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The suppressed sound of inhomogeneous flapping airfoils

机译：不均匀拍打翼型的抑制声

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We study the aeroacoustic problem of a flapping airfoil in a high-Reynolds and low-Mach regime, and propose novel means to reduce the far-field sound originated by the fluid-structure interaction. The present model consists of a two-dimensional, elastic airfoil, periodically actuated at the leading edge. The near-field is analyzed using thin airfoil theory, and a discrete-vortex wake-model. The derived airfoil motion and wake dynamics are then introduced as a source term to Powell-Howe's acoustic analogy, to project the sound towards a distant observer. Considering the elastic flapping configuration, we seek optimal material properties and a linear thickness distribution to reduce the sound of an otherwise rigid airfoil while retaining the lift amplitude of the rigid configuration. To this end, the aeroacoustic model is introduced into an optimization scheme, yielding an effective Pareto front, reflecting the trade-off between the competing objectives of lower sound levels and aerodynamic efficiency. Compared with the rigid heaving airfoil, over 10 [dB] sound reduction was obtained for the optimal flexible configuration with linear thickness distributions, producing the same lift amplitude. The optimal configurations are obtained by time shifting the motion- and wake-dipoles to antiphase mode, thus mitigating the total radiated sound, while keeping the lift-amplitude value. The opposing sound dipoles follow from the synchronized disposition of the airfoils' motion and circulation signals.
机译：我们研究了高雷诺和低马赫制度的扑翼翼型的空气声学问题，并提出了一种新颖的手段，以减少流体结构相互作用的远场声音。本模型由二维弹性翼型组成，周期性地在前缘致动。使用薄的翼型理论分析近场，以及一个离散涡旋唤醒模型。然后将衍生的翼型运动和唤醒动力学作为源术语引入Powell-Howe的声学类比，将声音投射到远处观察者。考虑到弹性拍打配置，我们寻求最佳材料性能和线性厚度分布，以减少诸如刚性构造的升力幅度的同时减小诸如刚性翼型的声音。为此，流动模型被引入优化方案，产生有效的帕累托前线，反映了较低声级和空气动力学效率的竞争目标之间的权衡。与具有线性厚度分布的最佳柔性配置获得超过10℃的刚性气象减少，产生相同的提升振幅。通过将运动和唤醒 - 偶极倍数移至反相模式，从而减轻总辐射声音，同时保持提升幅度值来获得最佳配置。相对的声音偶极级偏离翼型运动和循环信号的同步配置。

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