A parametric study of coupled‐mode flutter for MW‐size wind turbine blades

With the increasing size of offshore wind turbine rotors, the design criteria used for the blades may also evolve. Current offshore technology utilizes three relatively stiff blades in an upwind configuration. With the goal of minimizing the mass, there is an interest in the lightweight rotors that instead utilize two flexible blades oriented downwind. These longer blades are more flexible and thus susceptible to experience flow‐induced instability. Coupled‐mode flutter is one of the destructive aeroelastic instabilities that can occur in flexible structures subjected to aerodynamic loading. Because of variation in one of the system parameters, e.g., flow velocity, structural modes coalesce at a critical flow velocity, and coupled‐flutter occurs. In the present work, a parametric study is conducted in order to study the influence of the natural frequencies in the torsional and flapwise directions on the critical flutter speed for wind turbine blades. Three MW‐size wind turbine blades are studied using a three‐dimensional blade model, which includes coupled flapwise and torsional displacements. The results show that the three blades have very similar behavior as the system parameters vary. It is shown that the first torsional natural frequency and the ratio of the first torsional natural frequency to the first flapwise natural frequency are the most critical parameters affecting the onset of instability. Critical flutter speeds even lower than the blade rated speed can be observed for blades with low torsional natural frequencies. Copyright © 2015 John Wiley & Sons, Ltd.     

机械开关防抖动电路设计

机械开关防抖动电路采用触发器电路,在开关抖动的过程中将开关预期的稳定状态保存下来,使抖动过程对开关的状态没有任何影响.文中主要介绍机械开关防抖动的基本电路设计,并论述其工作原理.     

功率超声珩磨单颗磨粒耦合颤振模型

颤振是功率超声珩磨中极易产生的一种动态强烈自激振动现象,是影响工件加工质量和机床加工效率的重要因素之一.在对功率超声珩磨机理研究的基础上,以单颗磨粒工件系统为研究对象,分别建立了功率超声珩磨系统的两自由度非线性耦合颤振物理模型及数学模型,并确定了其微分方程表达式,为进一步寻求抑制、消除颤振策略提供了理论依据.     

高速磨床动态性能及变速磨削颤振的实验研究

为提高高速磨削表面质量,系统研究高速外圆磨床动力特性。通过设计动态性能试验,找到了样机的振动薄弱环节和主要振源,提出相应的改进措施。在改进后的高速外圆磨床上进行变速磨削颤振试验,结果证明变速磨削能在一定程度上抑制高速磨削颤振。     

模态局部化对T尾颤振特性的影响

模态局部化是弱耦合对称结构中一种不可预期的动力学现象。作为典型的工程对称结构,飞机T尾结构的地面振动试验中通常会发生强烈的模态局部化现象,使得T尾结构模态局部化研究成为T尾结构设计中一个重要的问题。基于失调T尾结构的气动弹性特性,提出了T尾颤振失调设计的概念,然后分析了平尾翼尖的质量失调设计和平尾根部的刚度失调设计,及其失调产生的模态局部化对T尾颤振特性的影响。算例结果表明:(1)失调产生的模态局部化对T尾结构的固有频率和振型产生较大影响。(2)在质量正失调时,T尾结构的模态局部化可以提高T尾颤振速度,而质量负失调时,T尾结构的模态局部化会降低T尾颤振速度。     

具有操纵面立方非线性机翼的混沌响应

以二元机翼-操纵面立方非线性系统为研究对象,基于能量方法和活塞理论建立了三自由度二维翼段-操纵面的运动微分方程,采用当量线性化方法计算出系统极限环颤振频率,然后将操纵面孤立成单自由度系统,借用现有的单自由度杜芬振子的混沌运动的解析条件来分析操纵面在极限环颤振频率下的响应情况,从而预估原系统的混沌运动存在区域,并用数值积分方法研究了系统的复杂动力学响应。结果表明:在理论分析所获得的混沌运动区域内,系统确实存在混沌运动,但从数值模拟的结果上看,在上述的区域内,系统还存在一些狭窄的周期窗口。     

带有结构刚度非线性的跨音速翼型颤振特性研究

以非定常纳维尔－斯托克斯方程为主管方程,计算翼型振荡的瞬态非定常气动力,并与带有结构刚度非线性的颤振方程耦合求解,用时间推进的方法,计算了带有结构刚度非线性的结构响应特性,及带有三次型刚度和间隙型刚度非线性的跨音速翼型库振特性。计算研究表明,岂于同时具有结构和气动非线性,导致了振荡极限环极为复杂的特性。     

超大跨径三塔悬索桥加劲梁吊装方案

为了寻找超大跨径三塔悬索桥建设中更为有利的加劲梁吊装方案,以主跨2 000 m三塔悬索桥为对象,运用参数分析结合有限元数值模拟的方法研究了主、边跨加劲梁不同吊装顺序对边塔鞍座偏移量、主缆线型、合龙位置、合龙段施工方法以及吊装过程中结构动力响应的影响.结果表明,超大跨径三塔悬索桥主跨加劲梁宜从跨中向索塔方向吊装,边跨加劲梁宜从锚碇向边塔方向吊装,该方案下主缆水平倾角小,扭转基频较大,颤振稳定性更好,且采用预偏合龙方法时所需要的牵引力小,因而按该方案吊装对桥梁建设更为有利.     

基于复Morlet小波和SVD的模态参数估计及其应用

与传统的分析技术相比,研究了基于复小波并结合SVD完成信号时频特征提取的参数化模态估计方法,该方法可获得更为准确的模态参数估计值。通过计算机仿真在低信噪比和模态密集情况下分析了该方法的数值特性,并应用某翼型颤振模型风洞试验验证了方法的工程实用价值。     

On-line identification, flutter testing and adaptive notching of structural mode parameters for V-22 tiltrotor aircraft

New algorithms and results are presented for flutter testing and adaptive notching of structural modes in V-22 tiltrotor aircraft based on simulated and flight-test data from Bell Helicopter Textron, Inc. (BHTI). For flutter testing and the identification of structural mode frequencies, dampings and mode shapes, time domain state space techniques based on Deterministic Stochastic Realization Algorithms (DSRA) are used to accurately identify multiple modes simultaneously from sine sweep and other multifrequency data, resulting in great savings over the conventional Prony method. Two different techniques for adaptive notching are explored in order to design an Integrated Flight Structural Control (IFSC) system. The first technique is based on on-line identification of structural mode parameters using DSRA algorithm and tuning of a notch filter. The second technique is based on decoupling rigid-body and structural modes of the aircraft by means of a Kalman filter and using rigid-body estimates in the feedback control loop. The difference between the two approaches is that on-line identification and adaptive notching in the first approach are entirely based on the knowledge of structural modes, whereas the Kalman filter design in the second approach is based on the rigid-body dynamic model only. In the first IFSC design, on-line identification is necessary for flight envelope expansion and to adjust the notch filter frequencies and suppress aero-servoelastic instabilities due to changing flight conditions such as gross weight, sling loads, and air speed. It is shown that by tuning the notch filter frequency to the identified frequency, the phase lag is reduced and the corresponding structural mode is effectively suppressed and stability is maintained. In the second IFSC design using Kalman filter design, the structural modes are again effectively suppressed. Furthermore, the rigid-body estimates are found to be fairly insensitive to both natural frequency and damping factor variations and therefore stability is maintained. The Kalman filter design might be a better choice when the rigid-body dynamics are well known because no adaptation is necessary in this case.