Birds are known for their extraordinary agility, maneuverability, flexibility and endurance during their flight, even under some adverse flying conditions. Bird wings have been the most inspirational element, attracting the attention of researchers to reveal the underlying physical mechanism of lift production as well as to apply the results into the artificial flying vehicles. This paper presents a systematic experimental investigation on a passive flow control of a NACA0012 airfoil using real feather flap which is installed on the suction or pressure surface. The focus of the present study is to determine the major role of a real feather flap in the aerodynamic performance of a NACA0012 airfoil at small attack angles (α). The feather flap width w and its installation position x in are varied from 0.27c to 0.8c and from 0.0 to 0.2c, respectively, where x in is measured from the leading edge of the airfoil, and c is the chord length of the airfoil. Detailed particle image velocimetry (PIV) measurements are conducted to understand the origin of the aerodynamic benefits introduced by the feather flap. The flap mounted on the suction side may have a positive impact only at large α, beyond the stall. On the other hand, when mounted on the pressure side, the feather flap is proved to be beneficial to improve the aerodynamic performance of the airfoil at small α (=  -4° to 8°). The lift C L and lift-to-drag ratio C L /C D are enhanced by 186% and 72%, respectively, for w  =  0.53c, x in   =  0.2c at α  =  2°. Time-averaged and instantaneous vorticities, time-averaged streamwise velocity, and lateral velocity around the flapped airfoil weaken, decrease and increase, respectively, compared with those around the plain airfoil, which are attributed to the increased C L and C L /C D .

中文翻译：

---

生物启发式羽毛襟翼被动控制翼型提升的实验研究。

鸟类以其在飞行过程中甚至在某些不利飞行条件下的非凡敏捷性，机动性，灵活性和耐力而闻名。鸟的翅膀一直是最鼓舞人心的元素，吸引了研究人员的注意力，以揭示提升生产的潜在物理机制，并将结果应用于人工飞行器中。本文提出了使用安装在吸力或压力表面上的真羽襟翼对NACA0012机翼进行被动流动控制的系统实验研究。本研究的重点是确定在小的迎角（α）下，真正的羽毛襟翼在NACA0012机翼的气动性能中的主要作用。羽毛襟翼宽度w及其安装位置x in分别从0.27c至0.8c和0.0至0.2c变化，其中x in是从机翼的前缘开始测量的，而c是机翼的弦长。进行了详细的颗粒图像测速（PIV）测量，以了解羽毛襟翼带来的空气动力学优势的起源。安装在吸力侧的襟翼可能仅在较大的α处（超出失速范围）产生积极影响。另一方面，证明当安装在压力侧时，羽毛襟翼有利于在小α（＝-4°至8°）下改善翼型的空气动力性能。对于w = 0.53c，在α= 2°时x in = 0.2c，升力CL和升阻比CL / CD分别提高了186％和72％。时间平均和瞬时涡度，时间平均流向速度和拍打翼型周围的横向速度分别减弱，降低和增加，