased the carbonate phase content and improved carbonation degree by up to 96% by increasing the availability of Mg(OH)₂ for carbonation. The dense formation of carbonates in seeded samples enabled improved microstructures and 28-day strengths of 64 MPa, which were 33% higher than unseeded samples.