During the flight of birds, it is often possible to notice that some of the primaries and covert feathers on the upper side of the wing pop-up under critical flight conditions, such as the landing approach or when stalking their prey (see Fig. 1) . It is often conjectured that the feathers pop up plays an aerodynamic role by limiting the spread of flow separation . A combined experimental and numerical study was conducted to shed some light on the physical mechanism determining the feathers self actuation and their effective role in controlling the flow field in nominally stalled conditions. In particular, we have considered a NACA0020 aerofoil, equipped with a flexible flap at low chord Reynolds numbers. A parametric study has been conducted on the effects of the length, natural frequency, and position of the flap. A configuration with a single flap hinged on the suction side at 70 % of the chord size c (from the leading edge), with a length of $$L=0.2c$$L=0.2c matching the shedding frequency of vortices at stall condition has been found to be optimum in delivering maximum aerodynamic efficiency and lift gains. Flow evolution both during a ramp-up motion (incidence angle from $$\alpha _0=0$$α0=0 to $$\alpha _s=20^\circ$$αs=20∘ with a reduced frequency of $$k= 0.12\, U_{\infty }/c$$k=0.12U∞/c, $$U_{\infty }$$U∞ being the free stream velocity magnitude), and at a static stalled condition ($$\alpha =20^\circ$$α=20∘) were analysed with and without the flap. A significant increase of the mean lift after a ramp-up manoeuvre is observed in presence of the flap. Stall dynamics (i.e., lift overshoot and oscillations) are altered and the simulations reveal a periodic re-generation cycle composed of a leading edge vortex that lift the flap during his passage, and an ejection generated by the relaxing of the flap in its equilibrium position. The flap movement in turns avoid the interaction between leading and trailing edge vortices when lift up and push the trailing edge vortex downstream when relaxing back. This cyclic behaviour is clearly shown by the periodic variation of the lift about the average value, and also from the periodic motion of the flap. A comparison with the experiments shows a similar but somewhat higher non-dimensional frequency of the flap oscillation. By assuming that the cycle frequency scales inversely with the boundary layer thickness, one can explain the higher frequencies observed in the experiments which were run at a Reynolds number about one order of magnitude higher than in the simulations. In addition, in experiments the periodic re-generation cycle decays after 3–4 periods ultimately leading to the full stall of the aerofoil. In contrast, the 2D simulations show that the cycle can become self-sustained without any decay when the flap parameters are accurately tuned.

中文翻译：

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PELskin 项目-第 V 部分：使用自激活可展开襟翼控制大迎角机翼周围的气流

在鸟类的飞行过程中，通常可能会注意到在关键的飞行条件下，例如着陆进近或跟踪猎物时，机翼上侧的一些主要和隐蔽的羽毛会弹出（见图 1） )。人们经常推测，弹出的羽毛通过限制流动分离的传播来发挥空气动力学作用。进行了结合实验和数值研究，以阐明决定羽毛自致动的物理机制及其在名义上的停滞条件下控制流场的有效作用。特别是，我们考虑了 NACA0020 机翼，它配备了低弦雷诺数下的柔性襟翼。已经对襟翼的长度、固有频率和位置的影响进行了参数研究。单个襟翼铰接在弦尺寸 c 的 70%（从前缘）的吸力侧的配置，长度为 $$L=0.2c$$L=0.2c，与失速处的涡流脱落频率相匹配已发现条件是提供最大空气动力学效率和升力增益的最佳条件。在斜升运动期间的流动演变（入射角从 $$\alpha _0=0$$α0=0 到 $$\alpha _s=20^\circ$$αs=20∘，频率降低 $$k = 0.12\, U_{\infty }/c$$k=0.12U∞/c, $$U_{\infty }$$U∞ 是自由流速度幅度），并且在静态停滞条件下（$$\ alpha =20^\circ$$α=20∘) 在有和没有皮瓣的情况下进行分析。在存在襟翼的情况下，观察到加速操作后平均升力显着增加。失速动态（即 升力超调和振荡）被改变，模拟揭示了一个周期性的再生循环，由一个前缘涡流组成，该涡流在襟翼通过期间提升襟翼，以及襟翼在其平衡位置放松时产生的弹射。襟翼运动依次避免升起时前缘涡流和后缘涡流之间的相互作用，并在向后放松时将后缘涡流推向下游。这种循环行为由升力围绕平均值的周期性变化以及襟翼的周期性运动清楚地显示出来。与实验的比较表明襟翼振荡的无量纲频率相似但稍高一些。通过假设循环频率与边界层厚度成反比，人们可以解释在雷诺数比模拟高一个数量级的实验中观察到的更高频率。此外，在实验中，周期性再生循环在 3-4 个周期后衰减，最终导致翼型完全失速。相比之下，2D 模拟表明，当襟翼参数被准确调整时，循环可以自我维持而没有任何衰减。