Effect of attack angle on the electromagnetic wave transmission characteristics in the hypersonic plasma sheath of a re-entry vehicle

The attack angle may greatly affect the hypersonic plasma sheaths around the re-entry vehicle,thereby affecting the transmission characteristics of electromagnetic(EM)waves in the sheaths.In this paper,we propose an integrated three-dimensional(3D)model with various attack angles and realistic flying conditions of radio attenuation measurement C-II(RAM C-II)re-entry tasks for analyzing the effect of the attack angle on the transmission characteristics of EM waves in the sheaths.It is shown that the electron density and collision frequency of the sheath on the windward side can be increased by an order of magnitude with the increase of the attack angle.Meanwhile,the thickness of the sheath on the leeward side is increased where the electron density and collision frequency are reduced.The EM waves are mainly reflected on the windward plasma sheath due to the cutoff effect,and the radio-frequency(RF)blackout is mitigated if the antenna is positioned on the leeward side.Thus,by planning the trajectory properly and installing the antenna accordingly during the re-entry,it is possible to provide an approach for mitigation of the RF blackout problem to an extent.

Scanning transmission electron microscopy: A review of high angle annular dark field and annular bright field imaging and applications in lithium-ion batteries

Scanning transmission electron microscopy（STEM） has been shown as powerful tools for material characterization,especially after the appearance of aberration-corrector which greatly enhances the resolution of STEM. High angle annular dark field（HAADF） and annular bright field（ABF） imaging of the aberration-corrected STEM are widely used due to their high-resolution capabilities and easily interpretable image contrasts. However, HAADF mode of the STEM is still limited in detecting light elements due to the weak electron-scattering power. ABF mode of the STEM could detect light and heavy elements simultaneously, providing unprecedented opportunities for probing unknown structures of materials. Atomiclevel structure investigation of materials has been achieved by means of these imaging modes, which is invaluable in many fields for either improving properties of materials or developing new materials. This paper aims to provide a introduction of HAADF and ABF imaging techniques and reviews their applications in characterization of cathode materials, study of electrochemical reaction mechanisms, and exploring the effective design of lithium-ion batteries（LIBs）. The future prospects of the STEM are also discussed.

Modification Method of Lightning Shielding Angle Calculation for Transmission Lines in Mountain Areas

Shielding angle is one of the main factors influencing lightning performance of transmission lines,which always stays in the focus of the design and the evaluation of lightning protection.A formula for the improved shielding angle is proposed for evaluating the lightning performance in different terrains.The digital elevation model(DEM) is used to obtain the micro-topography data,such as the slope gradient,slope aspect,etc.The following results are obtained by analyzing the influence of topography factors on the improved shielding angle:(1) improved shielding angle non-linearly increases with the increase of the slope gradient and the slope aspect,(2) improved shielding angle is more sensitive to the slope gradient than to the slope aspect,(3) the improved shielding angle in the mountain terrains is much greater than the designed shielding angle.This may be the reason why the designed shielding angle is limited into the rational range,while the shielding faults occur frequently.

传动直丝弓矫治器治疗AngleⅡ^1类错(牙合)的头影测量分析

目的采用传动直丝弓技术治疗青少年Angle Ⅱ1类错牙合,通过头影测量分析评价患者的矫治效果。方法选择40例(女24例,男16例)传动直丝弓技术矫治的Angle Ⅱ1类错牙合患者,平均年龄13.2岁。经X线头影测量分析,比较治疗前后软硬组织变化,对所得数据采用SPSS 19.0统计软件进行统计学处理。结果患者平均治疗时间12个月,治疗后均获得良好的覆牙合、覆盖及中性磨牙关系,侧貌也得到明显的改善。头影测量显示,U1-SN角、ANB角、U1-RL距、U6-RL距变小,L1-MP角、L1-RL距、L6-RL距变大,其他无显著变化。结论传动直丝弓矫治器利用口内支抗结合细丝轻力,快速倾斜移动牙齿,后期控制转矩并正轴,可快速有效矫治Angle Ⅱ1类错牙合。

Growth Kinetics of Nanocrystals and Nanorods by Employing Small-angle X-ray Scattering (SAXS) and Other Techniques

In this article, we report the results of our detailed investigations of the growth kinetics of zero-dimensional nanocrystals as well as one-dimensional nanorods by the combined use of small angel X-ray scattering （SAXS）, transmission electron microscopy （TEM） along with other physical techniques. The study includes growth kinetics of gold nanocrystals formed by the reduction of HAuCl4 by tetrakis（hydroxymethyl） phosphonium chloride in aqueous solution, of CdSe nanocrystals formed by the reaction of cadmium stearate and selenium under solvothermal conditions, and of ZnO nanorods formed by the reaction of zinc acetate with sodium hydroxide under solvothermal conditions in the absence and presence of capping agents. The growth of gold nanocrystals does not follow the diffusion-limited Ostwald ripening, and instead follows a Sigmoidal rate curve. The heat change associated with the growth determined by isothermal titration calorimetry is about 10 kcal·mol^-1 per I nm increase in the diameter of the nanocrystals. In the case of CdSe nanocrystals also, the growth mechanism deviates from diffusion-limited growth and follows a combined model containing both diffusion and surface reaction terms. Our study of the growth kinetics of uncapped and poly（vinyl pyrollidone） （PVP）-capped ZnO nanorods has yielded interesting insights. We observe small nanocrystals next to the ZnO nanorods after a lapse of time in addition to periodic focusing and defocusing of the width of the length distribution. These observations lend support to the diffusion-limited growth model for the growth of uncapped ZnO nanorods. Accordingly, the time dependence on the length of uncapped nanorods follows the L3 law as required for diffusion-limited Ostwald ripening. The PVP-capped nanorods, however, show a time dependence, which is best described by a combination of diffusion （L^3） and surface reaction （L^2） terms.

Approximate Calculation of Oblique Wave Transmission from A Single-Row of Rectangular Piles

Transmissions of oblique incident wave from a row of rectangular piles are analyzed theoretically. The incident angle of plane wave is taken as g = 90° , there then is the transmission coefficient ｜T｜ = 1 （This is a paradox）. In this paper, by means of the approximate relation between the transmitted and incident wave angle found from the shape of a slit, the paradoxical phenomenon is removed. On the basis of the continuality of the pressure and flux and the analysis of flow resistance at the row of rectangular piles, formulas of reflection and transmission coefficients are obtained. The transmission and reflection coefficients predicted by the present model quite agree with those of laboratory experiments in previous references

NOVEL METHOD OF REALIZING THE OPTIMAL TRANSMISSION OF THE CRANK-AND-ROCKERMECHANISM DESIGN

A novel method of realizing the optimal transmission of the crank-and-rockermechanism is presented. The optimal combination design is made by finding the related optimaltransmission parameters. The diagram of the optimal transmission is drawn. In the diagram, therelation among mini-mum transmission angle, the coefficient of travel speed variation, theoscillating angle of the rocker and the length of the bars is shown, concisely, conveniently anddirectly. The method possesses the main characteristic. That it is to achieve the optimaltransmission parameters under the transmission angle by directly choosing in the diagram, accordingto the given requirements. The characteristics of the mechanical transmission can be improved togain the optimal transmission effect by the method. Especially, the method is simple and convenientin practical use.

Mechanics Analysis of Overhead Transmission Lines Based On-line Monitoring

At present, the on-line monitoring is widely applied to the power line monitoring. In this paper, a new mechanical calculation model is established according to the on-line monitoring. And this model is based on the parameters that tension sensors and angle sensors on suspended points detect, and combines with the parameters of the wire itself, and also considers the deflection angel of wires due to wind. In this model, mechanics parameters of wires are turned into the new coordinate plane after deflection angel of wires due to wind, or windage yaw plane. A statics tension balance equation is built in the vertical direction of the new windage yaw plane. According to the theoretical analysis and algorithm, we verify the accuracy of this newly developed mechanical calculation model.

基于波形群灵敏角特征的输电线路故障单端行波辨识与测距

针对输电线路单端行波测距难以可靠、有效辨识第二个波头的不足,该文提出基于波形群灵敏角检测的单端行波辨识与测距方法。首先,对线路故障波过程的解析和规律分析,发现波头局部波形呈现灵敏角和非轴对称性的特点。进而提出利用Radon变换变角度投影检测最灵敏角并判别最灵敏角两侧非轴对称特性,实现单端故障初始行波及多次同极性行波群的可靠辨识。最后,基于多次同极性行波到达时差构造多测距算式,输出最合理测距结果。所提方法将时域波头辨识转化为波形图像空间域最灵敏变化方向和几何轴对称性的检测,抗干扰能力强,大幅提升检测可靠性和后续波头的识别能力,提升单端测距算式冗余性。大量实测和仿真数据测试表明,算法有效、可靠、直观,适用范围广,为单端行波分析故障测距及扩展应用提供了新思路。

无伴随运动2PRU-PRUR并联机构高维多目标优化设计

设计了一种无伴随运动的面对称一移动两转动2PRU-PRUR并联机构,应用旋量理论分析了机构自由度和运动特性。将末端执行器输出轴设置为偏置形式,可提高姿态能力。采用球面姿态角(方位角和倾摆角)描述该输出轴姿态,并推导出机构逆向位置符号解。以旋量理论为数学工具,给出局部和全域传递性能指标的定义和计算方法。在机构性能评价和工作空间分析的基础上,建立了机构尺度参数昂贵约束高维多目标优化模型,并采用多目标进化算法求解该问题。同时,为提高高维多目标优化算法在解决此模型时的性能,提出了一种矢量角高维多目标差分进化(Vector-angle-based many-objective Differential Evolution,VaDE)算法,可综合平衡个体的收敛性和分布性。应用VaDE求解机构高维多目标优化问题,得到多组Pareto备选解。研究结果可为机构尺度综合和实际应用提供参考。

导线倾角与覆冰过程扭转因素对覆冰形状特性影响研究

输电导线覆冰严重威胁电网运行安全,准确预测导线覆冰形状和增长趋势,可以有效预防次生灾害的发生.已有输电导线覆冰增长预测模型较少考虑覆冰偏心扭转对导线表面覆冰形状的影响.为此,综合考虑覆冰偏心、风速等对导线的耦合扭转特性,建立输电导线动态扭转覆冰预测模型.基于预测模型对导线表面覆冰形状进行预测模拟并与文献实验数据对比,验证了模型的有效性.利用预测模型,进一步讨论导线倾角及温度、风速和MVD(水滴中值直径)对冰形和扭转速度的影响.研究发现,导线迎风面覆冰厚度随着倾斜角度的增加,分层现象逐渐明显.与水平布置(零倾角)导线对比,倾斜角度为60°时,导线覆冰面积分别增加18.26%(覆冰125 min)、26.30%(覆冰245 min),扭转角度分别增加10.4°(覆冰125 min)、16.2°(覆冰245 min),冰形呈“沟壑”状.

频率可调谐的双偏振非对称传输器件

为了研究太赫兹超构材料的可调谐非对称传输特性,本文基于相变原理提出一种类S型的双层结构,采用有限积分算法求解其偏振传输特性。利用温度调控二氧化钒的相变特性,实现了频率可调谐的双偏振非对称传输效应。在宽角度入射范围内,45°线偏振光的非对称传输特性保持着较高的工作效率,倾斜入射角为60°时,效率优于0.4,且绝缘态和金属态下的谐振频率几乎保持不变,谐振频率的可调谐度为16.3%。本文所提出的频率可调谐双偏振非对称传输器件在偏振调控、太赫兹通信和太赫兹芯片领域具有潜在的应用前景。

盐渍土环境下考虑耐久性损伤的输电塔风振时程分析

长期服役于盐渍土环境下的输电塔耐久性损伤较为严重,当其材料力学性能退化到一定程度时,在强风作用下,输电塔可能会发生破坏,危害线路的安全运行。基于±800kV天中线特高压直流输电工程直线塔,考虑基础钢筋锈蚀、基础混凝土侵蚀、输电塔杆塔钢材锈蚀以及基础箍筋的约束效应,在ABAQUS有限元软件中建立塔-线-基础耦合模型,研究不同风攻角和材料耐久性损伤对塔身动力响应的影响。结果表明:90°风攻角下塔身顺风向位移最大,为最不利风向基本工况;90°风攻角下输电塔塔顶顺风向位移响应贡献主要以背景分量为主,但共振分量也有一定贡献,主要以输电塔1阶振型频率为主,且随着服役时间的增加,振型频率逐渐降低,相应响应的贡献程度逐渐提高;相同风荷载作用下,随着服役时间的增加,塔身位移逐渐增大,相同服役时间下,随着塔身高度的增加,塔身位移响应峰值增长速度变快,结构振动更加明显,材料耐久性损伤对输电塔的位移响应影响较大。

区域新能源并网暂态功角稳定分析与紧急控制策略优化

随着新型电力系统发展和新能源机组不断投产,南方区域将形成多区域新能源与邻近常规电源联合集中接入系统的场景。新能源集中接入常与常规电源共用输电通道,将影响区域系统暂态稳定特性及输送通道的传输能力,使南方区域系统的安全稳定运行和风险防控面临了前所未有的挑战。为深入研究区域新能源送出的稳定特性建立了区域新能源交流集中送出的等值系统模型,分析了新能源并入对区域系统对暂态稳定特性影响的主要机理。基于等面积法则对区域送出系统暂态过程的加、减速面积进行了定量分析,提出了紧急控制切机策略的协调优化方法,实现区域送出传统机组与新能源机组紧急控制切机量及控制代价的最小化的切机策略配置。最终,基于广东区域海风火电打捆送出系统为例,对分析结果的正确性进行了验证。

输电塔钢管角钢组合节点承载能力模拟分析与计算方法研究

依托输电线路实际工程,选取输电铁塔主材与横担连接处钢管角钢组合节点为研究对象,采用数值软件ABAQUS建立了钢管角钢组合节点精细化有限元模型,并通过静力加载试验验证了有限元模型的合理性;随后模拟分析了组合节点的传力机理和应力分布,明确了不同参数对组合节点受力特性的影响规律。结果表明,钢管、中环板与竖向板相交区域出现应力集中且最大应力达到屈服强度,并向四周扩散且逐渐减小,中环板和竖向板的厚度是影响组合节点极限承载力的关键因素。采用有效宽度法计算了中环板和竖向板的承载能力,建立了不同板件的内力占比计算式,基于此,提出了组合节点承载力计算方法。所提出的极限承载力计算方法与数值模拟结果的误差在13%以内,验证了计算方法的合理性,可为钢管角钢组合节点的结构设计与分析提供参考。

双摆线针齿传动齿廓特性与啮合特性分析

将双摆线齿廓运用到NN型少齿差行星齿轮传动领域,通过分析内外摆线轮齿廓的凹凸性、曲率和曲率半径,获得了内、外摆线齿廓的特性;通过图解法推导双摆线针齿传动中内、外摆线轮的压力角计算公式,获得了啮合过程压力角变化规律,以及双摆线针齿主要参数对于内、外摆线轮压力角的影响;通过对等距修形和移距修形进行研究,表明齿廓修形对齿形有明显影响。

基于多变量设计的打纬筘座急回特性分析及优化

为改进筘座后摆急回特性,通过打纬原理分析与机构学推演,提出了一种针对牵手栓最大曲线位移S_(m)的优化数学模型。利用构件长度(l_(c)、l_(h)、l_(r)、l_(f))和曲柄转角α构建多变量设计空间,实现了短牵手四连杆打纬的参数化功能模型开发,并通过机构学仿真对其运动位姿与质心轨迹进行分析验证。实例研究结果表明,曲柄长度l_(c)和转角α对目标函数F(X)的敏感度影响更大,多变量(DV_1、DV_2、DV_3)耦合可有效增强打纬筘座急回特性的设计可塑性;当牵手栓最大曲线位移Sm增大至71.41 mm时,极位夹角θ和行程速比系数K分别被提高了10.96°和0.131,筘座急回运动效果得到明显提升。优化结果符合杆长条件和机构传动要求,有助于四连杆打纬系统的设计创新和性能改进。

基于PMU的特高压直流输电系统孤岛运行控制方法

受特高压(Ultra-High Voltage,UHV)直流输电系统自身运行特点的影响,孤岛运行阶段的环流难以实现快速维稳,提出基于电源管理单元(Power Management Unit,PMU)的特高压直流输电系统孤岛运行控制方法。考虑输出电压的角度会受电压源换流器(Voltage Sourced Converters,VSC)的影响发生瞬时改变,提出有功-相角下垂控制方法,逆变器通过微型PMU的同步信号,确定派克变换的参考坐标系,将特高压直流输电系统孤岛运行阶段的目标维持频率作为控制基准。以调整特高压直流输电系统中所有分布式发电装置(Distributed Generation,DG)的相角参考值处于一致状态为导向,结合微型PMU测得的公共母线电压相角参数,控制有功补偿量。在测试结果中,环流达到0值后出现的偏离情况程度稳定在10.0 VA以内,并在测试后的0.8 s内达到0稳态。

输电直线杆塔风灾易损性的准静态分析方法

如何合理并高效地评估既有输电杆塔的抗风承载性能及安全裕度,对电网的风灾预测及运维管理具有重要的意义。该文提出一种考虑风向效应杆塔结构易损性评估的高效实用方法。首先基于风工程原理,采用准静态分析框架推导给出不同设计风速下的杆塔整体风荷载的极值概率分布;随后综合考虑输电塔结构参数的不确定性及初始缺陷,通过非线性静力推覆分析(nonlinear static pushover analysis,NSPA)考察杆塔结构的抗风承载能力性能及对应的破坏准则;进而依据随机抽样分析得到杆塔结构的抗风承载力的概率模型;最后采用极限状态函数计算不同风向角下的杆塔风灾易损性概率曲线。研究表明,基于首次超越概率理论可以方便得到准静态分析框架下的整体风效应概率分布;准静态的基底剪力响应不受结构非线性状态影响,可作为整体风荷载作用的评价指标;基于构件屈服破坏与整体破坏内在联系,可建立合理的轻微、严重和倒塌的破坏准则;风向角对杆塔的抗风承载性能和易损性均有着显著影响,杆塔在60°风向角下的易损概率最高。

取石网篮扭转传递性能测试装置的研发及应用

为精准评估取石网篮在体内取石时的扭转传递性能,模拟其在人体屈曲腔道内的取石效能,设计并研制了专门的扭转传递性能测试装置。测试过程中,为确保结果准确性,通过试验优选20圈、转速为50 r/min的参数和重型夹具进行试验,并结合三种弯曲通路,对取石网篮扭转传递性能进行体外模拟和定量测试。测试结果表明:当取石网篮扭转速度从50 r/min提高到100 r/min时,扭矩下降了37.6%;使用重型夹具进行测试时,扭转角传递系数的测试结果更加稳定。对比1145 mm和650 mm长度的取石网篮,前者在的扭矩与扭转角传递系数上分别降低15%和10%。360°弯曲通路下的扭矩和扭转角传递系数相较于伸直状态分别降低38%和14%。该测试装置能精确测量和表征取石网篮在不同条件下的扭转传递性能,可为网篮产品的进一步研发和临床应用提供定量化评价参考。