Covert-inspired flaps for lift enhancement and stall mitigation
Covert-inspired flaps for lift enhancement and stall mitigation
复制标题
DOI：
10.1088/1748-3190/abf3b3
复制
发表时间：
2021-03
影响因子：
3.4
通讯作者：
Chengfang Duan;Aimy A. Wissa
中科院分区：
文献类型：
--
作者：
Chengfang Duan;Aimy A. Wissa
尽管无人机（UAV）在军事和商业应用中发挥着越来越广泛的作用，但它们的适应性和敏捷性仍然不如鸟类等生物同类，尤其是在低雷诺数和中等雷诺数的情况下。鸟类使用的气动弹性装置系统（称为隐蔽羽毛）被认为是一种天然的流量控制装置，用于减轻气流分离、增强升力和延迟失速。这项研究展示了隐蔽式襟翼对两个具有不同失速特性的翼型的影响，其中雷诺数约为 105，小型无人机在其中运行。使用详细的实验和模拟来研究隐蔽襟翼如何影响表现出急剧或突然失速的翼型（即 NACA 2414 翼型）和表现出软或逐渐失速的翼型（即 E387(A) 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度对升力和阻力的影响，针对两种类型的襟翼，即自由移动襟翼和静态襟翼。结果表明，受静态隐蔽启发的襟翼可将失速延迟达 5°，并将失速后升力提高达 23%。然而，失速后升力改进特性和灵敏度很大程度上受到翼型选择的影响。对于软失速翼型（即E387（A）），失速起始延迟对襟翼偏转角的变化不敏感，并且当襟翼位置改变时襟翼失效。相比之下，对于急速失速翼型（即 NACA 2414），失速后升力改进可以使用襟翼偏转角进行调整，并且襟翼在广泛的弦向位置上保持有效。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较低。软失速翼型和锐失速翼型上襟翼引起的升力增强之间的区别表明，襟翼可以用作锐失速翼型的可调流量控制装置，而对于软失速翼型，它只能用作打开或关闭的失速缓解装置。
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 105, 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.