一种无人机空地MIMO信道的三维几何随机模型（英文）

提出一种针对无人机空地多输入多输出信道的三维非平稳几何随机模型.模型中无人机侧和地面站侧的散射体分别分布在两侧的2个圆柱体表面.为描述信道的非平稳性,所提模型考虑了无人机的旋转和加速度的影响.从理论上推导了对应时变参数的计算方法,包括无人机天线阵列角度、时延和最大多普勒频率.在此基础上,进一步推导了信道空时相关函数和多普勒功率谱密度等统计特性.最后,通过数值仿真验证了信道的统计特性.仿真结果表明：无人机和地面站的加速度越大,时间相关函数的值越小,信道在时域上的非平稳性越大;无人机旋转对空间相关性有重要影响,滚转运动对空间相关性的影响比俯仰运动更显著;无人机运动方向对多普勒功率谱密度有显著影响.     

非平稳信号的时变功率谱与Wigner分布

时频分析旨在构造一种时间和频率的联合密度函数，以揭示信号中所包含的频率分量随时间变化的特性。本文通过对平稳信号相关函数谱分解和wigner分布的研究，给出了非平稳随机信号的时变功率谱，并对其性质进行了详细的研究。     

拱型波纹屋盖风荷载数值模拟研究

基于谐波叠加法的基本原理,结合快速傅立叶变换,利用MATLAB编写了拱型波纹屋盖多点互相关的脉动风速程序.程序运行结果表明,结构各点模拟脉动风速时程的功率谱密度函数与目标函数、风速时程的自相关函数与目标自相关函数较为一致;相邻两点之间距离越近,互相关性越强,反之,则越弱.该研究结果对进一步研究拱型波纹屋盖结构风致振动时域分析具有重要意义.     

平稳序列谱函数积分谱窗估计的渐近性态(Ⅰ)

用积分周期图估计平稳序列的谱函数,无论对于正态的平稳序列还是非正态平稳序列都被证明具有优良的渐近性质。许重光用拟合自回归谱密度估计量 (x)的积分估计谱函数,也证明了此估计量具有优良的渐近性质。本文采用积分谱窗估计量估计谱函数,无论是Gauss序列还是非Gauss序列,都证明了其估计误差过程ζ_N(λ)= (F_N(λ)-F(λ))具有优良的渐近性质     

谱密度矩阵的对称谱窗估计

在平稳时间序列的谱分析中,估计谱密度的方法很多,谱窗估计法是常用的一种方法。采用这种方法的关键是选择好的窗函数,同时要控制窗口的大小——窗函数平滑周期的范围。评选窗函数要有一个好坏的标准,例如谢衷洁与程乾生[1]以分瓣率的高低作为评选时窗函数的标准来寻找最佳的时窗函数;作者在[3]和[4]中,适当控制某种对称窗口的大小,可使谱密度估计量具有渐近无偏和均方相合等优良性质,而对时间序列本身不作正态分布的假定。本文引进一个对称的谱窗矩阵函数来讨论多维平稳时间序列的谱窗估计。用这种窗函数来平滑周期图所得到的谱密度矩阵的估计量具有渐近无偏性和均方相合性,同时对偏的大小和渐近方差的阶进行了估计,得到了明显的解析表示式,所有结论都是在时间序列不必为高斯的情况下得到的。Hannan[5]、Rosenblatt[8]及作者[3][4]的有关结果可视为本文相应定理的特例。     

基于时域模型的路面平整度模拟

为了更加直观有效地描述路面平整度,提出了一种路面平整度的模拟方法,即随机正弦波法.该方法将路面平整度表示成大量具有随机相位的正弦或余弦之和,采用离散谱逼近目标随机过程,是一种离散化数值模拟路面平整度的方法.根据给定的路面功率谱系数,在时域路面随机位移输入的频率特征与给定的路面谱相一致的情况下,通过计算机模拟将路面平整度优化成为随机振动的等效信号,得到各等级下路面的平整度曲线.结果表明：随机正弦波法适用于实测道路谱的时域模拟,由于该方法数学推导严密,使用范围广泛,且模拟曲线直观,对于汽车平顺性研究具有十分重要的意义.最后,提出一个与路面功率谱系数相关的平整度指标——名义平整度指数.     

变变位齿轮传动的振动响应测试及分析

利用WS-5921/U60512型信号采集仪及加速度传感器,对变变位齿轮传动不同速度下的振动响应进行了测试及量化研究,得到了振动加速度信号的时域曲线及自功率谱密度曲线,并与变位齿轮的振动响应进行对比。研究表明:齿轮传动过程中,主动轮产生的振动能量基本相同,从动轮的振动主要来源于主、从动轮的啮合过程,变变位齿轮啮合过程中产生的振动能量小,传动更平稳;随着电机输入频率的增加,主、从动轮的振动能量逐渐增大,变变位齿轮传动最大自功率谱值对应的振动频率波动较小,约为100 Hz;变位齿轮、变变位齿轮从动轮的振动信号自功率谱密度的比值最大达到4∶1以上。     

浅谈噪声的统计特性及随机噪声

噪声的检测是一门新兴的学科，本扼要介绍了噪声的基本概念及统计特性，重点讨论了在噪声中占绝大多数的随机噪声的功率谱密度及相关函数。     

波纹管孔道压浆密实度超声仿真分析

利用超声波信号检测波纹管孔道中压浆密实度。运用COMSOL Multiphysics软件对空气泡不同长轴半径的波纹管孔道压浆模型进行了仿真,研究了不同空气泡长轴半径对接收透射信号波形及频谱的影响,研究了空气泡长轴半径对功率谱密度和平均功率的影响。数值结果分析表明,在空气泡小于波纹管范围内,空气泡长轴半径越大,接收透射信号的后续波幅就越大,频谱幅值越大,功率谱密度函数衰减越慢,平均功率越大。     

光波场的相干性

引入互相干函数来描述平稳场的二阶要干效应，并分别讨论了二阶相干效应中的时间相干性和空间相干性，对于振幅和相位均无规涨落的非平稳场，则有用了四阶或更高阶相关函数来描述。     

Response of a transmission tower-line system at a canyon site to spatially varying ground motions

Collapses of transmission towers were often observed in previous large earthquakes such as the Chi-Chi earthquake in Taiwan and Wenchuan earthquake in Sichuan, China. These collapses were partially caused by the pulling forces from the transmission lines generated from out-of-phase responses of the adjacent towers owing to spatially varying earthquake ground motions. In this paper, a 3D finite element model of the transmission tower-line system is established considering the geometric nonlinearity of transmission lines. The nonlinear responses of the structural system at a canyon site are analyzed subjected to spatially varying ground motions. The spatial variations of ground motion associated with the wave passage, coherency loss, and local site effects are given. The spatially varying ground motions are simulated stochastically based on an empirical coherency loss function and a filtered Tajimi-Kanai power spectral density function. The site effect is considered by a transfer function derived from 1D wave propagation theory. Compared with structural responses calculated using the uniform ground motion and delayed excitations, numerical results indicate that seismic responses of transmission towers and power lines are amplified when considering spatially varying ground motions including site effects. Each factor of ground motion spatial variations has a significant effect on the seismic response of the structure, especially for the local site effect. Therefore, neglecting the earthquake ground motion spatial variations may lead to a substantial underestimation of the response of transmission tower-line system during strong earthquakes. Each effect of ground motion spatial variations should be incorporated in seismic analysis of the structural system.     

Use of probabilistic and deterministic measures to identify unfavorable earthquake records

This study introduces measures to identify resonant （concentration of energy in a single or a few frequencies） or unfavorable earthquake ground motions. Probabilistic measures based on the entropy rate and the geometric properties of the power spectral density function （PSDF） of the ground acceleration are developed first. Subsequently, deterministic measures for the frequency content of the ground acceleration are also developed. These measures are then used for identifying resonance and criticality in stochastic earthquake models and 110 acceleration records measured at rock, stiff, medium and soft soil sites. The unfavorable earthquake record for a given structure is defined as the record having a narrow frequency content and dominant frequency close to the structure fundamental natural frequency. Accordingly, the measures developed in this study may provide a basis for selecting records that are capable of producing the highest structural response. Numerical verifications are provided on damage caused to structures by identified resonant records.     

Property of critical excitation for moment-resisting frames subjected to horizontal and vertical simultaneous ground motions

It has often been reported that, when building structures are subjected to near-fault earthquake ground motions, horizontal and vertical impulsive inputs may cause critical damage during the first few seconds. In practical design of building structures, however, the safety check, taking into account the effect of multi-component ground motions, is hardly conducted except the design of important structures such as high-rise buildings and nuclear power plants. Ftirthel＇more, it is not clear how the correlation of multi-component ground motions influences the actual safety of structures. In this paper, the detailed property of critical excitation is discussed in association with the relationship between the characteristics of ground motions and those of structures. The properties of various auto power spectral density （PSD） functions of the horizontal and vertical ground motions are investigated, and those of the critical cross PSD function of these two-directional ground motions are found by a devised algorithm in a feasible complex plane. A closed-form expression is derived from the critical relation of the auto PSD functions of the simultaneous inputs. This critical excitation method provides us with a new approach for earthquake-resistant design against the possible future earthquake which causes the critical damages to buildings.     

Hybrid slip model for near-field ground motion estimation based on uncertainty of source parameters

The hybrid slip model used to generate a finite fault model for near-field ground motion estimation and seismic hazard assessment was improved to express the uncertainty of the source form of a future earthquake.In this process, source parameters were treated as normal random variables, and the Fortran code of hybrid slip model was modified by adding a random number generator so that the code could generate many finite fault models with different dimensions and slip distributions for a given magnitude.Furth...     

Seismic response analysis of long-span suspen-dome under multi-support excitations

A practical suspen-dome project, Changzhou Gym roof, is adopted as an example and its transient analysis based on the multi-support excitations of the earthquake wave is carried out. Compared with the single support excitation, the position and value of the maximum stress under multi-support excitations both change and the amount of elements with obvious changes is large and more than 70% of the total. Moreover, when other terms are not changed, this influence will decrease as the span decreases, but increa...     

多点激励下多跨斜拉桥缩尺模型的地震响应研究．部分2：数值分析

研究目的：通过非线性动力有限元分析,模拟多塔斜拉桥在多点激励下的地震响应以及揭爪其在强震下的倒塌破坏模式。创新要点：解决了多塔斜拉桥的非线性动力数值模拟技术,并首次模拟了多塔斜拉桥在强震下的破坏模式。研究方法：首先采用隐式的Newmark-β法求解多点地震激励下的非线性动力方程（图3）,随后采用显式的中心差分格式对强震作用下桥梁的倒塌破坏过程进行数值仿真（图4）。重要结论：1．单主梁模犁的数值结果可以很好地反映地震模拟振动台试验的地震响应结果;2．在加速度峰值为4．0m／s2的E1。Centro波时,多塔斜拉桥模型只出现了中塔支座的破坏;3．在加速度峰值为4．0m／s2的江心波时,多塔斜拉桥模型先后出现了支序破坏、拉索与塔的连接破坏以及中塔上塔柱段的单元失效。     

立式单级离心泵的地震反应谱分析

为了在设计和安装中将离心泵的地震最不利方向避开地震频发的方向,本文将离心泵简化成有限元力学模型,采用有限元法对离心泵进行模态分析,计算出固有频率和振型。采用反应谱法进行分析,地震谱从不同方向进行输入,得到各个节点随地震谱输入方向的改变而变化的位移和载荷。通过分析数据结果,建议将该模型的45～75度方向设在频繁发生地震的方向上,以避免发生最大振动。     

Simulation of stochastic wind field for large complex structures based on modified Fourier spectrum

Simulation for stochastic wind field is very important in analyzing dynamic responses of large complex structures due to strong wind. The typical simulation method is the spectrum representation method (SRM), but the SRM has drawbacks of inferior precision in lower frequency and slow calculating speed. In view of this, the modified Fourier spectrum method (MFSM) is introduced into the simulation of stochastic wind field in this paper. In this method, phase information of wind velocity time history is determined by cross power spectral density (CPSD) between adjacent points, and the wind velocity time history with time and space correlation is generated by iterative modification for CPSD considering auto power spectral density (APSD). Simulation of the wind field for a long-span bridge is undertaken to verify the effectiveness of the MFSM. Simulation results of the SRM and the MFSM are compared. It can be concluded that the MFSM is more accurate and has higher calculation speed than the SRM.     

多天线蜂窝系统中的下行协作波束成型传输

在使用波束成型传输数据的蜂窝系统中,为提高下行链路的频谱效率,需要多小区多用户之间协作地产生波束.以多小区多用户下行链路的数学模型为基础,计算了各用户波束发送滤波器、接收滤波器、发送功率三类变量的梯度,使这些变量不断沿梯度方向调整并投影至可行空间,从而提出了一种基于梯度投影的多小区多用户下行协作波束成型传输方案,并分析了该方案需要在相邻小区基站间交互的信息.仿真结果表明,该方案在典型场景与参数配置下可达到约5 bit/（s·Hz·cell）的平均频谱效率性能.因此,多小区多用户之间的有效协作有助于提高蜂窝系统频谱效率性能.     

s=1／2xxz反铁磁链的Spin—Peierls相变研究

应用玻色化的自洽微扰论方法,给出了ｓ＝１／２ｘｘｚ反铁磁链的Ｓｐｉｎ－Ｐｅｉｅｒｌｓ系统的基态能量和准粒子元激发能谱的解析近似式,了模型基态能量密度与基态准粒子激发能隙随晶格畸变大小的关系受各向异性作用参量的影响。