Modified slow-fast analysis method for slow-fast dynamical systems with two scales in frequency domain

A modified slow-fast analysis method is presented for the periodically excited non-autonomous dynamical system with an order gap between the exciting frequency and the natural frequency.By regarding the exciting term as a slow-varying parameter,a generalized autonomous fast subsystem can be defined,the equilibrium branches as well as the bifurcations of which can be employed to account for the mechanism of the bursting oscillations by combining the transformed phase portrait introduced.As an example,a typical periodically excited Hartley model is used to demonstrate the validness of the method,in which the exciting frequency is far less than the natural frequency.The equilibrium branches and their bifurcations of the fast subsystem with the variation of the slow-varying parameter are presented.Bursting oscillations for two typical cases are considered,which reveals that,fold bifurcation may cause the the trajectory to jump between different equilibrium branches,while Hopf bifurcation may cause the trajectory to oscillate around the stable limit cycle.

Power Balance of Multi-Harmonic Components in Nonlinear Network

The main harmonic components in nonlinear differential equations can be solved by using the harmonic balance principle. The nonlinear coupling relation among various harmonics can be found by balance theorem of frequency domain. The superhet receiver circuits which are described by nonlinear differential equation of comprising even degree terms include three main harmonic components: the difference frequency and two signal frequencies. Based on the nonlinear coupling relation, taking superhet circuit as an example, this paper demonstrates that the every one of three main harmonics in networks must individually observe conservation of complex power. The power of difference frequency is from variable-frequency device. And total dissipative power of each harmonic is equal to zero. These conclusions can also be verified by the traditional harmonic analysis. The oscillation solutions which consist of the mixture of three main harmonics possess very long oscillation period, the spectral distribution are very tight, similar to evolution from doubling period leading to chaos. It can be illustrated that the chaos is sufficient or infinite extension of the oscillation period. In fact, the oscillation solutions plotted by numerical simulation all are certainly a periodic function of discrete spectrum. When phase portrait plotted hasn’t finished one cycle, it is shown as aperiodic chaos.

Friction coupling vibration characteristics analysis of aviation hydraulic pipelines considering multi factors

As the power transmission system of an aircraft,a hydraulic pipeline system is equivalent to the " blood vessel" of the aircraft. With the development of aircraft hydraulic system to high pressure,high speed and high power ratio,the fluid-structure interaction vibration mechanism of hydraulic pipeline is more complex and the influence of friction coupling on vibration cannot be ignored. The fluid-structure interaction of hydraulic pipeline will lead to system vibration,lower reliability of system operation and even pipeline rupture. Taking a hydraulic pipeline of C919 aircraft wingtip as the research object,a 14-equation model of fluid-structure interaction vibration considering friction coupling effect is established in this paper. The effects of friction and fluid parameters on the pipeline fluid-structure interaction vibration characteristics are studied and verified by experiments. The research results will provide theoretical guidance for the analysis of the pipeline fluid-structure interaction vibration and have important theoretical significance and great engineering value for promoting the localization process of large aircraft.

Radio-frequency transistors from millimeter-scale graphene domains

Graphene is a new promising candidate for application in radio-frequency （RF） electronics due to its excellent elec- tronic properties such as ultrahigh carrier mobility, large threshold current density, and high saturation velocity. Recently, much progress has been made in the graphene-based RF field-effect transistors （RF-FETs）. Here we present for the first time the high-performance top-gated RF transistors using millimeter-scale single graphene domain on a SiO2/Si substrate through a conventional microfabrication process. A maximum cut-off frequency of 178 GHz and a peak maximum os- cillation frequency of 35 GHz are achieved in the graphene-domain-based FET with a gate length of 50 nm and 150 nm, respectively. This work shows that the millimeter-scale single graphene domain has great potential applications in RF devices and circuits.

Frequency-domain generelaized singular peruturbation method for relative error model order reduction

A new mixed method for relative error model order reduction is proposed. In the proposed method the frequency domain balanced stochastic truncation method is improved by applying the generalized singular perturbation method to the frequency domain balanced system in the reduction procedure. The frequency domain balanced stochastic truncation method, which was proposed in [15] and [17] by the author, is based on two recently developed methods, namely frequency domain balanced truncation within a desired frequency bound and inner-outer factorization techniques. The proposed method in ttiis paper is a carry over of the frequency-domain balanced stochastic truncation and is of interest for practical model order reduction because in this context it shows to keep the accuracy of the approximation as high as possible without sacrificing the computational efficiency and important system properties. It is shown that some important properties of the frequency domain stochastic balanced reduction technique are extended to the proposed reduction method by using the concept and properties of the reciprocal systems. Numerical results show the accuracy, simplicity and flexibility enhancement of the method.

Scale effects on non-cavitation hydrodynamics and noise of highly-skewed propeller in wake flow

Regarding the scale effects on propeller＇s noncavitation hydrodynamics and hydroacoustics, three similar 7bladed highly-skewed propellers in the wake flow are addressed with diameters of 250, 500 and 1 000 mm, respectively. The discrete line-spectrum noise and its standardized spectrum level scaling law, together with the total sound pressure level are analyzed. The non-cavitation noise predictions are completed by both the frequency domain method and the time domain method. As a fluctuated noise source, the time-dependent fluctuated pressure and normal velocity distribution on propeller blades are obtained by the unsteady Reynolds-averaged Navier-Stokes （ URANS ） simulation. Results show that the pressure coefficient distribution of three propellers on the 0.7R section is nearly superposed under the same advance ratio. The periodic thrust fluctuation of three propellers can exactly reflect the tonal components of the axial passing frequency （APF） and the blade passing frequency （BPF）, and the fluctuation enhancement from the small to the middle propeller at the BPF is greater than that from the middle to the big one. By the two noise prediction methods, the increment of the total sound pressure level from the small to the big propeller differs by 2.49 dB. Following the standardized scaling law, the spectrum curves of the middle and big propellers are nearly the same while significantly differing from the small one. The increment of both the line-spectrum level and the total sound pressure increases with the increase in diameter. It is suggested that the model scale of the propeller should be as large as possible in engineering to reduce the prediction error of the empirical scalin~ law and weaken the scale effects.

Bayesian seismic multi-scale inversion in complex Laplace mixed domains

Seismic inversion performed in the time or frequency domain cannot always recover the long-wavelength background of subsurface parameters due to the lack of low-frequency seismic records. Since the low-frequency response becomes much richer in the Laplace mixed domains, one novel Bayesian impedance inversion approach in the complex Laplace mixed domains is established in this study to solve the model dependency problem. The derivation of a Laplace mixed-domain formula of the Robinson convolution is the first step in our work. With this formula, the Laplace seismic spectrum, the wavelet spectrum and time-domain reflectivity are joined together. Next, to improve inversion stability, the object inversion function accompanied by the initial constraint of the linear increment model is launched under a Bayesian framework. The likelihood function and prior probability distribution can be combined together by Bayesian formula to calculate the posterior probability distribution of subsurface parameters. By achieving the optimal solution corresponding to maximum posterior probability distribution, the low-frequency background of subsurface parameters can be obtained successfully. Then, with the regularization constraint of estimated low frequency in the Laplace mixed domains, multi-scale Bayesian inversion inthe pure frequency domain is exploited to obtain the absolute model parameters. The effectiveness, anti-noise capability and lateral continuity of Laplace mixed-domain inversion are illustrated by synthetic tests. Furthermore,one field case in the east of China is discussed carefully with different input frequency components and different inversion algorithms. This provides adequate proof to illustrate the reliability improvement in low-frequency estimation and resolution enhancement of subsurface parameters, in comparison with conventional Bayesian inversion in the frequency domain.

STRONGLY RESONANT BIFURCATIONS OF NONLINEARLY COUPLED VAN DER POL-DUFFING OSCILLATOR

In this paper, the strongly resonant bifurcations of a nonlinearly coupled Van der Pol-Duffing Oscillator by the classical multi-scale method are studied. It is shown that there exist periodic motions of a single oscillator, frequency-looking and quasi-periodic motions of two oscillators when the parameters vary. Meanwhile, some numerical results are given to test the theoretical ones.

Dynamic Positioning Control of Surge−Pitch Coupled Motion for Small-Waterplane-Area Marine Structures

For general dynamic positioning systems,controllers are mainly based on the feedback of motions only in the horizontal plane.However,for marine structures with a small water plane area and low metacentric height,undesirable surge and pitch oscillations may be induced by the thruster actions.In this paper,three control laws are investigated to suppress the induced pitch motion by adding pitch rate,pitch angle or pitch acceleration into the feedback control loop.Extensive numerical simulations are conducted with a semi-submersible platform for each control law.The influences of additional terms on surge−pitch coupled motions are analyzed in both frequency and time domain.The mechanical constraints of the thrust allocation and the frequency characters of external forces are simultaneously considered.It is concluded that adding pitch angle or pitch acceleration into the feedback loop changes the natural frequency in pitch,and its performance is highly dependent on the frequency distribution of external forces,while adding pitch rate into the feedback loop is always effective in mitigating surge−pitch coupled motions.

考虑多尺度纹理特征的红外传感图像频域增强

红外传感图像质量容易受探测器和传输距离影响,导致图像亮度和对比度较低、轮廓细节模糊等问题。为此,提出了考虑多尺度纹理特征的红外传感图像频域增强方法。引入残差学习策略,基于多尺度纹理特征搭建多尺度卷积神经网络模型,进行图像去噪。对去噪后图像进行傅里叶变换,获取红外传感图像的低频图像和高频图像。针对低频图像部分,调节图像灰度和对比度以增强低频分量。针对高频图像部分,利用Log算子和Laplace算子增强图像细节及边缘。加权融合两者处理结果,选取Gamma校正调节对比度,增强高频分量。融合两种增强后图像,实现红外传感图像频域增强。实验结果表明,该方法峰值信噪比高于43,信息熵大于8,边缘强度超过82,对比度熵大于8.1,平均梯度大于8。

融合时频特征的多源无监督域自适应轴承故障诊断方法

无监督域自适应已成为多工况下轴承故障诊断的一种重要方法。然而,现有多源无监督域自适应方法往往忽略不同视角信号对于跨域故障诊断的贡献,不足以全面表达轴承的故障特征。此外,这些方法的不同源域对同一目标域的预测结果存在差异。为此,提出一种融合时频特征的多源无监督域自适应(time-frequency features fused multi-source unsupervised domain adaptation,TFFMUDA)轴承故障诊断方法。该方法以时域和频域信号为输入,通过特征耦合机制实现两种故障特征的互补,并利用分类器对齐策略增强了不同源域对于同一目标域的诊断一致性。通过实际轴承故障案例的试验结果表明,所提方法相较于现有无监督域自适应轴承故障诊断方法能获得更清晰的故障类决策边界并具有更好的目标域诊断精度。

基于改进Faster R-CNN的红外目标检测算法

为提升红外目标的检测精度,提出了一种引入频域注意力机制的Faster R-CNN红外目标检测算法。首先,针对红外图像边缘模糊和噪声问题,设计了一种并行的图像增强预处理结构;其次,在Faster R-CNN中引入频域注意力机制,设计了一种新型红外目标检测主干网络;最后,引入路径增强金字塔结构,融合多尺度特征进行预测,利用底层网络丰富的位置信息,提升检测精度。在红外飞机的数据集上进行实验,结果表明,改进后的Faster R-CNN目标检测框架比以ResNet50为主干的算法的AP提升了7.6%。此外,与目前主流算法对比,本文算法提高了红外目标的检测精度,验证了算法改进的有效性。

基于空耦冲击回波法的饰面瓷砖层间脱空识别研究

针对饰面瓷砖脱空掉落的现象,建立了空气-饰面瓷砖层间脱空有限元模型,分析了钢球半径、激励速度以及信号采集位置等因素对饰面瓷砖层间脱空检测效果的影响,确定了最优值,然后利用特征识别参数对饰面瓷砖粘结层的脱空程度进行评估,并开展足尺试验对有限元模型的有效性进行验证。结果表明:钢球半径建议取9 mm,激励速度建议取1 m·s^(-1),信号采集点与激励点的横向距离不宜小于60 mm,垂向距离取5 mm;黏结层脱空会使冲击回波信号加强。因此,时域图中的信号波动性增强、衰减速度减缓,频域图中的声压级增大,主频段前移并分化为代表结构整体振动和局部振动的两个频段;并且随着脱空程度的增加,频域面积增大,通过计算频域面积可有效评估饰面瓷砖黏结层的脱空状况。研究结果为饰面瓷砖的无损检测提供了参考。

融合多尺度和频域特征的目标身份识别技术

拍摄设备、角度和光线的差异以及相似目标的干扰,给跨设备目标身份识别任务带来了严峻挑战。针对识别过程存在的类内差异和类间相似问题,提出了一种融合多尺度和频域特征的识别模型,在主干网络中加入注意力机制提高模型对高辨识特征的关注度;在分支网络中,设计了一种基于注意力的多尺度扩张融合模块对不同深度特征进行多粒度采样融合,增强网络的空间映射能力;在后处理阶段构造自学习的频域卷积模块,实现多尺度和频域特征的融合,利用频域信息提高度量相似目标的准确率。经过实验,算法在Veri776和VehicleID数据集的平均均值精度(mAP)和首次命中精度(Rank-1)分别获得了81.60%、97.20%和90.50%、85.30%,结果优于近些年主流方法,能够满足跨设备的多目标身份识别要求。

低照度下依据频域划分尺度的Retinex增强算法

针对经典多尺度Retinex算法在低照度下产生光晕、过增强、细节丢失等问题,提出了依据频率划分尺度的Retinex增强算法。改进后的算法将图像转换到频域分别对图像进行低通、带通、高通滤波后得到三幅图像,将滤波后图像的特点和Retinex算法中尺度因子对增强效果的影响进行有机结合,利用模糊集合将三个尺度的图像进行加权归一,突出了图像细节,解决了过增强产生光晕的问题。以低照度的图像做为数据源,采用峰值信噪比、均方误差和熵作为质量评价标准验证改进后的算法。

用于地下未爆弹探测的频域电磁传感器设计

未爆弹药通常位于地表以下,探测过程复杂,干扰多。利用频率域电磁法探测未爆弹时,需考虑激励和接收线圈之间的相互耦合。论文利用关于一次场对称的接收结构,使流入和流出接收线圈的磁通相互抵消,从而减小一次场对响应的影响。通过仿真研究了各接收线圈的响应信号,提出了基于传感器的水平和竖直定位方法。最后参照仿真尺寸搭建实验平台,对三种不同尺寸的未爆弹药的探测深度可达1.2 m。验证了水平定位方法的可行性,竖直定位误差在10 cm以内。

基于HIE-FDTD方法的电磁热耦合分析快速计算方法研究

基于混合显隐式时域有限差分(hybrid implicit-explicit finite-difference time-domain,HIE-FDTD)方法,提出了一种电磁热耦合分析快速计算方法。仿真电磁场时,采用HIE-FDTD方法求解Maxwell旋度方程;计算温度场时,采用时域有限差分(finite-difference time-domain,FDTD)方法求解热传导方程。由于HIE-FDTD方法只对1维空间偏导数采用隐式差分方法进行离散,因此时间步长不受电磁目标精细结构所在方向网格大小的限制。采用电磁热耦合分析快速计算方法对十字缝隙频率选择表面的电磁热耦合效应进行分析,并将仿真结果与采用传统FDTD方法、CST软件以及COMSOL软件的仿真结果进行对比,所有结果均相互符合。十字缝隙频率选择表面算例结果表明:在保证计算精度的前提下,采用HIE-FDTD方法的仿真时间仅为传统FDTD方法仿真时间的1/6,有效提高了电磁热耦合仿真的计算效率。

补给船靠泊方式对浮式码头水动力性能影响研究

浮式码头作为远海油气作业的后勤补给基地,通过补给船往返平台和码头之间,可有效提升补给效率和可靠性。而浮式码头的水动力性能和运动性能直接影响到补给作业的安全性,在补给船靠泊工况下,是个典型的多浮体水动力问题。本文基于三维势流理论,采用水动力分析软件AQWA对不同数量的补给船靠泊下的浮式码头进行水动力分析和频域运动响应分析,比较分析了不同泊位、不同数量的补给船靠泊下,对浮式码头水动力和运动响应的影响,为浮式码头设计和运营提供参考。

基于音频特征的水车室工作状态异常检测

水电站的水车室包含轴承和顶盖等机械设备,受水力因素影响,水车室的异常工作会带来较大的安全隐患,基于大数据分析的精确维护对于水车室的可靠运行至关重要。针对水车室的异常工作状态,通过模型训练、特征工程和分类模型的开发等过程,采用STFT、Log-Mel、MFCC等方法对音频数据进行了预处理,建立了基于音频数据的异常检测模型,并对溪洛渡水电站水车室工作状态进行了异常检测。结果表明,Log-Mel方法具有有效性。研究结果不仅降低了异常检测的成本,还为水电机组的健康监测提供了参考。

基于频率域峭度谱迭代阈值的多尺度形态学轴承故障诊断方法

在复杂恶劣的环境中,存在滚动轴承故障信息难以准确提取的问题。因此,引用多尺度数学形态学对滚动轴承故障诊断开展研究。由于小尺度形态学滤波能较好的保存信号的细节特征,大尺度形态学滤波能有效抑制噪声,因此为了能更好的兼顾噪声抑制和故障特征信息保存,采用迭代阈值的方法选取尺度范围,运用频率域峭度谱方法计算出阈值,然后通过迭代自适应获取最佳尺度区间。多尺度形态学信号重构加权方法引用加权多尺度形态梯度算法,该方法能保证小尺度具有较小的权重,大尺度具有大权重。通过仿真和实验表明:多尺度形态学有效检测出滚动轴承故障信号,深度挖掘滚动轴承故障特征信息。