Nonlinear flow-induced instability of an elastically mounted pitching wing
Nonlinear flow-induced instability of an elastically mounted pitching wing
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弹性安装的俯仰翼的非线性流动引起的不稳定性
DOI：
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发表时间：
2020
影响因子：
3.7
通讯作者：
K. Breuer
中科院分区：
文献类型：
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作者：
Yuanhang Zhu;Yunxing Su;K. Breuer
摘要实验研究了循环水隧道中弹性俯仰翼的非线性流致失稳问题。利用网络物理控制系统对有限跨度机翼的结构参数进行了仿真和调节。在一个小的固定阻尼下，我们系统地改变了不同惯性值的机翼刚度，以测试系统的稳定性边界。我们观察到，对于高惯性机翼，系统动力学通过一个亚临界分岔，从稳定的不动点分岔到小振幅振荡，然后是大振幅极限环振荡（LCOs），该分岔具有滞回双稳定性和振幅突变。在这种情况下，机翼的俯仰频率锁定在其结构频率上，振荡受惯性力支配，对应一种结构模态。力场和流场测量表明前缘存在二次涡。随着机翼惯性的减小，双稳区宽度减小。在足够低的惯性下，俯仰幅值随刚度平滑变化，无滞回，呈现超临界分岔。在此条件下，没有出现锁定现象，俯仰频率保持相对恒定，且低于结构频率。力分解显示流体力占主导地位，表明流体动力模式。二级LEV缺失。结果表明，结构模态和水动力模态的大振幅LCOs的发生与柯西数有关，结构模态LCOs的发生与无因次速度有关。我们详细研究了亚临界跃迁；我们发现这种过渡取决于机翼的静态特性，并且在过渡的早期阶段就开始出现次级LEV。最后，我们采用能量方法来绘制系统的稳定性，并解释了在不同惯性值下观察到的两种不同类型的分支的存在性。
Abstract We experimentally study the nonlinear flow-induced instability of an elastically mounted pitching wing in a circulating water tunnel. The structural parameters of the finite-span wing are simulated and regulated using a cyber-physical control system. At a small fixed damping, we systematically vary the stiffness of the wing for different inertia values to test for the stability boundaries of the system. We observe that, for a high-inertia wing, the system dynamics bifurcates from stable fixed points to small-amplitude oscillations followed by large-amplitude limit-cycle oscillations (LCOs) via a subcritical bifurcation, which features hysteretic bistability and an abrupt amplitude jump. Under this condition, the pitching frequency of the wing locks onto its structural frequency and the oscillation is dominated by the inertial force, corresponding to a structural mode. Force and flow field measurements indicate the presence of a secondary leading-edge vortex (LEV). As the wing inertia decreases, the width of the bistable region shrinks. At a sufficiently low inertia, the pitching amplitude changes smoothly with the stiffness without any hysteresis, revealing a supercritical bifurcation. Under this condition, no lock-in phenomenon is observed and the pitching frequency remains relatively constant at a value lower than the structural frequency. Force decomposition shows dominating fluid force, indicating a hydrodynamic mode. The secondary LEV is absent. We show that the onset of large-amplitude LCOs in both the structural mode and the hydrodynamic mode scales with the Cauchy number, and the LCOs in the structural mode collapse with the non-dimensional velocity. We examine the subcritical transition in detail; we find that this transition depends on the static characteristics of the wing, and the secondary LEV starts to emerge at the early stage of the transition. Lastly, we adopt an energy approach to map out the stability of the system and explain the existence of the two distinct types of bifurcations observed for different inertia values.
影响因子：
64.8
作者：
Jafferis, Noah T.;Helbling, E. Farrell;Wood, Robert J.
通讯作者：
Wood, Robert J.