An accurate singularity-free geometrically exact beam formulation using Euler parameters

Wind-induced nonlinear oscillations of twin-box girder bridges are very sensitive to the aerodynamic shape of the deck (i.e., slot width ratio (SWR) and wind fairing shape) due to the complicated flow characteristics around the bridge deck. This paper presents a fully integrated finite element (FE) model in time domain, involving a nonlinear aerodynamic force model and a bridge FE model, to allow the investigation of nonlinear oscillation behaviors of long-span twin-box girder bridges with various SWRs and wind fairing shapes. The parameters in integrated FE model were firstly identified by using CFD simulation, and then, the proposed model was validated by conducting wind tunnel testing using sectional models and full-bridge aeroelastic models. It demonstrates that the developed integrated model has the capability of simulating the nonlinear flutter behaviors of twin-box girder bridges with various aerodynamic shapes. Furthermore, the prediction results show that the wind fairing shape has significant impact on the degree of freedom participation in coupled oscillation and failure modes, as well as flutter performance of the bridges. In addition, there is an increase in amplitudes of the limit cycle oscillations with the increase in the SWR of the twin-box girder bridges, and the relationships between the bending-torsional coupled oscillation, failure modes, and SWR of the bridges with anti-symmetric wind fairings are opposite to those with symmetric wind fairings.     

The Acoustic Analogy and the Prediction of the Noise of Rotating Blades

The acoustic analogy was introduced into acoustics by Lighthill in 1952 to understand and predict the noise generated by the jet of an aircraft turbojet engine. The idea behind the acoustic analogy is simple but powerful. The entire noise generation process is mathematically reduced to the study of wave propagation in a quiescent medium with the effect of flow replaced by quadrupole sources. In jet noise theory, Lighthill was able to obtain significant and useful qualitative results from the acoustic analogy. The acoustic analogy has influenced the theoretical and experimental research on jet noise since the early 1950s. This paper, however, focuses on another area in which the acoustic analogy has had a significant impact, namely, the prediction of the noise of rotating machinery. The governing equation for this problem was derived by Ffowcs Williams and Hawkings in 1969. This equation is a wave equation for perturbation density with three source terms, which have become known as thickness, loading, and the quadrupole source terms, respectively. The Ffowcs Williams–Hawkings (FW–H) equation has been used for the successful prediction of the noise of helicopter rotors, propellers, and fans. Several reasons account for the success and popularity of the acoustic analogy. First, the problems of acoustics and aerodynamics are separated. Second, because the FW–H equation is linear, powerful analytical methods from linear operator theory can be used to obtain closed-form solutions. Third, advances in digital computers and computational fluid dynamics algorithms have resulted in high-resolution near-field aerodynamic calculations that are suitable for noise prediction. We present some of the mathematical results for noise prediction based on the FW–H equation, including examples for helicopter rotors. In particular, we discuss the prediction of blade-vortex interaction noise and high-speed impulsive noise of helicopter rotors. For high-speed propellers, we briefly discuss the derivation of a singularity-free solution of the FW–H equation for a supersonic panel on a blade. Received 24 January 1997 and accepted 30 May 1997     

冲击射流中反馈环机理的再研究

冲击射流的噪声抑制对于研究短程起飞和垂直起降飞行器(STOVL)是极其重要的. 为了研究冲击射流噪声尤其是冲击单音与涡结构尺度之间的关系以及反馈声波的上传方向,采用小波分析技术和``声类比'方法来分析冲击单音的传播方向. 研究中用到的冲击射流的速度场由PIV技术给出,冲击单音的频率通过噪声场的测量获得. 利用双正交小波变换来提取冲击射流速度场中含有的波动信息,结合冲击单音的频率特性对噪声场进行研究.研究结果表明大尺度结构是冲击单音的``拟声源'. 此外,还可以看出大尺度涡结构产生的反馈声波一部分向喷嘴出口处传播,形成反馈环;另一部分反馈声波向四周传播.      

The use of plasma actuators for bluff body broadband noise control

Experiments were conducted using plasma actuators to control broadband noise generated by a bluff body flow. The motivation behind the study was to explore the potential of plasma actuators to reduce landing gear noise during approach phase of an aircraft. The control effectiveness of both dielectric barrier discharge and sliding discharge plasma actuators were tested in laboratory environment, using a representative bluff body consisting of a circular cylinder and an oblique strut. Noise measurements were taken in an anechoic chamber using a phased microphone array and far-field microphones. Results showed that the upstream directed plasma forcing, located at ±90 deg on the upstream cylinder with respect to the approaching flow, could effectively attenuate the broadband noise radiated from the wake flow interaction with the downstream strut. With the same AC electrical power consumption, the sliding discharge with additional DC voltage was found to be more effective due to its elongated plasma distribution and higher induced flow momentum. Measurements using particle image velocimetry suggested that the flow speed impinging on the downstream strut was reduced by the upstream plasma forcing, contributing to the reduced noise.     

飞行器非线性气动伺服弹性力学

现代飞行器日益呈现结构轻质化、控制系统宽通带和高权限的发展趋势. 因此, 非定常气动力、柔性结构和主动控制系统三者间的耦合力学成为重要的研究领域. 自20世纪80年代起, 航空界开始关注受控飞行器的气动弹性稳定性以及主动控制问题, 但对气动/结构的非线性效应、控制回路时滞对受控飞行器动力学行为的影响规律研究尚不充分. 研究这些影响规律不仅涉及非线性、高维数、多变参数和时滞效应等难题, 而且必须面对空气动力、飞行器结构、驱动机构、控制系统之间的强耦合问题. 其中的前沿难题是: 发展非线性气动伺服弹性动力学建模理论, 揭示上述因素诱发受控气动弹性振动的动力学机理, 开展气动伺服弹性控制风洞实验. 本文针对非线性气动伺服弹性力学所涉及的非线性非定常气动力建模、非线性结构动力学、气动伺服弹性控制律设计、气动伺服弹性实验, 总结相关研究现状和最新进展, 特别是近年来作者学术团队的研究成果, 并对进一步研究给出若干建议.      

Passage of a subsonic flow around V-shaped wings

In view of the problems involved in the design of hypersonic aircraft great interest has arisen in recent years as to the behavior of wings in fast supersonic flows. Two main approaches have been used: a study of hypersonic flow around traditional wings, and a search for new configurations with optimum aerodynamic properties. Aerodynamic [1, 2], heat-transfer [3], and stability investigations (for V-shaped wings in super- and hypersonic flows) belong to the latter category. Before attaining supersonic flight the aircraft has to overcome the range of subsonic velocities. In this connection it is important to study flow around V-shaped wings at M < 1. Little research has been devoted to flow around such configurations at subsonic velocities, principal attention having been directed at the study of rapid flow around aircraft configurations with V-shaped wings or tails. The results of analytical and numerical calculations allowing for the interference of transient aerodynamic forces acting on a V-shaped and mutiple-fin tail group in combination with the fuselage were presented in [4, 5]. An experimental study of V-shaped wings as regards the influence of the wing dihedral angle on the aerodynamic characteristics of a model aircraft was presented in [6, 7].Translated from Zhurnal Prikladnoi Mekhaniki i Technicheskoi Fiziki, No. 4, pp. 102–106, July–August, 1975.     

噪声有源控制系统的性能预测

研究有源噪声控制中系统性能的实验预测方法,通过原理分析表明利用原始声场和次级声通道传递函数测试数据,能够预测有源噪声控制系统的性能并优化设计系统;结合飞机舱内的应用实际,给出了有源噪声控制系统性能预测和优化设计的基本方法和过程。该方法已被应用于国产运七飞机舱内噪声有源控制实验,取得了满意的结果。     

Sound transmission through laminated composite cylindrical shells using analytical model

Composite structures are often used in aircraft because of advantages offered by a high strength to weight ratio. Sound transmission through an infinite laminated composite cylindrical shell is studied in the context of the transmission of airborne sound into aircraft interior. The shell is immersed in an external fluid medium and contains an internal fluid, and airflow in an external fluid medium moves with a constant velocity. The different parameters were used to see how laminate specification affected noise transmission. An exact solution is obtained by solving the vibration equation of laminated composite shell and acoustic wave equations simultaneously. Transmission losses (TLs) obtained from numerical solution are compared with those of other authors. The effects of different source condition, structural properties and flight conditions on TL are studied for a range of values, especially, incident angle of the plane wave, Mach number and flight altitude of aircraft, stack sequences, angle of warp and damping.     

飞机结构气动弹性分析与控制研究

随着主动控制技术的发展,飞机结构设计理念已由提高结构刚度的被动设计转变为随控布局的主动设计.主动设计理念不再刻意回避气动弹性问题,而是采用主动控制技术实时调节结构气动弹性,进而减轻结构重量、优化飞机性能. 在飞机随控布局主动设计中,必须深入分析结构与气流之间的耦合,才能更好发挥气动弹性主动控制技术的作用. 从20 世纪80 年代起,航空科技界对该问题进行了长期研究,对飞机结构-空气动力-主动控制相互耦合后的关键力学问题有了深入理解. 然而,已有研究多基于简化模型,导致研究结果难以直接应用于工程. 本文将针对气动弹性动态问题,综述空气动力非线性、控制面间隙非线性、时滞诱发失稳、颤振主动抑制、突风载荷减缓、风洞实验验证等方面的国内外研究进展,重点介绍近年来作者团队所提出的若干方法及相关算例和风洞实验. 最后,指出今后一个时期值得研究的若干气动弹性分析与控制问题.      

翼型连续变形过程中非定常气动特性研究

可变形飞行器的优越性,使其成为近年来研究者关注的热点。当前对于可变形飞行器气动方面的研究工作主要集中于采用数值模拟或风洞实验,开展可变形飞行器气动布局概念和不同变形模态气动特性研究,而关于连续变形过程中非定常气动特性的研究则很少,几为空白。本文在无粘不可压(Ma=0.2)流场中,数值模拟了儒可夫斯基翼型连续改变厚度、弯度所引起的气动特性变化,分析了不同变形幅度、变形速度以及变形加速度对非定常升力系数的影响情况,这也为以后的三维数值模拟提供了基础。     

非稳态温度场热弹塑性蠕变有限元分析

本文分析航空发动机主要部件在飞行一次循环加载过程中的应力与应变状态.考虑到瞬态效应,它们在升机与降机时应力变化是剧烈的,为此编制了非稳态温度场的热弹塑性蠕变分析专用程序(简称TEPCAB),该程序包括各向同性强化与随动强化两种模型.经考核后对某涡盘采用不同加载方案进行应力分析与比较.所得结论可供工程设计者参考.     

Stochastic optimal control of flexible aircraft taxiing at constant or variable velocity

Active control landing gears are used to alleviate vibration during aircraft taxiing. A nonlinear stochastic dynamics model is established, considering the aircraft body pitch movement and elastic vibration excited by the random runway. The equivalent linearization method is adopted to ensure the model linearity near the balance point, and the Gaussian random process of the runway is generated from the Gaussian white noise using a shape filter. Based on the stochastic optimal control theory, the LQG controller is designed along with weighted quadratic performance index for a better ride comfort, shock absorption, road holding and least energy expenditure. The algebraic Riccati and Lyapunov equations are solved to obtain stationary response while taxiing aircraft at a constant velocity and the differential Riccati and Lyapunov equations are solved to obtain the nonstationary response while taxiing aircraft at a variable velocity. The aircraft dynamic responses are obtained through the runway random process modeled by Monte Carlo method. Simulation results show that active control landing gear can give a better ride comfort, shock absorption and road holding performance no matter whether taxiing is at constant or variable velocity.     

复合材料层板构件气弹振动控制实验研究

飞机局部复合材料构件存在着因涡流作用诱发的弹性振动问题。本文首先研究了表面粘贴型压电元件对复合材料构件的传感和驱动原理;针对结构待控模态的要求,采用了D-准则优化确定同位压电传感/驱动器在构件中的布置方案;应用自适应振动前馈控制原理和方法,构造了闭环控制系统;分别利用正弦和方波信号激发气动声场,对玻纤/环氧圆筒构件进行激振和振动控制实验;结果表明：构件主要待控模态的振动得到了有效抑制,但也出现了高阶模态被激发的问题,导致结构辐射噪声上升。     

翼吊式双发民机机体/动力装置一体化数值分析

介绍了多块网格技术与流场分区求解方法在翼吊式双发民机机体／动力装置一体化研究中的应用。数值求解Ｅｕｌｅｒ方程模拟复杂组合体绕流。采用边界层方程／Ｅｕｌｅｒ方程耦合迭代技术进行翼面粘性修正。为保持Ｅｕｌｅｒ注解中计算网格固定,粘流／无粘流耦合迭采用表面源模型。该方法对某民用飞机模型跨音速绕流流场进行了数值模拟,机翼表面计算压力分布与实验吻合良好。     

A method for the identification of dynamic constraint parameters in multi-supported flexible structures

In this paper, a new method for identifying the dynamical parameters of local constraining supports such as mass, stiffness, and damping was developed through combining the measured frequency transfer functions and structural modification techniques. Since measurement noise often leads to erroneous identifications, regularization techniques have been implemented to reduce noise amplification in the inverse problem. The developed technique has been validated by numerical tests on a multi-supported flexible structure, which can be seen as an idealized electricity generator rotor shaft. The results are satisfactory for noise-free data as well as under realistic noise levels. The sensitivity of the identified support features to noise levels is asserted through a parametric study     

Frost formation mechanism analysis and frost growth prediction on ground aircraft

Frost predictions are needed to help the deicing operation decide. The mechanism of frost formation on aircraft surface under icing conditions has been analyzed. A simple theoretical frost growth prediction model by heat and mass transfer analysis has been presented. It produces a method to forecast the frost growth tendency. An experimental system for atmospheric frost reproduction is also presented. Effects of aircraft surface temperatures, air temperature on the frost growth is evaluated by this model.     

Safe regions with partial control of a chaotic system in the presence of white Gaussian noise

Noise is present in a wide variety of engineering systems, and it can play a significant role in influencing system dynamics. Under the influence of noise, it has been shown that the response of many discrete-time dynamical systems can be moved away from a particular region. In the present study, the partial control scheme constructed for a chaotic system is applied for confining the trajectories inside a particular region despite the presence of white noise. The proposed algorithm is independent of the dimension of the system. As an illustration, the partial control method has been applied to restrict the response of a Duffing oscillator to a certain state-space region. Different noise forms are considered and numerical results are presented to illustrate the effectiveness of this control method.     

复合材料胶接修理损伤金属结构的研究现状

损伤金属飞机结构的复合材料补片胶接修理技术是一项经济而有效的结构延寿方法．详细 叙述了这种修理技术的优缺点及其技术要点,涉及的内容包括胶粘剂的选择、补片材料和参数设计、 表面处理、无损检测和试验验证等．介绍了国内外对该项先进修理技术的研究现状和在飞机结构上 的实际使用情况．评述了现有的理论分析和试验研究方法及其结果．      

摩擦噪声研究的现状和进展

评述了近２０年来摩擦噪声的理论和实验研究进展,指出在摩擦噪声研究领域取得的主要进展表现在制动系统摩擦尖啸噪声的动力学分析下,而对摩擦同观结构影响的研究不多,许多问题还有待深入研究,为了更好地认识摩擦噪声机理和探讨控制摩擦噪声的措施,建议应从摩擦学与动力学交叉的角度来研究和分析摩擦噪声。