Dynamic instability characteristics of laminated composite plates subjected to partial follower edge load with damping

The study of vibration and dynamic instability behaviour of laminated composite plates subjected to partially distributed non-conservative follower forces is presented by using the finite element technique. The first-order shear deformation theory is used to model the plate, considering the effects of shear deformation and rotary inertia. The modal transformation technique is employed to the resulting equilibrium equation for subsequent analysis. Structural damping is introduced into the system in terms of equivalent viscous damping to study the significance of damping on stability characteristics. The effects of load width, boundary condition, aspect ratio, ply orientation, direction control of the load and damping parameters are considered for the stability behaviour of the plates. The results show that under follower loading, the system is susceptible to instability due to flutter alone or due to both flutter and divergence, depending on system parameters.     

预张力膜片风振反应的试验研究

针对张拉膜片在确定的预应力水平下的风振反应来考察它的模态和风振特性．采用膜面预应力检测仪测量了试验中膜片的预应力水平。膜结构的振动形式经常是以高阶振型为主．基频随预应力水平的增大而增大。预应力水平越高，各阶模态对应的阻尼比相差越小；反之，气动阻尼越集中在低阶模态。与风速和预应力水平相比．膜面倾角对膜面位移风振系数的影响更为显著．并对不同倾角的膜面位移风振系数提出了建议值。部分工况下膜片出现颤振。膜面倾角和较低的预应力水平是诱发颤振的主要因素。     

Output-only modal analysis of wind turbine tower based on vibration response under emergency stop

The identification technique of output-only modal parameters is proposed for the large wind turbine tower under emergency stop. Compared with the response of regular operating conditions, the immediate tower structural response under emergency stop much more resembles a state of free vibration, which is more appropriate for the modal identification of the wind turbine tower. The vibration response is measured in the nacelle, which is easy to perform in the field modal test. The variational mode decomposition (VMD) is applied to decompose the vibration response into several band-limited intrinsic mode functions. The free responses of decomposed functions are extracted by applying the random decrement technique (RDT). Finally, the modal damping ratio and natural frequency are identified from each free modal response by using the Hilbert transform method. Simulations and a 1.5 MW wind turbine field modal test results verify the effectiveness of the proposed identification method. The main modal parameters of wind turbine, including weak modes, are effectively extracted by using output-only vibration responses under emergency stop. The modal parameter identification method is provided for the large wind turbine structure under the engineering condition.     

Modal parameter estimation and monitoring for on-line flight flutter analysis

The clearance of the flight envelope of a new airplane by means of flight flutter testing is time consuming and expensive. Most common approach is to track the modal damping ratios during a number of flight conditions, and hence the accuracy of the damping estimates plays a crucial role. However, aircraft manufacturers desire to decrease the flight flutter testing time for practical, safety and economical reasons by evolving from discrete flight test points to a more continuous flight test pattern. Therefore, this paper presents an approach that provides modal parameter estimation and monitoring for an aircraft with a slowly time-varying structural behaviour that will be observed during a faster and more continuous exploration of the flight envelope. The proposed identification approach estimates the modal parameters directly from input/output Fourier data. This avoids the need for an averaging-based pre-processing of the data, which becomes inapplicable in the case that only short data records are measured. Instead of using a Hanning window to reduce effects of leakage, these transient effects are modelled simultaneously with the dynamical behaviour of the airplane. The method is validated for the monitoring of the system poles during flight flutter testing.     

A numerical integration method for the determination of flutter speeds

A method of determining the flutter speed of a structural system using a numerical integration method is described. The response of the system to a random disturbance is determined at various wind speeds. From these the system dampings are calculated and the wind speed for which damping is zero is the flutter speed.The method has been developed for application to suspension bridge problems in which the flutter modes are not known functions and to configurations involving structural and aerodynamic nonlinearities.     

Extraction of the damping ratio of a beat wave using envelope logarithmic decrement

A phenomenon called “beat” occurs in bell structures with slight asymmetry, wherein the vibration and the sound periodically fluctuate with respect to strength. In oriental bells, beat makes the bell sound grand and lively. The audience wants to hear the bell sound with a strong beat for as long as possible. The modal damping ratio is used as a physical factor to evaluate the duration of the beat sound. For low damping, the modal damping ratio can be extracted using the logarithmic decrement method with a single-frequency time signal. However, because the beat wave includes two adjacent frequencies, the conventional logarithmic decrement method cannot be applied. In this study, a method for extracting the damping ratio from the envelope data of beat waves was proposed and verified experimentally. Finally, the method was applied to evaluate the duration of the beat sound of the 2018 Winter Olympic bell.

     

模态试验中柔性支承特性的识别及其对试验结果影响的修正

对模态试验中采用的柔性支承的特征识别和支承对模态参数影响修正方法作了研究。     

Output-only analysis of structures with closely spaced poles

In the framework of the operational modal analysis, several approaches have been developed for estimating the modal parameters, i.e., natural frequencies, damping ratios, and mode shapes. Specifically, a technique capable to evaluate the biased (i.e., unscaled by a constant or an almost constant function) frequency response functions, FRFs, has been proposed. Assuming that only the responses of the structure are disposable, the technique allows one to estimate biased FRFs starting from the power spectral densities, PSDs, and applying the Hilbert transform. This paper deals with the estimates of the modal analysis parameters mentioned above. It is possible to obtain each single mode shape, from the singular vectors achieved by applying the singular value decomposition to the FRF matrix evaluated at the spectral line corresponding to the selected natural frequency. A special attention will be devoted to structures with coupled modes, i.e., closely spaced modes. Once the FRFs have been obtained, the natural frequencies and damping ratios could be achieved either in the frequency domain or in the time domain. Experimental tests, carried out on beams, plates and on the AB-204 helicopter blade, will be presented.     

减振镗杆的动态特性分析

介绍了深孔镗削加工过程中产生振颤的机理,建立了减振镗杆的动力学模型,在此基础上优化减振系统的参数,并且对减振镗杆进行瞬态动力学和谐响应分析,分析结果表明该减振系统具有良好的减振性能。     

EVALUATION OF DAMPING NON-PROPORTIONALITY USING IDENTIFIED MODAL INFORMATION

This paper focuses on quantification of damping non-proportionality present in a discrete vibratory system. The study assumes that the information available is a set of identified system eigenvalues and eigenvectors and that the system parameters such as mass, stiffness, and damping matrices are unknown a priori. This set of modal parameters may be incomplete. The investigation is concentrated on how two existing analytical indices can be utilised when the modal damping matrix is not available. The quantification procedure starts with extraction of normal modes using a known algorithm. It is shown that two matrices, by-products of the normal model extraction, can be used to study damping non-proportionality. The first matrix is a scaled modal damping matrix. The paper shows that the indices developed from the scaled modal damping matrix preserves the properties of the indices based on the analytical modal damping matrix. The second matrix is a complex matrix which is obtained by expanding complex modes into the subspace of real modes. The off-diagonal elements of the complex matrix indicate coupling between modes due to damping non-proportionality. Based on this characteristic, three new indices are proposed. Numerical examples are presented to illustrate the use of the new indices and to compare them with the indices that are described in literature.     

The inception of OMA in the development of modal testing technology for wind turbines

Wind turbines are immense, flexible structures with aerodynamic forces acting on the rotating blades at harmonics of the turbine rotational frequency. These harmonics are comparable to the modal frequencies of the structure. Predicting and experimentally measuring the modal frequencies of wind turbines have been important to their successful design and operation. Performing modal tests on wind turbine structures over 100 m tall is a substantial challenge, which has inspired innovative developments in modal test technology. For wind turbines, a further complication is that the modal frequencies are dependent on the turbine rotation speed. The history and development of a new technique for acquiring the modal parameters using output-only response data, called the Natural Excitation Technique (NExT), will be reviewed, showing historical tests and techniques. The initial attempts at output-only modal testing began in the late 1980s with the development of NExT in the 1990s. NExT was a predecessor to Operational Modal Analysis (OMA), developed to overcome these challenges of testing immense structures excited with natural environmental inputs. We will trace the difficulties and successes of wind turbine modal testing from 1982 to the present.     

The determination of modal damping ratios and natural frequencies from bispectrum modeling

We present a new signal processing and testing technique by using a higher statistical moment, the bispectrum, to determine the damping ratio and natural frequency of offshore structures excited by both unexpected Gaussian forces and known non-Gaussian driving forces. Due to unexpected exciting forces, such as turbulence, in the ocean, environment, the transfer functions of offshore structures are not determined through operating a known driving force and measuring its response. In order to overcome this problem, some of the existing techniques try to model the unexpected forces as white Gaussian forces or almost white Gaussian forces and determine the modal parameters from the response only. Others try to average the input and output to suppress unexpected parts. Our method uses third-order moments to keep the influence of the unexpected Gaussian forces away from the determination of the transfer function of the structure which has linear properties. We model the third-order moment property of the response function with a bispectral model. The modal parameters can be calculated from the estimated model's coefficients. The method has been proven by a number of simulations.     

基于环境激励的港口起重机工作模态分析

针对港口起重机难以施加激振力的问题,采用自然环境激励技术(NExT)对港口起重机进行了工作模态分析.在自然风的随机激励下,对某港口40t位起重机进行了模态试验.采用响应点与参考点之间的互功率谱的幅值图代替集总传递函数的幅值图进行了模态分析.由所有测点和参考点间的互功率谱,通过拟合各响应点同参考点之间的互功率谱来识别起重...     

Measurement and correlation of high frequency behaviors of a very flexible beam undergoing large deformation

A correlation method of high frequency behaviors of a very flexible beam undergoing large displacement is presented. The suggested method based on the experimental modal analysis leads to more accurate correlation results because it directly uses the modal parameters of each mode achieved from experiment. First, the modal testing and the parameter identification method are suggested for flexible multibody dynamics. Due to the flexibility of a very thin beam, traditional testing methods such as impact hammer or contact type accelerometer are not working well. The suggested measurement with high speed camera, even though the test beam is very flexible, is working well. Using measurements with a high speed camera, modal properties until the 5th mode are measured. And After measuring each damping ratio until the 5th mode, a generic damping model is constructed using inverse modal transformation technique. It’s very interesting that the modal transformation technique can be also applied even in the ANCF simulation which uses the global displacement and finite slope as the nodal coordinates. The results of experiment and simulation are compared until the 5th mode frequency, respectively, by using ANCF forced vibration analysis. Through comparison between numerical simulation and experiment, this study showed that the proposed generic damping matrix, modal testing and parameter identification method is very proper in flexible multibody dynamic problems undergoing large deformation.     

Identification of modal parameters using the wavelet transform

The wavelet transform is used as a time-frequency representation for the determination of modal parameters such as natural frequencies, damping ratios and mode shapes of a vibrating system. It is shown that using a particular form of the son wavelet function, results are improved compared to those obtained with the traditionally Morlet wavelet function. The accuracy of this new technique is confirmed by applying it to a numerical example and to ambient vibration measurements of a tower excited by wind.     

基于H_∞控制和μ控制的二元机翼颤振主动抑制分析

针对二元机翼颤振主动抑制问题,对二元翼段颤振控制律的设计以及风洞实验验证展开了深入研究。首先,基于Theodorsen非定常气动力理论建立了该翼段模型的运动状态方程。其次,基于鲁棒控制理论设计了H_∞控制器和μ控制器,分别与二元翼段气动弹性系统共同组成了机翼颤振主动控制系统。最后,采用数值仿真和风洞实验相结合的方式,对H_∞控制器和μ控制器的颤振控制效果进行了验证。研究结果表明:H_∞控制器和μ控制器都能有效抑制颤振的发生,可将机翼颤振临界速度由15.45 m/s提高到27 m/s以上。且μ控制器在颤振主动抑制中的颤振抑制收敛时间更短,颤振抑制风速上限更高,具有更好的鲁棒稳定性和鲁棒性能。     

基于递归理论的泵站压力管道运行状态监测

为了准确地识别建筑结构的模态参数,提出了一种基于多重信号分类算法（multiple signal classification,简称MUSIC）、经验小波变换（empirical wavelet transform,简称EWT）和同步提取小波变换（synchroextracting transform ,简称SET）的结构模态参数识别方法。首先,通过MUSIC-EWT对实测振动信号进行分解;其次,使用SET对单模态信号进行去噪处理;然后,采用自然环境激励技术（natural excitation technique,简称NExT）得到单模态信号的自由衰减响应;最后,利用Hilbert变换（hilbert transform,简称HT）和曲线拟合获得结构的自振频率和阻尼比。通过三层框架结构的数值模拟验证了该方法的准确性和鲁棒性。利用该方法对台风“达维”作用下广州中信广场的实测加速度数据进行分析,并将估计的结构模态参数和其他识别方法的分析结果进行对比,进一步证明了该方法的准确性和鲁棒性。     

基于递归理论的泵站压力管道运行状态监测

针对不利因素导致的管道运行异常问题,提出一种基于递归理论的泵站管道运行状态监测方法。首先,通过振动传感器提取压力管道关键部位的实测信息,并将同一位置不同方向的数据信息进行融合,得到一组反映结构整体动力特性的综合数据;其次,利用伪近临法与互信息法分别选取相空间重构参数m和τ;最后,绘制并计算代表管道动力特性的递归图及递归量化指标。将该方法应用于景泰川工程二期七泵站管道运行监测,通过设置不同的运行工况进行验证,结果表明：机组开关瞬间与稳定运行工况下,管道结构振动信号的递归图呈现不同模式,递归量化指标-确定性、对角线平均长度L、递归率及递归熵也呈现明显差异,能有效区分管道振动状态。该方法为压力管道的无损动态监测提供了新思路。     

Smoothing of power spectral densities by successive autocorrelation: An application to flight flutter tests

A simple modal parameters mxtraction method (natural frequencies and damping ratios) from response power spectral densities (PSD) due to random excitations is presented. Firstly, the noisy PSD are smoothed using the ‘successive autocorrelation method’. The inverse of the resulting smoothed displacement-PSD is then fitted in the frequency domain using a hysteretically damped sdof receptance model.The validity and efficiency of the method is shown using a small plate. The method is then applied to the delicate flight flutter testing problem.     

Wind tunnel experiment for wind breakage of Actinidia deliciosa P. shoots

This study investigates the wind-breakage biomechanics of Actinidia deliciosa P. using varying degrees of free-stream turbulence and different physical parameters of plant shoots. Two different turbulent boundary layers (TBL) of wind were simulated inside a wind tunnel test section. The relationship between the load capability and the actual load of shoots was evaluated by using a specific failure criterion. The responses of the shoots to the on-coming wind velocity were estimated quantitatively using three physical parameters: the speedspecific drag, the E-value, and the factor of safety. The E-value of shoots exposed to a highly turbulent condition (TBL type B) is higher than those exposed to a less turbulent one (TBL type A). As the value of the length-diameter ratio (L/D) increases, the peak location of the speed-specific drag shifts towards higher wind speed and the E-value likewise increases. In addition, with increasing L/D, the factors of safety dwindle regardless of wind speed. The trend in the relationship between L/D and the factor of safety for both TBL types is similar.