失谐叶片-轮盘结构系统振动局部化问题的研究进展

对近２０年来国内外关于失谐叶片－轮盘结构系统振动局部化问题研究的进展进行了较为详细的评述和讨论,文中首先说明了振动局部化问题的基本概念,然后对失谐叶片－轮盘结构系统模态局部化和动态响应局部化在分析模型、求解方法及其基本性质和规律等方面的研究进展进行了较为全面的评述,最后提出了今后应深入研究的问题．     

失谐弱耦合卫星天线结构振动分析及预测控制

为了研究弱耦合卫星天线结构的振动控制,建立了该结构的简化计算模型,并针对该模型研究了弱耦合卫星天线结构动力学性能的特殊性:结构失谐时的振动模态局部化现象;针对失谐前后的结构,采用预测控制方法进行了振动控制,并与二次线性最优控制(LQR)方法的振动抑制效果进行了对比. 仿真结果表明:弱耦合星载天线结构参数的微小失谐会导致结构振动产生明显的模态局部化;采用预测控制方法进行结构振动控制的效果明显优于LQR控制方法,且在失谐导致的模型失配时,预测控制方法对结构振动亦有较好的抑制;在进行此种结构的振动主动控制时必须考虑到结构失谐的影响.      

失谐周期结构中振动局部化问题的研究进展

周期结构在工程中有很多应用实例, 其具有频率通带和禁带等特殊力学性质. 失谐可使周期结构的力学特性产生本质变化, 即失谐周期结构中存在振动局部化现象.局部化破坏了周期结构模态的规则性, 在外激励下会使结构某些部位的响应幅值过大, 产生能量积聚, 甚至导致结构发生疲劳破坏. 因此分析失谐周期结构中振动和能量的传播方式与规律具有重要的理论与实际意义, 可以为重要子结构的振动控制和减振设计提供理论依据. 针对一维直线型周期结构、循环周期结构以及二维周期结构等, 综述了其中的振动局部化问题的研究现状,主要集中于力学模型的建立、振动局部化问题的研究内容、分析方法和主要研究结果等, 并提出了值得进一步研究的问题.      

大型离心叶轮振动模态局部化特性研究

为了研究目前工程中大型流体机械离心叶轮出现的局部疲劳破坏的机理,基于现有的有限元分析方法,结合模态分析等动力学理论以及叶轮所承受的气流激励,对其动力学特性进行了研究。着重研究了离心叶轮这类周期循环对称性结构具有的不同于非周期循环结构的特殊动力学性质以及该特殊的动力学特性与叶轮疲劳破坏的联系。研究发现其存在频率通带(passbands)、频率禁带(stopbands)现象,并出现了振动模态局部化现象。另外,叶轮的动力学特性对其周期性结构失谐特别敏感,该类失谐可来自由制造误差、材料和使用中磨损出现的不均匀等多种因素。对于协调结构,在一定条件下(如系统具有高密集模态),很小的失谐量(1%)就可使结构振动模态产生急剧变化,从而出现振动模态局部化现象。对于所研究的机组,当进口预旋器导致的流体激振频率(1166.7Hz)接近叶轮的由第12阶～18阶固有频率组成的禁带(1020.3～1054.5 Hz)时,数值分析结果显示叶片进口部位出现了振幅较大的振动,与该机组实际破坏的部位相符。研究结果表明所使用的振动模态局部化分析方法能够揭示叶轮发生疲劳破坏的原因,即是一类共振型疲劳破坏现象。     

T尾结构振动的模态局部化判据研究

用峰值振幅比定义局部化度,用平尾刚度与垂尾刚度的比值定义耦合度.基于T尾结构的质量失调模型,从模态峰值振幅比、失调耦合比、常规摄动和近频摄动4个角度,提出4个不同的局部化判据来预测T尾结构模态局部化的发生.对一个T尾结构模型局部化振动的数值分析结果表明:(1)T尾结构系统一般具有弱耦合性,小量的失调就可以使T尾结构发生模态局部化;(2)T尾结构一旦发生模态局部化,不但使对称一弯模态和反对称一弯模态的振型发生较大改变,而且其模态频率也将改变,模态频率的改变在失调量的正负区间内具有唯一性;(3)算例验证了4个模态局部化判据的可行性和有效性,为T尾结构的模态局部化分析和设计提供了依据.     

失谐周期结构中弹性波与振动的局部化

基于弹性波传递矩阵方法，研究了失谐周期结构中弹性波与振动的局部化问题．给出了结构中弹性波传递矩阵的一般表达式，采用奇异值分解方法，分别计算了谐和与失谐周期结构中的局部化因子，并对其进行了分析讨论．对周期结构中波传播与振动局部化的分析方法可用于结构的优化设计．     

结构参数对飞机高平尾振动模态局部化的影响

针对结构参数对高平尾布局尾翼振动横态局部化影响进行了研究。考虑到失调本身的不可预知性,在实际结构的基础上设计了质量正失调、质量负失调、刚度正失调、刚度负失调四种可能出现的失调尾翼模型。当失调量为±0.5时,通过模态分析法得到了翼梁和翼肋厚度不同的情况下结构的振动模态局部化度,并讨论了结构自身参数对不同失调原因引起的模态局部化的影响。结果表明:垂尾梁厚度的增加会提高振动模态局部化程度,而平尾梁厚度的影响与之相反,翼肋参数对模态局部化基本无影响;与前梁厚度相比,后梁厚度对结构振动模态局部化的影响较大;当垂尾后梁厚度增加到2.0mm时,0.5kε=-的失调模型的局部化度从0.342变为0.971,这种显著变化在结构设计中需引起注意;此外,结构参数对质量正失调和刚度负失调产生的模态局部化的影响强于质量负失调和刚度正失调。     

轴流泵参数化设计及应力与模态特征分析

以船用轴流式喷水推进泵为对象,探索了轴流泵参数化设计、水动力性能、静强度和结构声学特征分析的数值途径。轴流泵叶轮采用升力法设计,导叶采用流线法设计,叶片三维造型在NUMECA参数化设计平台中完成。轴流泵水动力性能校核由粘性CFD计算完成,CFD计算同时提取得到叶片分布式水动力载荷。叶片静强度校核由ANSYS有限元计算叶片应力和应变特征完成,应力分析时同时考虑水动力载荷、重力载荷和离心力载荷。叶片结构声学特征分析由NASTRAN有限元计算叶片模态振型和振型频率完成。计算结果表明,轴流泵扬程和功率满足设计指标,效率达87.13%;叶轮叶片形变相对于叶顶间隙来说为极小量,可忽略不计,叶片存在局部应力集中现象,最大应力小于许用应力,满足静强度要求;叶片前四阶振型特征与分析经验一致,且振型频率远离轴频、叶频及其谐频特征频率,能够避免共振产生。     

失谐周期弹性支撑多跨梁中的波动局部化

分析研究了失谐周期弹性支撑多跨梁中的波动局部化问题,采用传递矩阵方法给出了系统的传递矩阵,采用Wolf提出的计算Lyapunov指数的方法,确定了局部化因子,作为算例,给出了结构中局部化因子的数值结果,分析了跨长的失谐程度、线弹簧和抗弯弹簧的无量纲刚度对弹性波局部化的影响。     

振型一阶导数的高精度截尾模态展开法

模态展开法是计算振型一阶导数的常用方法,然而,当高阶模态被截断时,它不能给出精确解,甚至会产生很大的截断误差。本文研究被截断的高阶模态对振型导数贡献的定量计算问题,证明了被截断模态的贡献可以用已知的低阶模态和系统矩阵来显式表达,给出了计算方法,并用数值例子说明了本文方法的有效性和正确性。     

2010城市地质环境与可持续发展论坛（二号通知）

用峰值振幅比定义局部化度,用平尾刚度与垂尾刚度的比值定义耦合度. 基于T 尾结构的质量失调模型,从模态峰值振幅比、失调耦合比、常规摄动和近频摄动4个角度, 提出4个不同的局部化判据来预测T尾结构模态局部化的发生. 对一个T尾结构模型局部化 振动的数值分析结果表明: (1) T尾结构系统一般具有弱耦合性,小量的失调就可以使T尾 结构发生模态局部化; (2) T尾结构一旦发生模态局部化,不但使对称一弯模态和反对称一 弯模态的振型发生较大改变,而且其模态频率也将改变,模态频率的改变在失调量的正负区 间内具有唯一性; (3) 算例验证了4个模态局部化判据的可行性和有效性,为T尾结构的模 态局部化分析和设计提供了依据.     

Localization in Nonlinear Mistuned Systems with Cyclic Symmetry

In forced systems with cyclic symmetry localization can occur due toparameter uncertainties. Often, Monte-Carlo simulations are used to findregions, where the system response is sensitive to parameteruncertainties. These simulations require a large computation time.Therefore, an approximate method to calculate the envelopes of thefrequency response functions is developed in this paper. An example of anonlinear system with cyclic symmetry is a bladed disk assembly withfriction dampers. Friction dampers can be installed underneath the bladeplatforms of turbine blades. Due to dry friction and the relative motionbetween blades and dampers, energy is dissipated, which results in areduction of blade vibration amplitudes. By optimizing the mass of thefriction dampers, the best damping effects are obtained, which lead toan increase in the reliability of the turbine. In this paper, thecalculated response of a mistuned bladed disk assembly with frictiondampers is discussed. An approximate method is developed to calculatethe envelopes of the corresponding frequency response function forstatistically varying eigenfrequencies of the blades. Regions wherelocalization can occur with a high probability, are calculated by thismethod.     

非线性系统模态分叉与模态局部化现象

运用匹配法和多尺度法对一个两自由度非线性系统进行了研究，详细分析了非线性系统的模态分叉和局部化现象     

基于子集模拟法的风机叶片可靠度分析

风机叶片是风机机组中最重要的组件之一,其可靠性对整个发电系统的健康运行十分关键。传统的蒙特卡罗法可靠度计算效率较低,本文将子集模拟法应用到风机叶片的可靠度计算中,通过合理选取中间失效事件临界值和建议概率密度函数,采用Metropolis算法模拟产生一系列的样本,极大地提高了抽样效率。结果表明,子集模拟法在风机叶片可靠度计算中精度较高,且结构的失效概率越小,相比于传统蒙特卡罗法效率更高,可以极大地减轻计算压力。     

Nonlinear modeling of tuned liquid dampers (TLDs) in rotating wind turbine blades for damping edgewise vibrations

Tuned liquid dampers (TLDs) utilize the sloshing motion of the fluid to suppress structural vibrations and become a natural candidate for damping vibrations in rotating wind turbine blades. The centrifugal acceleration at the tip of a wind turbine blade can reach a magnitude of 7–8g. This facilitates the use of a TLD with a relatively small fluid mass and with feasible geometric dimensions to mitigate the lightly-damped edgewise vibrations effectively. In the present paper, modal expansions are carried out directly on the velocity field and the free surface of the sloshing liquid in the rotating coordinate system. A formulation has been proposed leading to coupled nonlinear ordinary differential equations, which have been obtained through the Galerkin variational approach together with the modal expansion technique. Two models, with one sloshing mode and three sloshing modes, have been studied in the numerical simulation. It is shown that the one-mode model is able to predict the sloshing force and the damped structural response accurately, since the primary damping effect on the structure is achieved by the first sloshing mode of the fluid. Although it is unable to predict the fluid free-surface elevation equally well, the one-mode model can still be utilized for the design of TLD. Parametric optimization of the TLD is carried out based on the one-mode model, and the optimized damper effectively improves the dynamic response of wind turbine blades.     

Singular Characteristics of Nonlinear Normal Modes in a Two Degrees of Freedom Asymmetric Systems with Cubic Nonlinearities

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Aeroelastic analysis of large horizontal wind turbine baldes

A nonlinear aeroelastic analysis method for large horizontal wind turbines is described. A vortex wake method and a nonlinear finite element method (FEM) are coupled in the approach. The vortex wake method is used to predict wind turbine aerodynamic loads of a wind turbine, and a three-dimensional (3D) shell model is built for the rotor. Average aerodynamic forces along the azimuth are applied to the structural model, and the nonlinear static aeroelastic behaviors are computed. The wind rotor modes are obtained at the static aeroelastic status by linearizing the coupled equations. The static aeroelastic performance and dynamic aeroelastic responses are calculated for the NH1500 wind turbine. The results show that structural geometrical nonlinearities significantly reduce displacements and vibration amplitudes of the wind turbine blades. Therefore, structural geometrical nonlinearities cannot be neglected both in the static aeroelastic analysis and dynamic aeroelastic analysis.     

Application of bamboo laminates in large-scale wind turbine blade design

From the viewpoint of material and structure in the design of bamboo blades of large-scale wind turbine, a series of mechanical property tests of bamboo laminates as the major enhancement materials for blades are presented. The basic mechanical characteristics needed in the design of bamboo blades are briefly introduced. Based on these data, the aerodynamic-structural integrated design of a 1.5 MW wind turbine bamboo blade relying on a conventional platform of upwind, variable speed, variable pitch, and doubly-fed generator is carried out. The process of the structural layer design of bamboo blades is documented in detail. The structural strength and fatigue life of the designed wind turbine blades are certified. The technical issues raised from the design are discussed. Key problems and direction of the future study are also summarized.     

Supercritical and subcritical dynamic flow-induced instabilities of a small-scale wind turbine blade placed in uniform flow

There is a growing interest in extracting more power per turbine by increasing the rotor size in offshore wind turbines. As a result, the turbine blades will become longer and therefore more flexible, and a flexible blade is susceptible to flow-induced instabilities. In order to design and build stable large wind turbine blades, the onset of possible flow-induced instabilities should be considered in the design process. Currently, there is a lack of experimental work on flow-induced instabilities of wind turbine blades. In the present study, a series of experiments were conducted and flow-induced instabilities were observed in wind turbine blades. A small-scale flexible blade based on the NREL 5 MW reference wind turbine blade was built using three-dimensional printing technique. The blade was placed in the test section of a wind tunnel and was subjected to uniform oncoming flow, representing the case of a parked wind turbine blade. The blade׳s tip displacement was measured using a non-contacting displacement measurement device as the oncoming wind speed was increased. At a critical wind speed, the blade became unstable and experienced limit cycle oscillations. The amplitude of these oscillations increased with increasing wind speed. Both supercritical and subcritical dynamic instabilities were observed. The instabilities were observed at different angles of attack and for blades both with and without a geometric twist. It was found that the blade twist had a significant influence on the observed instability: a blade without a twist experienced a strong subcritical instability.