沿LiNbO3 y方向的准纵波及其超声非线性系数

本文给出了沿LiNbO₃ y方向的准纵波的非线性运动方程及其解.由于LiNbO₃晶体的二阶弹性常数c₁₄比c₁₁和 c₄₄小很多,沿y方向的准纵波非常接近纯模式,为用超声二次谐波发生技术测量晶体的超声非线性系数和三阶弹性常数提供了一个新的传播模式.对于LiNbO₃(或其它具有3m对称的晶体)从这个传播模式的超声非线性系数,可以确定一个包括c₂₂₂,c₂₄₄,c₁₁₄,c₁₂₄和c₄₄₄5个三阶弹性常数的组合.在准静态近似下,求得了与这个传播模式有关的二阶和三阶弹性常数与压电耦合的关系.用超声二次谐波发生技术测量了该模式的超声非线性系数,并求得了相应的三阶弹性常数的组合.实验和利用其他作者发表的数据计算的结果相符合.     

复杂网络上的意见动力学对谣言传播的影响

通过在SIR(susceptible-infected-recovered)模型中引入抑制者对谣言的辟谣机制研究了在线社交网络上的意见动力学对谣言传播的影响.在这一模型中,节点可以与自身的邻居组成1个群,传播者可以通过该群传播信息,抑制者也可以在此群中对信息发表意见进行辟谣.辟谣机制在降低未知者对于谣言的接受概率的同时也可以促使传播者向抑制者转变.本文采用ER(Erd?s-Rényi)随机网络、无标度网络以及真实的社交网络研究了抑制者的沉默概率对于谣言传播范围的影响.首先发现,谣言传播的过程以传播者的峰值为界可以分为两个阶段,即谣言自由传播的前期以及抑制者和传播者互相制衡的后期;其次,谣言的传播会随着抑制者的沉默概率的增大而突然暴发.在谣言暴发阈值之下,沉默概率的增大不会导致谣言传播范围显著增大,这是由于未知者在感知到谣言并转变为传播者后又迅速转变为抑制者;而当沉默概率达到谣言暴发阈值时,抑制者将不能控制传播者对谣言的传播从而导致抑制者的降低和谣言的暴发;最后,无标度上的谣言自由传播的前期阶段比随机网络持续的时间更短,从而使无标度上的谣言更难以暴发.本文的模型综合考虑了意见动力学和谣言传播的相互作用,更加真实地模拟了真实世界社交网络中的谣言传播过程.为谣言传播的控制和干预提供了一些有用的思路和见解.     

壁面结构对非定常薄膜流动表面波特性的影响

采用VOF方法,在入口处引入周期扰动,对沿矩形结构底板下落的二维非定常薄膜流动进行直接数值模拟.计算结果表明,不同频率的外加周期扰动在流场中激发出不同波长的自由表面波.当壁面结构的波长与之相比较小时,由于壁面结构引起的静态波骑行在外加扰动形成的行进波上.若这两个波长相近,非线性效应将促进附近的表面波之间的合并.当薄膜无...     

Transmission-mode GaN photocathode based on graded Al_xGa_(1-x)N buffer layer

We create a GaN photocathode based on graded Alx Ga1-x N buffer layers to overcome the influence of buffer-emission layer interface on the photoemission of transmission-mode GaN photocathodes.A gateshaped spectral response with a 260-nm starting wavelength and a 375-nm cut-off wavelength is obtained.Average quantum efficiency is 15% and short wavelength responses are almost equivalent to long wavelength ones.The fitted interface recombination velocity is 5×104 cm/s,with negligible magnitude,proving that the design of the graded buffer layers is efficient in obtaining good interface quality between the buffer and the emission layer.     

Scintillation reduction using multi-beam propagating technique in atmospheric WOCDMA system

We propose employing multi-beam propagating technology to mitigate the influence of atmospheric scintillation to the wireless optical code division multiple access (WOCDMA) system and then deduce the bit error rate (BER) formulas of systems in weak and strong scintillations,respectively.According to simulation experiment results,multi-beam propagation can improve the system performance very well compared with single-beam propagating technique.Moreover,the more beams we use,the better the performance we get.When the received optical power is-30 dBm,the BER of the system employing four beams is 5 and 1 dB lower than that of using single-beam propagating technique in weak and strong scintillations,respectively.     

基于FPGA和DSP的超声波风向风速测量系统

本文研究了数字化超声风速测量系统,提出并实现了完整的系统设计方案。该方案的核心是基于现场可编程门阵列(FPGA)和数字信号处理器(DSP)的硬件系统架构和基于包络重心法的信号处理算法。该设计可实现超声信号的高速数据采集和复杂的信号处理算法。测试结果表明所实现的超声风速测量系统具有体积小、反应速度快、抗干扰能力强和分辨率高等优点,且具有成本优势,并保持较低的功耗。     

基于ANSYS提取单相单向换能器耦合模参数

基于有限元理论,给出了基于ANSYS提取单相单向换能器（Single Phase Unidirectional Transducers,SPUDT）耦合模参数的方法。首先,结合声表面波在压电介质中的传播原理,给出了压电有限元分析方程和对应的ANSYS分析步骤。然后针对SPUDT结构,给出了周期性近似分析的理论模型,利用耦合场分析对其进行模态分析和谐响应分析,最后结合两者的结果来计算SPUDT的耦合模参数。本文给出基底材料为压电晶体128°YX-LiNbO₃和YZ-LiNbO₃,电极材料为铝的三种不同SPUDT的计算结果,理论结果与Hashimoto和Hartmann文献报道的结果相吻合,并且和实验测试的结果基本一致。从而给出了提取SPUDT耦合模参数的一种通用、有效的方法。     

三层单负材料结构中电磁波模分析

本文分析了由负介电常数材料和负磁导率材料交替排列组成的一维三层结构中的电磁波模式,包括传播模及波导模.传播模对应于入射的行波模,着重研究了发生共振隧穿时场分布和透射谱随入射角度的变化关系,发现改变结构参数时共振隧穿频率会劈裂成两个,且这两个隧穿模的频率间距随着中间层厚度的减小而逐渐加大.波导模是指在结构两边的半无限真空中以倏逝场形式存在的电磁模式,分析了它的存在条件即色散关系,发现这种模在材料的交界面处会出现较强的局域场. 关键词： 单负材料 共振隧穿 传播模 波导模     

Characterizing the properties of ultrasonics propagating on the surface of materials with defect by Laser-ultrasonics method

A laser ultrasonics method is used to characterize the propagation properties of surface wave traveling on the surface of materials with sub-surface defect in 2D.Linear and nonlinear propagation properties of ultrasonics caused by the defects have been detected in experiment. A theoretical model is proposed and used to study the linear and nonlinear properties of ultrasonics caused by the defect.The numerical results indicate that the nonlinear ultrasonic wave will be excited when a finite amplitude ultrasonics propagates on the surface of materials with sub-surface defect.The theoretical analysis confirms that the nonlinear wave is caused by the "clapping"of the interface of defect instead of the mode conversions of ultrasonics.     

Analysis of ultrasonic guided waves propagating in long bones and cortical thickness based on time-frequency methods

The ultrasonic signals in long bones contain multiple guided modes which are mutual superposed.The velocities of guided waves in long bones are very sensitive to cortical thickness (CTh).In this paper,Hilbert-Huang transform(HHT) was proposed to analyze multi-mode guided waves,which can decompose superposed waves into many independent modes.Then the group velocity of each mode was obtained at corresponding frequency,which was compared with the results of short time Fourier transform(STFT).The CTh was also obtained by comparing with the theoretical calculation.The results showed that the experimental determined thickness was in agreement with the actual CTh,indicating that measuring the velocity of the guided mode can be used to estimate the CTh.The HHT is an effective method to identify multimode guided waves.