University of Illinois at Urbana-Champaign, 105 S. Mathews Ave., Urbana, IL 61801, United States of America.
Bioinspir Biomim. 2021 Jun 22;16(4). doi: 10.1088/1748-3190/abf3b3.
Even though unmanned aerial vehicles (UAVs) are taking on more expansive roles in military and commercial applications, their adaptability and agility are still inferior to that of their biological counterparts like birds, especially at low and moderate Reynolds numbers. A system of aeroelastic devices used by birds, known as the covert feathers, has been considered as a natural flow-control device for mitigating flow separation, enhancing lift, and delaying stall. This study presents the effects of a covert-inspired flap on two airfoils with different stall characteristics at Reynolds numbers in the order of 10, where small scale UAVs operate. Detailed experiments and simulations are used to investigate how the covert-inspired flap affects lift and drag on an airfoil that exhibits sharp or sudden stall (i.e. the NACA 2414 airfoil) and one that exhibits soft or gradual stall (i.e. an E387(A) airfoil). The effects of the flap chord-wise locations and deflection angles on lift and drag is investigated, through wind tunnel experiments, for two types of flaps namely, a freely-moving flap and a static flap. Results show that the static covert-inspired flap can delay stall by up to 5° and improve post-stall lift by up to 23%. However, the post-stall lift improvement characteristics and sensitivities are highly affected by the airfoil choice. For the soft stall airfoil (i.e. E387(A)), the stall onset delay is insensitive to changing the flap deflection angle, and the flap becomes ineffective when the flap location is changed. In contrast, for the sharp stall airfoil (i.e. NACA 2414), the post-stall lift improvements can be tuned using the flap deflection angle, and the flap remains effective over a wide range of chord-wise locations. Numerical studies reveal that the lift improvements are attributed to a step in the pressure distribution over the airfoil, which allows for lower pressures on the suction side upstream of the flap. The distinctions between the flap-induced lift enhancements on the soft and sharp stall airfoils suggest that the flap can be used as a tunable flow control device for the sharp stall airfoil, while for the soft stall airfoil, it can solely be used as a stall mitigation device that is either on or off.
尽管无人机 (UAV) 在军事和商业应用中扮演着越来越广泛的角色，但它们的适应性和敏捷性仍不如鸟类等生物对应物，尤其是在低中和中雷诺数下。鸟类使用的一种气动弹性装置系统，称为隐蔽羽，被认为是一种天然的流动控制装置，可减轻流动分离、提高升力和延迟失速。本研究介绍了一种隐蔽启发式襟翼对两种具有不同失速特性的翼型的影响，这两种翼型的雷诺数在 10 左右，属于小型无人机的工作范围。详细的实验和模拟用于研究隐蔽启发式襟翼如何影响具有急剧或突然失速（即 NACA 2414 翼型）和具有软或逐渐失速（即 E387(A) 翼型）的翼型的升力和阻力。通过风洞实验研究了襟翼弦向位置和偏转角对两种襟翼（即自由移动襟翼和固定襟翼）的升力和阻力的影响。结果表明，静态隐蔽启发式襟翼可使失速延迟高达 5°，并使失速后的升力提高高达 23%。然而，失速后升力改善特性和敏感度受翼型选择的影响很大。对于软失速翼型（即 E387(A)），襟翼偏转角的变化对失速延迟不敏感，并且当襟翼位置改变时襟翼失效。相比之下，对于急剧失速翼型（即 NACA 2414），可以通过襟翼偏转角来调整失速后的升力提高，并且在宽范围的弦向位置上襟翼仍然有效。数值研究表明，升力的提高归因于翼型上压力分布的阶跃，这使得襟翼上游吸力侧的压力降低。在软失速和急剧失速翼型上襟翼引起的升力增强之间的区别表明，襟翼可以用作急剧失速翼型的可调流量控制装置，而对于软失速翼型，它只能用作开或关的失速缓解装置。
