Fourier变换的基本格式与PSD非参数估计

以传统Fourier变换为基础的非参数估计方法是振动工程研究中PSD估计的重要方法。文章以周期图法为例,研究Fourier变换基本格式的不同对PSD非参数估计结果的影响。首先介绍了Fourier变换常见的几种基本格式及其对应的PSD非参数估计计算公式;然后基于PSD的物理内涵,分析、证明多种不同形式的PSD估计公式的同时存在是一种人为误解,只有惟一的一种PSD估计结果才是具有典型物理意义的;最后给出了Fourier变换的一种通用格式及其PSD估计计算公式。     

Analysis of precipitable water vapor from GPS measurements in Chengdu region: Distribution and evolution characteristics in autumn

The rainfall process of Chengdu region in autumn has obvious regional features. Especially, the night-time rain rate of this region in this season is very high in China. Studying the spatial distribution and temporal variation of regional atmospheric precipitable water vapor (PWV) is important for our understanding of water vapor related processes, such as rainfall, evaporation, convective activity, among others in this area. Since GPS detection technology has the unique characteristics, such as all-weather, high accuracy, high spatial and temporal resolution as well as low cost, tracking and monitoring techniques on water vapor has achieved rapid developments in recent years. With GPS–PWV data at 30-min interval gathered from six GPS observational stations in Chengdu region in two autumns (September 2007–December 2007 and September 2008–December 2008), it is revealed that negative correlations exist between seasonally averaged value of GPS–PWV as well as its variation amplitude and local terrain altitude. The variation of PWV in the upper atmosphere of this region results from the water vapor variation from surface to 850 hPa. With the help of Fast Fourier Transform (FFT), it is found that the autumn PWV in Chengdu region has a multi-scale feature, which includes a seasonal cycle, 22.5 days period (quasi-tri-weekly oscillation). The variation of the GPS–PWV is related to periodical change in the transmitting of the water vapor caused by zonal and meridional wind strengths’ change and to the East Asian monsoon system. According to seasonal variation characteristics, we concluded that the middle October is the critical turning point in PWV content. On a shorter time scale, the relationship between autumn PWV and ground meteorological elements was obtained using the composite analysis approach.     

周期对称性在叶盘结构瞬态响应求解中的应用

研究了利用结构周期对称性计算叶盘结构瞬态响应的方法.首先用ANSYS得到扩充扇区的刚度和质量矩阵,通过周期对称边界上节点的对应关系得到扩充扇区到基本扇区的转换矩阵.利用叶盘结构中各扇区所受载荷的特点,得到任意时刻载荷的空间分布,并对其进行Fourier展开,以得到周期对称模型的载荷.采用Newmark法进行时域积分,利...     

用于失谐叶盘动力学特性分析的减缩计算方法

提出了一种用于失谐叶盘动力学特性分析的减缩计算方法.该方法采用基于周期对称的谐波平衡法,建立非线性的迭代方程,能够基于单扇区的矩阵求解整个失谐叶盘的强迫响应,且不存在任何截断误差,因而效率高于部件模态综合法但精度并不降低.从周期对称结构理论出发给出了方法原理的数学推导,在此基础上通过一个失谐叶盘有限元模型算例对该方法的效率与精度进行了讨论.结果表明:相较部件模态综合法,周期对称谐波平衡法矩阵维度减小59.5%,计算复杂度减少93.4%,计算时间节省57.4%.     

基于PD处理技术的雷达弱信号检测方法

针对脉冲跟踪测量雷达普遍存在的远距离弱信号检测能力不足的问题,基于PD（PulseDoppler,脉冲多普勒）雷达的杂波抑制与信号处理技术,提出脉冲雷达中的弱信号检测方法;详细分析了在脉冲跟踪测量雷达中应用PD处理技术的可行性,得出在一定约束条件下,可将PD处理技术运用于脉冲跟踪测量雷达,并提出了具体实现方法;最后,通过数据仿真和脉冲雷达实际测量数据进行验证,结果表明该方法能有效提高信噪比。     

事故车辆油漆物证分析-比对系统的建立

在已建立的事故车辆物理信息和光谱信息的油漆样本数据库的基础上,建立油漆物证比对系统。根据相关系数所表现的特征,实现对未知样本属性范围的确定,找出数据库中与其相关度较高的部分油漆样本,再根据数据库中同一车辆信息具有同一序号的特点,找出与油漆对应的车辆,即与案件现场最接近的嫌疑车辆,为肇事逃逸案件的破获提供数据依据。     

Reliability analysis based on a novel density estimation method for structures with correlations

Estimating the Probability Density Function (PDF) of the performance function is a direct way for structural reliability analysis,and the failure probability can be easily obtained by integration in the failure domain.However,efficiently estimating the PDF is still an urgent problem to be solved.The existing fractional moment based maximum entropy has provided a very advanced method for the PDF estimation,whereas the main shortcoming is that it limits the application of the reliability analysis method only to structures with independent inputs.While in fact,structures with correlated inputs always exist in engineering,thus this paper improves the maximum entropy method,and applies the Unscented Transformation (UT) technique to compute the frac tional moments of the performance function for structures with correlations,which is a very efficient moment estimation method for models with any inputs.The proposed method can precisely estimate the probability distributions of performance functions for structures with correlations.Besides,the number of function evaluations of the proposed method in reliability analysis,which is determined by UT,is really small.Several examples are employed to illustrate the accuracy and advantages of the proposed method.     

有限长Ramanujan-Fourier快速变换及频率估计

近来出现的Ramnujan-Fourier变换(RFT,Ramanujan Fourier Transformation)是以"Ramanujan和"为基向量的算术变换,该变换可提供分数频率分辨力.首先分析了有限长Ramanujan频谱特点,给出了基向量分布情况,推导了该变换的快速算法,比较了有限长RFT与快速傅里叶变换的乘法计算量;其次,给出了利用RFT的递归峰值检测频率估计算法,并分析了RFT的频率分辨率和适用特点,在非高斯噪声条件下,仿真比较了RFT与傅里叶变换对信号进行频率估计的性能,得到在信噪比为-20 dB的非高斯噪声情况下,频率估计的归一化均方误差可以达到 10-3.      

铁木辛柯梁固有振动频率的边界元解法

根据傅里叶变换推导具有两个广义位移的铁木辛柯梁固有振动的基本解.利用加权残量方法,从控制微分方程出发建立边界积分方程,进而根据边界条件得到频率方程,采用代数特征值方法和影响系数方法求解频率,并分析了两种方法的特点.以杆为例证明了对于一维均匀结构,对不同的边界条件利用边界元方法(BEM,Boundary Element Method)都可以得到精确频率.将铁木辛柯梁的BEM结果与有限元结果和精确解进行了比较.     

基于递推傅里叶变换的飞行器参数在线辨识方法

在线辨识在现代飞行控制系统设计中扮演越来越重要的角色,飞行器模型的在线更新使得人们可以采用更智能的控制方法。基于计算精度和速度的考虑,在线辨识方法通常以递推方式进行,主要分为时域和频域两大类方法。在建立飞行器系统模型结构的基础上,利用频域递推傅里叶变换及最小二乘方法,实现对气动及控制偏导数的在线辨识。针对某飞机纵向通道的在线辨识仿真验证该方法有效,且计算速度和收敛速度快,辨识结果与参数真实值之间的一致性好,方法对传感器噪声有较强的适应性。最后,分析比较了各种典型激励信号对辨识结果的影响,为进行实际在线辨识试验提供了参考依据。