The Effects of the Longitudinal Axis of Loading upon Bending, Shear and Torsion of a Thin-Walled Cantilever Channel Beam

Three aluminium channel sections of US standard extruded dimension are mounted as cantilevers with x-axis symmetry. The flexural bending and shear that arise with applied axial torsion are each considered theoretically and numerically in terms of two longitudinal axes of loading not coincident with the shear centre. In particular, the warping displacements, stiffness and stress distributions are calculated for torsion applied to longitudinal axes passing through the section’s centroid and its web centre. The stress conversions derived from each action are superimposed to reveal a net sectional stress distribution. Therein, the influence of the axis position upon the net axial and shear stress distributions is established compared to previous results for each beam when loading is referred to a flexural axis through the shear centre. Within the net stress analysis is, it is shown how the constraint to free warping presented by the end fixing modifies the axial stress. The latter can be identified with the action of a ‘bimoment’ upon each thin-walled section.

Monitoring of Real-Time Complex Deformed Shapes of Thin-Walled Channel Beam Structures Subject to the Coupling Between Bi-Axial Bending and Warping Torsion

Structural health monitoring(SHM)is a research focus involving a large category of techniques performing in-situ identification of structural damage,stress,external loads,vibration signatures,etc.Among various SHM techniques,those able to monitoring structural deformed shapes are considered as an important category.A novel method of deformed shape reconstruction for thinwalled beam structures was recently proposed by Xu et al.[1],which is capable of decoupling complex beam deformations subject to the combination of different loading cases,including tension/compression,bending and warping torsion,and also able to reconstruct the full-field displacement distributions.However,this method was demonstrated only under a relatively simple loading coupling cases,involving uni-axial bending and warping torsion.The effectiveness of the method under more complex loading cases needs to be thoroughly investigated.In this study,more complex deformations under the coupling between bi-axial bending and warping torsion was decoupled using the method.The set of equations for deformation decoupling was established,and the reconstruction algorithm for bending and torsion deformation were utilized.The effectiveness and accuracy of the method was examined using a thin-walled channel beam,relying on analysis results of finite element analysis(FEA).In the analysis,the influence of the positions of the measurement of surface strain distributions on the reconstruction accuracy was discussed.Moreover,different levels of measurement noise were added to the axial strain values based on numerical method,and the noise resistance ability of the deformation reconstruction method was investigated systematically.According to the FEA results,the effectiveness and precision of the method in complex deformation decoupling and reconstruction were demonstrated.Moreover,the immunity of the method to measurement noise was proven to be considerably strong.

A Non-Stationary Beam-Enabled Stochastic Channel Model and Characterization over Non-Reciprocal Beam Patterns

The multiple-input multiple-output(MIMO)-enabled beamforming technology offers great data rate and channel quality for next-generation communication.In this paper,we propose a beam channel model and enable it with time-varying simulation capability by adopting the stochastic geometry theory.First,clusters are generated located within transceivers'beam ranges based on the Mate?rn hardcore Poisson cluster process.The line-of-sight,singlebounce,and double-bounce components are calculated when generating the complex channel impulse response.Furthermore,we elaborate on the expressions of channel links based on the propagation-graph theory.A birth-death process consisting of the effects of beams and cluster velocities is also formulated.Numerical simulation results prove that the proposed model can capture the channel non-stationarity.Besides,the non-reciprocal beam patterns yield severe channel dispersion compared to the reciprocal patterns.

Theory of Flexural Shear, Bending and Torsion for a Thin-Walled Beam of Open Section

Aspects of the general Vlasov theory are examined separately as applied to a thin-walled channel section cantilever beam under free-end end loading. In particular, the flexural bending and shear that arise under transverse shear and axial torsional loading are each considered theoretically. These analyses involve the location of the shear centre at which transverse shear forces when applied do not produce torsion. This centre, when taken to be coincident with the centre of twist implies an equivalent reciprocal behaviour. That is, an axial torsion applied concentric with the shear centre will twist but not bend the beam. The respective bending and shear stress conversions are derived for each action applied to three aluminium alloy extruded channel sections mounted as cantilevers with a horizontal principal axis of symmetry. Bending and shear are considered more generally for other thin-walled sections when the transverse loading axes at the shear centre are not parallel to the section = s centroidal axes of principal second moments of area. The fixing at one end of the cantilever modifies the St Venant free angular twist and the free warping displacement. It is shown from the Wagner-Kappus torsion theory how the end constrained warping generates an axial stress distribution that varies with the length and across the cross-section for an axial torsion applied to the shear centre. It should be mentioned here for wider applications and validation of the Vlasov theory that attendant papers are to consider in detail bending and torsional loadings applied to other axes through each of the centroid and the web centre. Therein, both bending and twisting arise from transverse shear and axial torsion applied to each position being displaced from the shear centre. Here, the influence of the axis position upon the net axial and shear stress distributions is to be established. That is, the net axial stress from axial torsional loading is identified with the sum of axial stress due to bending and axial stress arising from constrained warping displacements at the fixing. The net shear stress distribution overlays the distributions from axial torsion and that from flexural shear under transverse loading. Both arise when transverse forces are displaced from the shear centre.

Spatial Correlation Characteristics Analysis of Multi-Beam Channels of Mobile Satellite System

Due to the influence of scatterers around the receiving antenna, the multipath signal in satellite mobile communication systems is correlated with each other which would influence the system performance. There is no systematic standard on the channel modelling of the wideband satellite channel at present, so the study of the modelling of the wideband satellite channel is of great importance. In this paper, firstly we created a multi-beam model which can figure out the antenna gain of the nth component beam. Secondly, we combined the characteristics of multi-beam satellite channel and the distribution of the scatterers, and set up a three-dimension random channel model. This model is more realistic for satellite communication system since it considers the height of scatterers. According to the channel models, we had the formula of spatial correlation coefficient. We used the formula to calculate the relationship between spatial correlation coefficient and the interval of antennas. The result shows that the spatial correlation exists and cannot be ignored while modeling for mobile satellite system.

Wave growth rate in a cylindrical metal waveguide with ion-channel guiding of a relativistic electron beam

This paper addresses the formulae and numerical issues related to the possibility that fast wave may be grown when a relativistic electron beam through an ion channel in a cylindrical metal waveguide. To derive the dispersion equations of the beam-wave interaction, it solves relativistic Lorentz equation and Maxwell＇s equations for appropriate boundary conditions. It has been found in this waveguide structure that the TM0m modes are the rational operating modes of coupling between the electromagnetic modes and the betatron modes. The interaction of the dispersion curves of the electromagnetic TM0m modes and the upper betatron modes is studied. The growth rates of the wave are obtained, and the effects of the beam radius, the beam energy, the plasma frequency, and the beam plasma frequency on the wave growth rate are numerically calculated and discussed.

深厚冲积层薄基岩条件下沿空留巷支护技术研究

为解决某矿16041工作面原支护方案沿空留巷变形大、破坏严重难题,提出了顶板采用“16#槽钢梁+注浆锚索”、采空区侧采用“36U型钢+注浆锚杆”不对称耦合支护优化方案,基于理论分析研究围岩结构破坏形式,通过单元荷载法计算了留巷顶板与采空区侧围岩应力,最后开展工业性应用,验证优化后方案的合理性。研究表明:优化后支护方案顶板和采空区侧计算单元面荷载分别为351.85 kPa和206.53 kPa;注浆锚索和注浆锚杆承载力分别为263.89 kN和199.08 kN,均小于注浆锚索(锚杆)极限承载力。根据对留巷数据监测,最大顶板下沉量为35.6 mm,较原支护方案减小了1.40倍;采空区侧最大移近量为29.6 mm,与较原支护方案减小了4.13倍;最大注浆锚索承载力为253 kN,与理论计算值误差为4.1%;最大注浆锚杆承载力为174 kN,与理论计算值误差为12.6%。位移量与支护承载力均在工程允许范围内,优化后方案支护效果良好。

装配式槽钢-混凝土组合梁受弯性能研究

为研究采用螺栓连接的装配式槽钢-混凝土组合梁(PSCCB)的受弯性能,制作了7个不同宽跨比、梁高和剪切连接间距的PSCCB试件,对其进行四点弯曲测试。基于ABAQUS软件建立PSCCB有限元模型,通过试验结果验证模型的可靠性,并分析螺栓间距、螺栓预紧力、抗剪连接件间距、剪跨比、宽跨比、槽钢强度和混凝土强度对PSCCB受弯性能的影响。试验结果表明:高强度螺栓能确保PSCCB具有较高的整体性和良好的强度和延性;随着宽跨比增大,PSCCB抗弯承载力和延性分别提高8%~19%和11%~21%;穿孔钢板剪力连接件在槽钢和混凝土板之间提供高效的组合作用,部分抗剪连接与完全抗剪连接PSCCB的延性、强度、刚度差异少于10%;与梁高320 mm的PSCCB相比,梁高400 mm的PSCCB的抗弯承载力提高96%,但延性降低10%。分析结果说明:有限元分析结果与试验结果较吻合,所建立的模型能准确反映PSCCB的受弯性能;随着螺栓间距和预紧力的增大,PSCCB受弯承载力和初始刚度基本保持不变;适当增加抗剪连接程度和宽跨比能有效提升PSCCB的受弯承载力和初始刚度;增加槽钢强度能显著提高PSCCB的受弯承载力,而混凝土强度变化对其受弯承载力影响较小。根据试验和有限元参数分析,提出PSCCB受弯性能设计方法,受弯承载力和跨中挠度计算值与试验及有限元分析结果吻合良好,可为PSCCB的工程设计提供参考。

腹板高度对简支槽型梁力学特性的影响

以阿联酋铁路二期项目B标段跨度48.5 m混凝土简支槽型梁为工程背景,针对采用变截面适应大跨度、重载的受力需求,讨论了腹板高度变化对该重载铁路槽型梁空间变形、道床板双向受力和剪力滞效应等空间力学行为的影响规律。利用Midas FEA NX软件建立了4种不同变截面参数的槽型梁空间实体有限元模型,分析了自重+二期恒载、预应力、单线和双线列车荷载等工况。结果表明:增加两侧腹板的高度,可降低槽型梁道床板的竖向挠度,但会加大腹板上翼缘内倾变形;预应力荷载作用下情况相反。对于槽型梁道床板受力而言,腹板高度越高可降低道床板的纵向应力,但对横向应力无明显影响。在竖向荷载作用下,越靠近跨中位置,剪力滞效应越明显。在预应力荷载作用下,腹板高度变化对剪力滞效应的影响取决于预应力筋布置位置;当仅布置在腹板内时,剪力滞效应不受腹板高度影响。

可重构智能超表面在卫星导航系统中的应用展望

高中低轨复杂大型星座是下一代全球卫星导航系统(GNSS)的发展方向,尤其是低轨卫星的大量部署,对卫星段和地面段的波束扫描天线在成本、重量及功耗等方面提出了更高要求,传统的电扫相控阵天线和机械扫描天线均无法较好地满足需求。近些年,可重构智能超表面(RIS)因其可低成本、低功耗地控制波束传播方向而被极大关注,并在下一代移动通信中被视为能够提升网络容量和覆盖范围的主要关键技术之一,利用RIS来解决卫星导航系统中的天线问题具有较大的潜力,但同时也面临诸多挑战。基于下一代卫星导航系统的发展需求,本文简要介绍了RIS的基本原理及目前的发展现状,重点分析了RIS在卫星导航系统中的应用前景,并提出了需要解决的核心关键技术。

基于射束通道和生成对抗网络的调强放射治疗剂量预测方法

目的基于射束通道实现调强放射治疗(放疗)剂量的精准预测。方法选取行调强放疗的鼻咽癌Ⅲ期患者137例。随机抽取102例为训练集,24例为验证集,11例为测试集。基于Pix2pix模型,引入剂量预计算结果作为射束通道,预测患者个体化放疗剂量分布。采用剂量体积直方图(DVH)、处方剂量参数和计划综合评分对预测结果进行评估。结果11个测试病例的预测剂量和人工计划剂量综合评分分别为70/60、40/50、10/30、70/60、70/60、80/80、80/70、70/60、60/70、80/60和60/60。随机选取4个测试病例,DVH显示,模型对脑干和脊髓的预测剂量曲线整体位于人工计划剂量曲线的左侧,而对眼球和腮腺的预测剂量曲线整体位于人工计划剂量曲线的右侧。结论基于射束通道和Pix2pix模型,可对鼻咽癌放疗患者进行有效剂量预测,从而为放射物理师的计划设计目标提供参考。

利用均匀密度等离子体通道加速质子的辐射压力增强方案

提出了辐射压力加速与激光尾波场加速相结合的高能质子加速方案.该方案中在临界密度薄靶后端增加一个均匀密度等离子体通道获得了峰值能量为22.2 GeV、截止能量为36.4 GeV和电荷量为0.67 nC的高能质子束,与质子在纯辐射压力加速中相比,其高能质子的截止能量可以提升2个数量级.结果证实了通过在近临界密度薄靶后面连接一个均匀密度等离子体通道,等离子体通道中激光激发的尾波场可以捕获经辐射压力过程预加速的质子并维持长时间的加速,最终获得高能质子,本文还研究了不同均匀密度等离子体通道中质子的加速情况,发现密度越高,则被加速质子的峰值能量和截止能量越高,电荷量也越大.该组合加速方案对高能质子束的产生与应用具有一定的指导意义.

基于时间调制多波束阵列的单通道全向测向方法

基于多波束阵列的测向方法具有测向精度高的优势,但各波束都需要独立的射频通道,导致其测向系统复杂且成本高,通道之间的幅相不一致也会增大测向误差。本文提出了一种基于时间调制多波束阵列的单通道全向测向方法,通过时间调制技术将多波束阵列测向系统简化为单通道系统,利用接收信号的谐波特征获得不同波束的接收信号强度实现高精度测向,同时降低了系统的复杂度及成本。仿真结果表明在相同条件下,本文提出的方法比基于多波束阵列的多通道测向方法具有更小的测向误差。经实测验证,本文的方法在4~5 GHz频带内的测向误差小于1.3°,低于现有的多波束阵列测向误差。

方位多波束弧形阵列SAR成像方法研究

弧形阵列合成孔径雷达(Synthetic Aperture Radar,简称SAR)是一种广域观测的新体制微波成像系统,相比于线性阵列合成孔径雷达,弧形阵列SAR能够有效克服常规成像雷达前视、侧视、下视等观测视角单一的问题,弥补了常规成像模式的不足。方位多波束技术是星载合成孔径雷达系统获取高分辨宽覆盖测绘能力的重要技术途径。本文首先构建了多波束弧形阵列SAR系统的方位向多通道信号模型,并通过传统的方位多通道重建方法进行成像仿真,成像结果出现虚假目标,由于传统的多通道重建方法适用于大多数线性阵列,所以通过分析回波斜距的数学几何模型推导出了多波束弧形阵列SAR相比于线性阵列SAR多了一个附加相位,与弧形阵列天线结构相结合,附加相位由子孔径角度引起,因此方位多通道回波数据中相位误差是由多通道弧形阵列SAR的子孔径角度引起,从而导致多通道不均衡,所以传统的多通道重建方法不能够直接用于弧形阵列SAR的成像处理中,为了实现弧形阵列SAR系统的静止目标聚焦成像,提出了适用于弧形阵列SAR系统的方位多通道重建方法,在进行多通道重构和合并之前,对方位通道间的附加相位进行补偿,避免方位欠采样,然后根据弧形阵列方位多通道脉冲响应重构方位多通道数据,该方法可以很好的抑制了虚假目标,最后通过仿真实验验证了该方法的可行性。

模块式四芯光纤信道分束器及其性能测试方法

针对传统多芯光纤分束器与多芯光纤连接方式的不足,如拆卸困难和测试方案需额外引入测试分束器等问题,采用双包层光纤束拉锥法制备了四芯光纤信道分束器。为了方便器件的测试和连接,在单模光纤和四芯光纤的尾端分别制备了LC/PC型的活动连接器,从而形成了单个模块式的四芯光纤信道分束器;同时,提出了一种针对该模块式器件的镜像反射式测试方法。这种测试方法仅需要一个LC/PC活动连接器式反射器,就可以轻松完成器件的插入损耗和芯间串扰的测试。测试结果表明,所制备的四芯光纤信道分束器性能优异,其最大插入损耗和最大芯间串扰分别为0.84 dB和-46.32 dB。

基于LOFAR的超宽带数字阵列处理技术研究

低频射电阵列(Low-Frequency Array,LOFAR)是一个高分辨的低频射电望远镜阵列,采用了多种先进的技术,如宽带天线、多波束窄带检测、序贯检测思想等。近年来,由于其在高分辨天体图像观测上具有明显的优势,受到了广泛关注。在此背景下,重点分析了基于LOFAR的超宽带数字阵列处理技术。

三类视距无多径空间信道对通信对抗的影响分析

通信与通信对抗攻防双方都十分关注信号在空间信道中的传输特性,这对于对抗效能的影响极其巨大。针对三类视距无多径空间信道中信号传输的衰减特性进行了定量的分析与对比,展现了采用高增益天线定向窄波束互指的通信双方所具有的较强反侦察与抗干扰能力;而且阵列天线波束方向图固定方向置零与自适应调零从理论上为通信收发双方提供了更强的通信电子防御手段。所以为了提升侦察干扰的有效性,通信对抗必然会向分布式协同方向发展,这样才能实现对抗博弈的平衡。所揭示的上述技术演变趋势为通信对抗的未来发展规划研究提供了重要参考。

面向下一代网络的近场通信:理论、应用与挑战

从电磁理论出发,分析了近场通信不可忽视的电磁特性和波束特性,以及对现有通信系统性能的影响。利用近场效应,阐释了几种典型近场应用场景以及如何实现更高的性能指标。最后,为实现近场通信容量的提升,介绍了一些实现方法与架构,并对近场信道估计的两种技术进行展望,指出了未来研究方向。本研究可为未来近场通信的研究提供参考。

卫星通信中基于流量均衡的跳波束分簇构型研究

星上资源有限且珍贵,而在真实的通信场景中,地面终端用户地理位置分布和卫星通信业务需求各不相同,因此需要进行动态星上资源管理,并设计灵活高效的资源管理方案。本文利用跳波束技术提供波束级别灵活的资源管理,解决高通量卫星产生的点波束覆盖地面小区之间业务需求量不均衡的问题。首先,针对跳波束卫星存在共信道干扰的问题,提出了基于频率模式切换的跳波束簇间干扰避免策略;然后,提出均匀分簇和非均匀分簇两种分簇构型策略,针对均匀分簇均衡效果差、不适用于地面动态变化的问题和共信道干扰问题,提出了基于频率模式切换的跳波束簇间干扰避免策略的非均匀分簇构型方案;最后,对干扰避免策略进行仿真,同时基于多种智能优化算法对不同的分簇策略进行验证,证明所提分簇构型的可行性,并表明无距离限制的非均匀分簇算法的流量均衡能力较传统分簇算法更强。

铁路小曲线半径槽型梁转体平衡控制技术研究

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