CHAOTIC MOTIONS AND LIMIT CYCLE FLUTTER OF TWO-DIMENSIONAL WING IN SUPERSONIC FLOW

Based on the piston theory of supersonic flow and the energy method, the flutter motion equations of a two-dimensional wing with cubic stiffness in the pitching direction are established. The aeroelastic system contains both structural and aerodynamic nonlinearities. Hopf bifurcation theory is used to analyze the flutter speed of the system. The effects of system parameters on the flutter speed are studied. The 4th order Runge-Kutta method is used to calculate the stable limit cycle responses and chaotic motions of the aeroelastic system. Results show that the number and the stability of equilibrium points of the system vary with the increase of flow speed. Besides the simple limit cycle response of period 1, there are also period-doubling responses and chaotic motions in the flutter system. The route leading to chaos in the aeroelastic model used here is the period-doubling bifurcation. The chaotic motions in the system occur only when the flow speed is higher than the linear divergent speed and the initial condition is very small. Moreover, the flow speed regions in which the system behaves chaos axe very narrow.     

Synthesis and Structure Characterization of a Tetranuclear Manganese(Ⅲ) Complex:[Mn4O2(O2CMe)6(MeOH)2(dbm)2]·2MeCOOH·2CH2Cl2

The title compound, [Mn4O2(O2CMe)6(MeOH)2(dbm)2]·2MeCOOH·2CH2Cl2 (Hdbm = dibenzoylmethane), has been synthesized and structurally determined by single-crystal X-ray diffraction. The crystal belongs to triclinic, space group P(-l), with a = 10.729(3), b = 12.269(3), c =13.085(4) (A), a = 106.367(3), β = 107.643(2), γ = 94.771(2)°, V = 1547.9(7) (A)3, Z = 1,C50H64Cl4Mn4O24, Mr= 1410.57, Dc= 1.513 g/em3, F(000) = 724, Rint = 0.0147, T = 293(2) K and μ = 1.046 mm-1. The fimal R = 0.0359 and wR = 0.0938 for 5791 observed reflections with I ＞ 2( )I).The structure of the complex consists of one [Mn4(μ3-O)2]8+ core with four coplanar Mn atoms disposed in an extended "butterfly-like" arrangement and two O atoms triply bridging each "wing",and the peripheral ligation is provided by six μ2-MeCO2-, two terminalμ2-dbm- groups at the two hydrogen bonding interactions are found within the structure of the compound.     

柔性扑翼的气动特性研究

以往扑翼的气动力计算研究都很少考虑扑翼的柔性,而在鸟的扑翼动作中,在外加气动力和鸟自身的扑动力作用下,扑翼的柔性变形相当大。本文在原有匀速刚性模型的基础上,提出考虑了扑翼扑动速率变化和形状变化的扑翼分析模型,使之更接近鸟翼柔性扑动真实情况。通过计算分析气动特性发现,控制适当的话,柔性变形能大大改善扑翼的气动性能。本文通过模拟鸟扑翼的柔性运动,计算了时柔性扑翼气动力以及平均升力系数和平均推力系数随着扑动角、倾斜角等参数变化的情况,从而从气动的角度解释了为什么鸟在不同的飞行阶段扑翼规律各不相同,并为柔性扑翼飞行器的设计提供了理论依据。     

Aeroelastic analysis of swept pre-twisted wings

In this paper, aeroelastic modeling of aircraft wings with variations in sweep angle, taper ratio, and variable pre-twist angle along the span is considered. The wing structure is modeled as a classical beam with torsion and bending flexibility. The governing equations are derived based on Hamilton’s principle. Moreover, Peters’ finite state aerodynamic model which is modified to take into account the effects of the wing finite-span, the wing sweep angle, and the wing pre-twist angle, is used to simulate the aerodynamic loads on the wing. The coupled partially differential equations are discretized to a set of ordinary differential equations using Galerkin’s approach. By solving these equations the aeroelastic instability conditions are derived. The results are compared with some experimental and analytical results of previous published papers and good agreement is attained. Effects of the wing sweep angle, taper ratio, bending to torsional rigidity, and pre-twist angle on the flutter boundary in several cases are studied. Results show that these geometrical and physical parameters have considerable effects on the wing flutter boundary.     

两种湍流模型时域颤振计算方法研究

采用时域计算分析方法进行了机翼跨音速颤振特性研究。在结构运动网格的基础上,采用格点格式有限体积方法进行空间离散和双时间全隐式方法进行时间推进求解雷诺平均N-S方程。针对流动粘性分别应用了SST湍流模型和SSG雷诺应力模型,通过对跨音速标模算例AGARD445.6机翼的计算结果与实验值的对比分析,其中应用SST湍流模型得到的颤振速度与实验值最为接近,特别是在跨音速段平均相对误差在3%以内;并且计算结果整体上反映了跨音速颤振"凹坑"物理特性,验证了方法的有效性。     

随机激励下受热翼面的模态频率特性试验研究

现代高速飞行器结构热模态频率特性试验研究,对这类飞行器设计校核和飞行安全具有重要意义。根据飞行过程中遭受的气动加热特性设计了瞬态热环境模拟系统,同时,根据高温环境的特点对测试中的激励和测量方式进行了重新设计,成功地将普通激振器应用于高温结构模态试验,最终将热环境模拟系统与振动测试系统组合,形成一套考虑瞬态热影响的热模态试验系统,实现了瞬态热环境下结构模态的地面测试。对一个切尖三角翼测量了各个加热区的温度随加热时间的变化,验证了加热温度控制的精确性;在纯随机激励下对测得的激励和振动响应信号采用短时傅里叶变换(Short Time Fourier Transformation,STFT)进行时变模态参数辨识,获得了前四阶模态频率随加热时间的变化,并与结构有限元数值计算结果进行了比较,试验与计算结果吻合得很好,验证了该试验方法对热模态测试问题的有效性和准确性。通过分别对瞬态和稳态热环境下结构模态频率试验和计算结果的分析,探讨了结构瞬态温度场对模态频率影响的机理,揭示了结构内部存在的热应力和材料属性的变化,是决定模态频率随加热时间变化趋势的内在原因。     

飞机机翼气动外形优化设计研究

本文首先对某飞机原机翼外形进行了详尽的气动分析计算,然后确定了设计思路和方案,探讨了后掠角变化对机翼气动性能的影响,研究选定了减小外翼后掠角的机翼新平面形状,采用先进的CFD软件优化机翼的气动设计,根据不同设计思想进行了多个机翼的外形优化,包括新的翼剖面和弯扭配置,最后将优化设计结果与原机翼进行了对比,对比结果表明,以Q5-M2T和Q5-N2T为代表的优化结果取得了十分理想的改进效果,优化机翼提高了气动性能,机翼升阻比提高了20%-30%,满足了飞机载弹量增大后性能仍可以全面提高的设计要求。     

耦合时间精度对模拟飞行器自由运动特性的影响

将亚迭代技术引入流体动力学和刚体动力学方程的耦合求解,获得细长三角翼极限环运动的规律.探讨耦合时间精度对飞行器非定常运动特性的影响,细长三角冀的大迎角自由滚运动最终形成极限环振荡的周期性自维持运动,不同攻角自由滚振幅阶跃式的变化特点较好地吻合了自由滚试验的规律.对于多系统耦合问题,亚迭代耦合求解(耦合时间精度为二阶)对物理时间步长的依赖性不明显;而存在一阶时间滞后的解耦推进方法的数值结果强烈地依赖于物理时间步长选取,稍大的时间步长将导致非物理的数值结果.     

低雷诺数反齐默曼机翼的气动特性

通过PISO方法求解非定常不可压N-S方程,研究小展弦比反齐默曼机翼在低雷诺数下的流场特征,并分析其对气动特性的影响.结果显示前缘分离涡在反齐默曼机翼上表面形成一对集中涡.分析表明这对集中涡是影响反齐默曼机翼气动特性的主要因素,给机翼提供了较大的非线性升力和较大的失速攻角,前缘涡之间的相互影响使得机翼出现非定常现象和大攻角的非对称现象.与矩形翼相比,反齐默曼机翼有较好的稳定性.