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.     

An efficient method for nonlinear aeroelasticy of slender wings

This paper aims the nonlinear aeroelastic analysis of slender wings using a nonlinear structural model coupled with the linear unsteady aerodynamic model. High aspect ratio and flexibility are the specific characteristic of this type of wings. Wing flexibility, coupled with long wingspan can lead to large deflections during normal flight operation of an aircraft; therefore, a wing in vertical/forward-afterward/torsional motion using a third-order form of nonlinear general flexible Euler–Bernoulli beam equations is used for structural modeling. Unsteady linear aerodynamic strip theory based on the Wagner function is used for determination of aerodynamic loading on the wing. Combining these two types of formulation yields nonlinear integro-differentials aeroelastic equations. Using the Galerkin’s method and a mode summation technique, the governing equations will be solved by introducing a numerical method without the need to adding any aerodynamic state space variables and the corresponding equations related to these variables of the problem. The obtained equations are solved to predict the aeroelastic response of the problem. The obtained results for a test case are compared with those of some other works and show a good agreement between results.     

Nonlinear Analysis of Store-Induced Limit Cycle Oscillations

Flight tests of modern high-performance fighter aircraft reveal the presence of limit cycle oscillation (LCO) responses for aircraft with certain external store configurations. Conventional linear aeroelastic analysis predicts flutter for conditions well beyond the operational envelope, yet these store-induced LCO responses occur at flight conditions within the flight envelope. Several nonlinear sources may be present, including aerodynamic effects such as flow separation and shock-boundary layer interaction and structural effects such as stiffening, damping, and system kinematics. No complete theory has been forwarded to accurately explain the mechanisms responsible. This research examines a two degree-of-freedom aeroelastic system which possesses kinematic nonlinearities and a strong nonlinearity in pitch stiffness. Nonlinear analysis techniques are used to gain insight into the characteristics of the behavior of the system. Numerical simulation is used to verify and validate the analysis. It is found that when system damping is low, the system clearly exhibits nonlinear interaction between aeroelastic modes. It is also shown that although certain applied forcing conditions may appear negligible, these same forces produce large amplitude LCOs under specific realizable circumstances.     

An Investigation of Internal Resonance in Aeroelastic Systems

Although the study of internal resonance in mechanical systems has been given significant consideration, minimal attention has been given to internal resonance for systems which consider the presence of aerodynamic forces. Herein, the investigators examine the possible existence of internal resonances, and the related nonlinear pathologies that such responses may have, for an aeroelastic system which possesses nonlinear aerodynamic loads. Evidence of internal resonance is presented for specific classes of aeroelastic systems, and such adverse response indicates nonlinearities may lead to aeroelastic instabilities that are not predicted by traditional (linear) approaches.     

Effects of actuator nonlinearity on aeroelastic characteristics of a control fin

The influences of actuator nonlinearities on actuator dynamics and the aeroelastic characteristics of a control fin were investigated by using iterative V-g methods in subsonic flows; in addition, the doublet-hybrid method (DHM) was used to calculate unsteady aerodynamic forces. The changes of actuator dynamics induced by nonlinearities, such as backlash or freeplay, and the variations of flutter boundaries due to the changes of actuator dynamics were observed. Results show that the aeroelastic characteristics can be significantly dependent on actuator dynamics. Thus, the actuator nonlinearities may play an important role in the nonlinear aeroelastic characteristics of an aeroelastic system. The present results also indicate that it is necessary to seriously consider the influence of actuator dynamics on the flutter characteristics at the design stage of actuators to prevent aeroelastic instabilities of aircraft or missiles.     

平流层飞艇空气动力估算

本文采用有限基本解方法与工程估算方法相结合的气动力工程计算方法,用以计算平流层飞艇的气动力。将飞艇所受的气动力分成飞艇艇身和尾翼所受气动力两部分,每一部分的气动力按照无粘性流产生的线性气动力和粘性引起的非线性气动力分别进行计算。根据势流理论对飞艇艇身线性气动力进行分析计算,由于飞艇艇体是旋成体,故根据Allen的横流阻力理论对其所受的非线性气动力进行计算;尾翼的线性气动力采用有限基本解方法进行计算,非线性气动力用Polhamus-Lamar吸力比拟方法估算。该方法中考虑了由于尾翼安装在体上后,处于艇体产生的上洗流场中,尾翼气动力的变化和尾翼对艇身气动力的干扰作用。通过算例的计算与实验结果比较得出该方法可以快速、准确的计算飞艇所受的气动力。     

Effects of multiple structural nonlinearities on limit cycle oscillation of missile control fin

Aeroelastic analyses are performed for a 2-D typical section model with multiple nonlinearities. The differences between a system with multiple nonlinearities in its pitch and plunge spring and a system with a single nonlinearity in its pitch are thoroughly investigated. The unsteady supersonic aerodynamic forces are calculated by the doublet point method (DPM). The iterative V-g method is used for a multiple-nonlinear aeroelastic analysis in the frequency domain and the freeplay nonlinearity is linearized using a describing function method. In the time domain, the DPM unsteady aerodynamic forces, which are based on a function of the reduced frequency, are approximated by the minimum state approximation method. Consequently, multiple structural nonlinearities in the 2-D typical wing section model are influenced by the pitch to plunge frequency ratio. This result is important in that it demonstrates that the flutter speed is closely connected with the frequency ratio, considering that both pitch and plunge nonlinearities result in a higher flutter speed boundary than a conventional aeroelastic system with only one pitch nonlinearity. Furthermore, the gap size of the freeplay affects the amplitude of the limit cycle oscillation (LCO) to gap size ratio.     

跨音速极限环型颤振的高效数值分析方法

事先建立一个低阶的非线性、非定常气动力模型是开展非线性流场中气动弹性问题研究的一个捷径.基于CFD方法,通过计算结构在流场中自激振动的响应来获得系统的训练数据.采用带输出反馈的循环RBF神经网络,建立时域非线性气动力降阶模型.耦合结构运动方程和非线性气动力降阶模型,采用杂交的线性多步方法计算结构在不同速度(动压)下的响应历程,从而获得模型极限环随速度(动压)变化的特性.两个典型的跨音速极限环型颤振算例表明,基于气动力降阶模型方法的计算结果与直接CFD仿真结果吻合很好,与后者相比其将计算效率提高了1～2个数量级.     

平流层飞艇气动参数辨识

进入21世纪以来，随着科技的飞速发展，世界上掀起了研究和开发平流层平台的热潮。飞艇作为平流层平台可以实现无线通信、空间观测、大气测量以及军事侦查等目的。本文首先将飞艇所受的气动力分成由于来流速度产生的定常气动力和飞艇转动引起的非定常气动力两部分，通过理论分析建立了飞艇的气动力模型，从而得到需要辨识的气动参数。其次建立了以浮心为原点的六自由度非线性动力学模型和一种基于混合遗传算法的气动力系数辨识方法——混合遗传算法（遗传算法＋单纯型法）与极大似然法相结合的方法，并利用该方法对飞艇的气动参数进行辨识。通过仿真结果验证了该方法实用性和有效性。最后通过对气动参数的准确值与辨识值的分析比较，得出各个参数对飞艇运动性能的影响情况。     

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.