Rosti Marco E, Kamps Laura, Bruecker Christoph, Omidyeganeh Mohammad, Pinelli Alfredo
School of Mathematics, Computer Science and Engineering, City, University of London, London, EC1V 0HB UK.
Institute of Mechanics and Fluid Dynamics, Technical University of Freiberg, Freiberg, Germany.
Meccanica. 2017;52(8):1811-1824. doi: 10.1007/s11012-016-0524-x. Epub 2016 Sep 27.
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 (from the leading edge), with a length of [Formula: see text] 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 [Formula: see text] to [Formula: see text] with a reduced frequency of [Formula: see text], [Formula: see text] being the free stream velocity magnitude), and at a static stalled condition ([Formula: see text]) 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.
在鸟类飞行过程中，常常可以注意到，在诸如着陆进近或捕食猎物时的临界飞行条件下，翅膀上侧的一些初级飞羽和覆羽会弹出（见图1）。人们常常推测，弹出的羽毛通过限制气流分离的扩散起到空气动力学作用。进行了一项实验与数值相结合的研究，以阐明决定羽毛自驱动的物理机制及其在名义失速条件下控制流场的有效作用。具体而言，我们考虑了一个NACA0020翼型，在低弦雷诺数下配备了一个柔性襟翼。对襟翼的长度、固有频率和位置的影响进行了参数研究。已发现一种配置在提供最大空气动力学效率和升力增益方面是最优的，即单个襟翼铰接在吸力侧弦长尺寸的70%（从前缘起）处，其长度与失速条件下涡旋的脱落频率相匹配。分析了在襟翼存在和不存在的情况下，在斜坡上升运动期间（入射角从[公式：见原文]到[公式：见原文]，频率降低为[公式：见原文]，[公式：见原文]为自由流速度大小）以及在静态失速条件下（[公式：见原文]）的流动演变。在襟翼存在的情况下，观察到斜坡上升操纵后平均升力显著增加。失速动力学（即升力过冲和振荡）发生改变，模拟揭示了一个周期性再生循环，该循环由一个在通过时抬起襟翼的前缘涡和襟翼在其平衡位置松弛时产生的喷射组成。襟翼的运动反过来在抬起时避免了前缘涡和后缘涡之间的相互作用，并在松弛时将后缘涡向下游推动。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了这种循环行为。与实验的比较表明，襟翼振荡的无量纲频率相似但略高。通过假设循环频率与边界层厚度成反比，可以解释在实验中观察到的较高频率，实验运行时的雷诺数比模拟中的雷诺数高约一个数量级。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，二维模拟表明，当襟翼参数精确调整时，该循环可以无衰减地自我维持。
