Chordwise spacing aerodynamic detuning for unstalled supersonic flutter stability enhancement

Unstalled supersonic flutter is a significant problem in the development of advanced gas turbines because it restricts the high speed operating range of the engine. A new approach to passive control of unstalled supersonic flutter is aerodynamic detuning, defined as designed passage-to-passage differences in the unsteady aerodynamics of a blade row. In this paper, a mathematical model is developed to predict the unstalled torsion mode stability of an aerodynamically detuned turbomachine rotor operating in a supersonic inlet flow field with a subsonic axial component, with the aerodynamic detuning accomplished by alternate chordwise spacing of adjacent rotor blades. The unsteady aerodynamic moments acting on the blading are calculated in terms of influence coefficients. The stability enhancement associated with this alternate chordwise aerodynamic detuning is demonstrated utilizing an unstable twelve bladed rotor based on Verdon's Cascade B flow geometry. This model and unstable baseline rotor configuration are then used to show that axial spacing detuning leads to greater flutter stability enhancement than does circumferential spacing aerodynamic detuning. Finally, the trade-offs between structural damping, alternate chordwise aerodynamic detuning, and alternate circumferential aerodynamic detuning are considered.     

宽弦高速跨音风扇颤振特性研究

颤振是航空发动机、燃气轮机等运行安全的重要威胁,但颤振稳定性与流动结构之间的关系尚不清晰。本文使用行波法和影响系数法,对某宽弦复合掠型高速跨音风扇转子的一阶模态进行了颤振特性研究,计算了在100%转速下从堵塞点到近失速点的颤振表现。使用影响系数法时,分析了不同通道数的计算域对气动阻尼计算的影响,并与行波法得到的结果进行了对比。研究了流动结构与叶片表面气动阻尼之间的关系,旨在提高对流动致颤机理的认识。结果表明影响系数法和行波法均能对叶片的气动阻尼进行较好的预测;流动结构方面,激波、激波附面层分离、叶尖泄漏流以及吸力面前缘叶顶附近的非定常压力波动,对叶片的气动阻尼分布有较大的影响。     

Limit cycle oscillation of missile control fin with structural non-linearity

Non-linear aeroelastic characteristics of a deployable missile control fin with structural non-linearity are investigated. A deployable missile control fin is modelled as a two-dimensional typical section model. Doublet-point method is used for the calculation of supersonic unsteady aerodynamic forces, and aerodynamic forces are approximated by using the minimum-state approximation. For non-linear flutter analysis structural non-linearity is represented by an asymmetric bilinear spring and is linearized by using the describing function method. The linear and non-linear flutter analyses indicate that the flutter characteristics are significantly dependent on the frequency ratio. From the non-linear flutter analysis, various types of limit cycle oscillations are observed in a wide range of air speeds below or above the linear divergent flutter boundary. The non-linear flutter characteristics and the non-linear aeroelastic responses are investigated.     

Aerodynamic analysis of a supersonic cascade vibrating in a complex mode

An analysis is presented which has been used to predict the unsteady aerodynamic behavior of a finite supersonic cascade of airfoils forced in harmonic oscillation with airfoil-to-airfoil variations in amplitude. Theoretical predictions are compared with some recent experimental results² at a reduced frequency representative of actual fan or compressor flutter cases. The similarity of the experimental situation in the finite cascade to the flutter of a severely mistuned rotor is noted.     

Effects of hysteretic and aerodynamic damping on supersonic panel flutter of composite plates

The governing equation for the finite element analysis of the panel flutter of composite plates including structural damping is derived from Hamilton's principle. The first order shear deformable plate theory has been applied to structural modelling so as to obtain the finite element eigenvalue equation. The unsteady aerodynamic load in a supersonic flow is computed by using the linear piston theory. The critical dynamic pressures for composite plates have been calculated to investigate the effects of structural damping on flutter boundaries. The effects are dependent on fiber orientation because flutter mode can be weak or strong in the fiber orientation of composite plates. Structural damping plays an important role in flutter stability with low aerodynamic damping but would not affect the flutter boundary with high aerodynamic damping.     

Helicopter noise in hover: Computational modelling and experimental validation

The aeroacoustic characteristics of a helicopter rotor are calculated by a new method, to assess its applicability in assessing rotor performance in hovering. Direct solution of the Euler equations in a noninertial coordinate system is used to calculate the near-field flow around the spinning rotor. The far-field noise field is calculated by the Ffowcs Williams–Hawkings (FW–H) method using permeable control surfaces that include the blade. For a multiblade rotor, the signal obtained is duplicated and shifted in phase for each successive blade. By that means, the spectral characteristics of the far-field noise may be obtained. To determine the integral aerodynamic characteristics of the rotor, software is written to calculate the thrust and torque characteristics from the near-field flow solution. The results of numerical simulation are compared with experimental acoustic and aerodynamic data for a large-scale model of a helicopter main rotor in an open test facility. Two- and four-blade configurations of the rotor are considered, in different hover conditions. The proposed method satisfactorily predicts the aerodynamic characteristics of the blades in such conditions and gives good estimates for the first harmonics of the noise. That permits the practical use of the proposed method, not only for hovering but also for forward flight.     

A simplified method for the free vibration and flutter analysis of bridge decks

A simplified method for the free vibration and flutter analysis of bridge decks is presented. Bending-torsion coupled beam theory with warping stiffness included is used in the structural idealization of bridge decks in order to derive explicit formulae for natural frequencies and mode shapes. These are used to perform the flutter analysis. The time-dependent aerodynamic forces are modelled using Theodorsen type flat plate theory. Expressions for generalized mass, generalized stiffness and generalized aerodynamic force terms are derived in compact explicit form. The flutter problem is then formulated by summing algebraically the analytical expressions for generalized mass, generalized stiffness and generalized aerodynamic forces, and the associated flutter determinant is expanded in analytical form. Finally, the flutter speed and flutter frequency are thereby determined by using a standard root finding procedure. The method is demonstrated by numerical results. This is followed by some concluding remarks.     

Parameterized aeroelastic modeling and flutter analysis for a folding wing

To investigate the flutter characteristics of a folding wing with different configurations, a parameterized aeroelastic model is proposed. First, a parameterized structural model is established based on the substructure synthesis. Afterwards, the parameterized aerodynamic model is derived for each lifting surface using the so-called Doublet Lattice Method (DLM). The correctness of the resulting aeroelastic model is verified via NASTRAN. Finally, some aeroelastic simulations are performed using the proposed aeroelastic model. The results demonstrate that the flutter characteristics of the folding wing are very sensitive to the folding angle. With increasing folding angle, a transition between two unstable modes occurs. Such a transition results in a sudden change of flutter mode shapes and a jump of critical flutter frequency. Besides, there exists a region of folding angle, where the flutter behavior of the folding wing strongly depends on the structural damping.     

LIMIT CYCLE FLUTTER OF AIRFOILS IN STEADY AND UNSTEADY FLOWS

The limit cycle flutter of a two-dimensional wing with non-linear pitching stiffness is investigated. For modelling the aerodynamic forces of the wing steady linear and non-linear models as well as an unsteady model were used. The flutter speed was calculated using the harmonic balance method and by predicting Hopf bifurcation. Analytical solutions based on the centre manifold theory and normal forms were obtained as were results given by the harmonic balance method. The analytical solutions were compared with those obtained by numerical integration. The results show that the harmonic balance method can forecast flutter speed with a good accuracy while analytical solutions based on centre manifold theorem are accurate only in a small neighbourhood of the bifurcation point. The oscillation of the airfoil after flutter for two different models, linear and non-linear pitching stiffness were compared with each other and the flutter speeds for two linear steady and an unsteady aerodynamic model calculated. The obtained results show that flutter analysis based on the linear steady model is conservative only for the ratios of plunge frequency to pitch frequency lower than 1.     

钝前缘梯形翼高超声速风洞颤振试验

为了研究钝前缘翼面的高超声速颤振特性,获得典型翼面高超声速颤振参数以校验非定常气动力和CFD计算,采用具有简单结构动力学特性的钝前缘梯形翼模型,在中国航天空气动力技术研究院FD-07高超声速风洞进行了高超声速风洞颤振试验研究.模型为9 mm厚钝前缘梯形平板翼,采用夹层设计:中间层为钢板,提供模型主要刚度和质量特性;两侧为泡沫,起维形作用.试验模型采用悬臂支撑安装于风洞试验段,试验Mach数分别为4.95和5.95.试验固定Mach数,通过缓慢增加动压以使模型达到颤振临界点,采用小波时频谱分析时域响应,结果显示试验模型发生了弯扭耦合经典颤振.试验采用直接观测法获得了颤振动压、颤振频率和对应的试验密度、总温等颤振相关参数.采用壳单元建立了结构有限元模型,并采用统一升力面理论对模型进行了颤振计算分析,研究了气流密度、结构阻尼、Mach数对颤振计算的影响,并对试验结果与理论计算的偏差进行了讨论.分析认为,计算气流密度、计算结构阻尼、结构建模偏差、试验结果散布特性等因素均会构成计算值和试验值之间的偏差,但即便在计算中考虑上述因素,计算结果与试验值仍存在较大偏差.      

An application of system identification to flutter testing

It is shown that the equations of motion of an aeroelastic system may be derived from measured response data. The structural and aerodynamic terms are separated by analyzing response measurements at two values of kinetic pressure; the derived equations can then be used to calculate dynamic characteristics of the system at any chosen values of kinetic pressure. Two examples of the application of the analysis to the prediction of flutter characteristics are given.     

A high-efficiency aerothermoelastic analysis method

In this paper,a high-efficiency aerothermoelastic analysis method based on unified hypersonic lifting surface theory is established.The method adopts a two-way coupling form that couples the structure,aerodynamic force,and aerodynamic thermo and heat conduction.The aerodynamic force is first calculated based on unified hypersonic lifting surface theory,and then the Eckert reference temperature method is used to solve the temperature field,where the transient heat conduction is solved using Fourier’s law,and the modal method is used for the aeroelastic correction.Finally,flutter is analyzed based on the p-k method.The aerothermoelastic behavior of a typical hypersonic low-aspect ratio wing is then analyzed,and the results indicate the following:(1)the combined effects of the aerodynamic load and thermal load both deform the wing,which would increase if the flexibility,size,and flight time of the hypersonic aircraft increase;(2)the effect of heat accumulation should be noted,and therefore,the trajectory parameters should be considered in the design of hypersonic flight vehicles to avoid hazardous conditions,such as flutter.     

带有结构非线性的跨音速翼型颤振特性研究

以非定常N-S方程为主管方程,采用时间推进的方法,计算翼型振荡的瞬态非定常气动力,并与带有结构非线性的颤振方程耦合求解,计算了带有结构刚度非线性(间隙型,三次型刚度非线性)和结构阻尼非线性(三次型阻尼非线性)的结构响应特性和颤振特性.计算研究表明,由于同时具有结构和气动非线性,振荡极限环和气动力极为复杂.     

On the aerodynamic mechanism of torsional flutter of bluff structures

This paper is concerned with theoretical and experimental investigations on the aerodynamic mechanism of torsional flutter of bluff structures. In the experiment, measurements were made of the unsteady aerodynamic lifts and moments acting on two rectangular bar models which were forced to oscillate either in a torsional or in a heaving mode in a uniform wind tunnel flow. It is found that the fluid memory effect is, in marked contrast to the situation for transverse galloping, responsible for the onset of torsional flutter of bluff structures. The effect of the pivotal position on torsional flutter of bluff structures is also investigated.     

Numerical study on the correlation of transonic single-degree-of-freedom flutter and buffet

Transonic single-degree-of-freedom(SDOF) flutter and transonic buffet are the typical and complex aeroelastic phenomena in the transonic flow. In this study, transonic aeroelastic issues of an elastic airfoil are investigated using Unsteady Reynolds-Averaged Navier-Stokes(URANS) equations. The airfoil is free to vibrate in SDOF of pitching. It is found that, the coupling system may be unstable and SDOF self-excited pitching oscillations occur in pre-buffet flow condition, where the free-stream angle of attack(AOA) is lower than the buffet onset of a stationary airfoil. In the theory of classical aeroelasticity, this unstable phenomenon is defined as flutter. However, this transonic SDOF flutter is closely related to transonic buffet(unstable aerodynamic models) due to the following reasons. Firstly, the SDOF flutter occurs only when the free-stream AOA of the spring suspended airfoil is slightly lower than that of buffet onset, and the ratio of the structural characteristic frequency to the buffet frequency is within a limited range. Secondly, the response characteristics show a high correlation between the SDOF flutter and buffet. A similar "lock-in" phenomenon exists, when the coupling frequency follows the structural characteristic frequency. Finally, there is no sudden change of the response characteristics in the vicinity of buffet onset, that is, the curve of response amplitude with the free-stream AOA is nearly smooth. Therefore, transonic SDOF flutter is often interwoven with transonic buffet and shows some complex characteristics of response, which is different from the traditional flutter.     

叶顶射流对涡轮流场及气动性能影响

通过对带有前缘气膜冷却的C3X涡轮叶片的传热和气动试验结果进行计算验证,表明所使用计算方法在对带有冷却射流的跨音涡轮压力及温度进行的预测具有较高的精度,在此基础上针对某型发动机低压涡轮,通过CFD数值模拟研究其叶顶冷却射流对叶顶泄漏流及涡轮性能的影响规律,并通过详细分析叶顶流动揭示该规律产生的原因;然后通过改变不同叶顶...     

双转子涡喷发动机气动性能优化控制研究

航空发动机的控制规律作用巨大,它决定了发动机能否获得设定的稳态工作下性能指标,同时保证工作过程中的压气机和涡轮的气动稳定性。双转子涡喷发动机气动性能优化控制的目的就是有效地挖掘发动机的使用潜力。研究方法采用部件特性法对发动机进行稳态建模,并针对某双转子涡喷发动机的稳态模型进行三种不同稳态控制规律下的仿真,得到发动机性能参数的不同变化趋势,并对其进行了详细的分析。结果表明：保持低压转子转速不变的情况下,随着压气机进口总温的增加,高压转子转速上升,涡轮前温度升高,发动机推力增加;保持涡轮前温度不变的情况下,随着压气机进口总温的升高,低压压气机气动负荷变重,低压转子转速降低;高压转子转速也下降,但是下降幅度很小;燃油流量增加;保持高压转子转速不变的情况下,随着压气机进口总温的升高,燃油流量有一定的增加,低压转子转速有所降低;推力受多重因素的影响,推力值变化趋势较为复杂。     

Galloping of bundled power line conductors

This paper describes an experimental and analytical study of the galloping of a two-dimensional section model of a two-conductor bundle in which ice-accreted conductors are replaced by two identical square prisms, with both vertical and torsional movements allowed but the horizontal one blocked, in a uniform wind tunnel flow. Emphasis is placed on elucidating the vital role played by the aerodynamic coupling in the stability of bundled conductors. It is shown that, apart from galloping type flutter, two other types of instability, namely, torsional and classical type flutter, can also occur for bundled conductors. In particular, it is shown that the aerodynamic coupling can cause violent classical type flutter to occur when the resonant condition is approached.     

Fluid-structure interactions with both structural and fluid nonlinearities

In this study, we consider a class of nonlinear aeroelastic stability problems, where geometric nonlinearities arising from large deflections and rotations in the structure interact with aerodynamic nonlinearities caused by moving shocks. Examples include transonic panel flutter and flutter of transonic wings of high aspect ratio, where the presence of both structural and aerodynamic nonlinearities can have a dramatic qualitative as well as quantitative effect on the flutter behavior. Both cases represent inherently nonlinear fluid-structure problems, where neglecting either the structural or the fluid nonlinearities can lead to completely erroneous stability predictions. The results presented in this paper illustrate the rich and in some cases surprising flutter behaviors of transonic wings, and the inherent limitations of the von Kármán nonlinear plate model in strongly nonlinear fluid-structure interaction problems of this type.     

风力发电机叶轮的数值优化设计法

１引言目前，在风力机叶轮的气动设计方面，还没有其系统的设计模型和方法，只有一些针对某一方面的模型，这些模型还无法规纳成一套可靠的系统的设计模型［１］，Ｇｏｕｒｉｅｒｅｓ在他的风力机设计理论的书中也只能介绍几种简化的有关设计模型［２］。所用的设计方法主要还是以经验设计为主，不是全面系统地考虑叶轮的各种影响因素，如Ｈａｂａｌｉ［３］在他的设计工作中主要考虑了叶片翼型对叶轮运行的气动作用，ＶＯｕｔｓｉｎａｓ［４］只是对偏航工况下叶轮的气动特性及操作进行了分析研究，文献［５］也仅对叶轮功率等气动特性进行…