具有不对称间隙的二元机翼自激振动的数值分析
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摘要
在飞机设计中,结构非线性是不可避免的。结构非线性源于控制其表面的旧铰链,松散的控制器联动装置,材料性能和其它的来源。在本文中,我们研究了在不可压缩流中的一个有俯仰间隙非线性刚度的二元机翼颤振系统的非线性行为。机翼的运动方程可以写成具有八个变量的一阶常微分方程组,取无量纲气流速度与无量纲线性颤振速度的比值U*/U*L作为分岔参数。用Runge-Kutta法分别对无预荷载情况下和有预荷载情况下的不对称分段线性系统进行数值分析,研究了系统可能出现的各种周期运动的时间历程、分岔和混沌运动及初值条件对这些周期运动产生的影响,以及影响机翼颤振幅值的几个方面。本文从理论上对二元机翼颤振系统进行了较为全面的深入探讨,得出了具有指导意义的结论,对机翼的设计和动力学分析有一定的指导意义。
    利用大量的数值计算得到在不同条件下的全局和局部分岔图,通过数值分析发现,当系统无预荷载时,系统中存在复杂的运动形式,其中包括从P-1到P-2的倍周期分岔,P-4到P-2,P-2到P-1的倒分岔,并且发现了混沌运动形式。随着机翼速度的增大,不同的初始条件会导致平衡点、极限环振动、复杂的周期运动及混沌等不同的运动类型,不同的初始条件所对应的运动形式有所不同。在无预荷载情况下,当其它条件不变,只有系统中的间隙发生变化时,间隙降低,极限环振荡区域减小,混沌区域增大,发生混沌的可能性越大。而当系统有预荷载时,得到以下几种结论:①系统没有混沌现象,系统的解收敛为平衡点或周期1运动,且幅值的变化规律与初始条件无关;②增大频率比,减小间隙,均可减小极限环颤振的区域,而且对应于同一速度比时,系统收敛的平衡点或极限环颤振的幅值减小;③增大机翼质量密度比,可减小极限环颤振的区域,提高线性颤振速度,俯仰运动极限环颤振的幅值基本上不随μ改变,但另一方面,沉浮运动极限环颤振的幅值却随μ的增大而增大;④当预荷载、初偏值、间隙及初始条件均成倍增长时,系统分岔图的变化趋势基本一致,当系统收敛于极限环时,其俯仰运动极限环颤振的幅值也相应的成倍增长。⑤在某些算例中,极限环颤振的幅值在一定的速度比区域内,不是随着速度比的增大而增大,而是表现为平衡点运动。
In airplane design, structural nonlinearities cannot be avoided.The structural nonlinearities arise from worn hinges of control surfaces, loose control linkages, material behavior and various other sources. In this paper, the nonlinear behavior of a two-degree-of-freedom airfoil with pitch freeplay nonlinearity stiffness in incompressible flow is investigated. The aeroelastic equations-of-motion for the self-excited system can be formulated into a system of eight first-order ordinary differential equations. Taking the ratio of non-dimentional stream velocity to non-dimentional linear flutter speed as the bifurcation parameter, numerical analysis are presented for the asymmetry freeplay model with and without preload using a fourth-order Runge-Kutta method. The time history of some kinds of periodic motions, bifurcation, chaos, the influence by the initial conditions and some aspects affecting the flutter amplitude of airfoil are studied. In this paper, further and detailed investigations are given from theory. Some conclusions are obtained, which are benefit to airfoil design and dynamics analysis.
    Globe and local bifurcation diagrams are given after a great deal of numerical computation. Complicated motions are found in the system without preload. When U*/U*L increases from zero to 1 gradually, there are chaos, double-bifurcations from P-1 to P-2, inverse bifurcations from P-4 to P-2 and from P-2 to P-1. The responded motions are changed with the initial value. The LCO (Limit Cycle Oscillation) region is decreased and chaos region is increased when is decreased while other conditions are not changed.
    Some conclusions are attained for the system with preload.①The solution of system is converged to an equilibrium point or period-one LCO without chaos and the responded motions are not changed with the initial value;②The LCO region is decreased when increasing or decreasing and the amplitude of equilibrium point or LCO is reduced for a same speed ratio.③The LCO region is decreased and the linear flutter speed is raised when increasing. The amplitude of pitch motion isn’t changed withμ,but is found to increase withμincreasing;④The amplitude of pitch motion is doubled whenand initial conditions are all doubled. ⑤The solution of system is not increasing with increasing speed ratio but converged to an equilibrium point in some speed ratio region in some cases.
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