Numerical Analysis of Flow-Induced Vibration and Noise Generation in a Variable Cross-Section Channel

Flow channels with a variable cross-section are important components of piping system and are widely used in variousfields of engineering.Using afinite element method and modal analysis theory,flow-induced noise,mode shapes,and structure-borne noise in such systems are investigated in this study.The results demonstrate that the maximum displacement and equivalent stress are located in the part with variable cross-sectional area.The aver-age excitation force on theflow channel wall increases with theflow velocity.The maximum excitation force occurs in the range of 0–20 Hz,and then it decreases gradually in the range of 20–1000 Hz.Additionally,as theflow velocity rises from 1 to 3 m/s,the overall sound pressure level associated with theflow-induced noise grows from 49.37 to 66.37 dB.Similarly,the overall sound pressure level associated with the structure-borne noise rises from 40.27 to 72.20 dB.When theflow velocity is increased,the increment of the structure-borne noise is higher than that of theflow-induced noise.

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.

Modeling and analysis of piezoelectric beam with periodically variable cross-sections for vibration energy harvesting

A bimorph piezoelectric beam with periodically variable cross-sections is used for the vibration energy harvesting. The effects of two geometrical parameters on the first band gap of this periodic beam are investigated by the generalized differential quadrature rule （GDQR） method. The GDQR method is also used to calculate the forced vibration response of the beam and voltage of each piezoelectric layer when the beam is subject to a sinusoidal base excitation. Results obtained from the analytical method are compared with those obtained from the finite element simulation with ANSYS, and good agreement is found. The voltage output of this periodic beam over its first band gap is calculated and compared with the voltage output of the uniform piezoelectric beam. It is concluded that this periodic beam has three advantages over the uniform piezoelectric beam, i.e., generating more voltage outputs over a wide frequency range, absorbing vibration, and being less weight.

Bending and free vibrational analysis of bi-directional functionally graded beams with circular cross-section

The bending and free vibrational behaviors of functionally graded(FG)cylindrical beams with radially and axially varying material inhomogeneities are investigated.Based on a high-order cylindrical beam model,where the shear deformation and rotary inertia are both considered,the two coupled governing differential motion equations for the deflection and rotation are established.The analytical bending solutions for various boundary conditions are derived.In the vibrational analysis of FG cylindrical beams,the two governing equations are firstly changed to a single equation by means of an auxiliary function,and then the vibration mode is expanded into shifted Chebyshev polynomials.Numerical examples are given to investigate the effects of the material gradient indices on the deflections,the stress distributions,and the eigenfrequencies of the cylindrical beams,respectively.By comparing the obtained numerical results with those obtained by the three-dimensional(3D)elasticity theory and the Timoshenko beam theory,the effectiveness of the present approach is verified.

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.

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.

Focusing characteristics of the relativistic electron beam transmitting in ion channel

Based on the beam-plasma system model established in this paper,the trajectory of the electron beam in the ion channel is studied quantitatively through the envelope equation.Under different initial system parameters,the focusing transmission conditions of the beam in the ion channel are discussed.Then,a series of particle-in-cell simulations are performed,which generally versifies the theoretical results and shows some further details of the focusing behavior of the beam.It is found that the deceleration of some electrons around the focusing point or the beam-plasma interaction at the ion channel boundary will result in the generation of the residual electrons,which forms the electron return current that leads to the new instabilities influencing the focusing characteristics of the beam.

The propagation dynamics and stability of an intense laser beam in a radial power-law plasma channel

By containing ponderomotive self-channeling,the propagation behavior of an intense laser beam and the physical conditions are obtained theoretically in a radial power-law plasma channel.It is found that ponderomotive self-channeling results in the emergence of a solitary wave and catastrophic focusing,which apparently decreases the region for stable propagation in a parameter space of laser power and the ratio of the initial laser spot radius to the channel radius(RLC).Direct numerical simulation confirms the theory of constant propagation,periodic defocusing and focusing oscillations in the parameter space,and reveals a radial instability which prevents the formation of bright and dark solitary waves.The corresponding unstable critical curve is added in the parameter space numerically and the induced unstable region above the unstable critical curve covers that of catastrophic focusing,which shrinks the stable region for laser beams.For the expected constant propagation,the results reveal the need for a low RLC.Further study illustrates that the channel power-law exponent has an obvious effect on the final stable region and laser propagation,for example increasing this exponent can enlarge the stable region significantly,which is beneficial for guiding of the laser and increases the lowest RLC for constant propagation.Our results also show that the initial laser amplitude has an apparent influence on the propagation behavior.

Estimation of the Temperature in the Weld Penetration Channel in Electron Beam Welding

In this paper, the method of experimental estimation of the temperature in a penetration channel in electron beam welding is described on the basis of chemical elements concentration in the vapors above welding zone. The temperature of a vapor-gas phase in the penetration channel is determined when equating calculated and experimental concentrations of the elements.

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.

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。

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。

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.

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。

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。

H13模具钢电子束选区熔化热行为的模拟研究

为了提高H13模具钢在电子束选区熔化技术过程中成型件的表面质量,并探明其成型过程缺陷问题的原因,文章基于离散元法建立了随机分布的金属粉末床模型,对熔体的热流体动力学行为进行数值模拟。单道结果表明,当电子束功率从600 W提高到1 080 W,扫描速度为1.5 m/s时,单道熔池平均深度从15.2μm增加到49.2μm,单道熔池宽度从79.2μm增加到140.1μm,同时发现电子束功率为960 W,单熔道形貌表面质量最好。双道结果表明,Z型扫描路径会导致熔道的一端形貌粗糙,S型扫描路径熔道内易形成热量累积现象。当电子束能量较低时,会出现球化效应和单道不平直现象。当电子束能量较高时,熔道易出现热量累积现象,影响后续的熔道形貌和尺寸。

背对式槽钢组合梁均布荷载下的弯曲应力研究

研究了均布荷载作用下的冷弯薄壁背对式组合梁的弯曲应力分布,基于梁的经典弯曲理论和约束扭转理论,在考虑翘曲作用的影响下推导了构件在均布荷载作用下的弯曲应力理论公式。并通过有限元软件ABAQUS分析算例,将算例的仿真值和理论解析值进行对比,验证了该理论公式的合理性。结果表明,弯曲应力在横截面上呈现线性梯度分布,以构件OT1.2-86-S75为例,当旋转约束k_(φ)取值分别为10、100、1000 N时,翼缘处的应力梯度分别为3.225、2.465、1.732 MPa/mm,即翼缘处的应力梯度随着旋转约束的减小而增大;沿纵向在相邻螺钉之间呈现连续抛物线式分布;若螺钉间距相等,弯曲应力最大值出现在跨中,弯曲应力极大值随着与支座距离减小逐渐递减。