City, University of London, Northampton Square, London, EC1V 0HB, United Kingdom.
RWTH Aachen University, Templergraben 55, 52062 Aachen, Germany.
Bioinspir Biomim. 2020 Dec 21;16(2). doi: 10.1088/1748-3190/abc6b4.
This work describes a novel mechanism of laminar flow control of straight and backward swept wings with a comb-like leading edge (LE) device. It is inspired by the LE comb on owl feathers and the special design of its barbs, resembling a cascade of complex 3D-curved thin finlets. The details of the geometry of the barbs from an owl feather were used to design a generic model of the comb for experimental and numerical flow studies with the comb attached to the LE of a flat plate. Due to the owls demonstrating a backward sweep of the wing during gliding and flapping from live recordings, our examinations have also been carried out at differing sweep angles. The results demonstrate a flow turning effect in the boundary layer inboards, which extends downstream in the chordwise direction over distances of multiples of the barb lengths. The inboard flow-turning effect described here, counter-acts the outboard directed cross-span flow typically appearing for backward swept wings. This flow turning behaviour is also shown on SD7003 airfoil using precursory LES investigations. From recent theoretical studies on a swept wing, such a way of turning the flow in the boundary layer is known to attenuate crossflow instabilities and delay transition. A comparison of the comb-induced cross-span velocity profiles with those proven to delay laminar to turbulent transition in theory shows excellent agreement, which supports the laminar flow control hypothesis. Thus, the observed effect is expected to delay transition in owl flight, contributing to a more silent flight.
本文描述了一种利用梳状前缘（LE）装置对直线和后掠翼进行层流控制的新机制。它的灵感来自于猫头鹰羽毛的 LE 梳和其倒钩的特殊设计，类似于一连串复杂的三维弯曲薄翼片。猫头鹰羽毛倒钩的几何细节被用来设计一种通用的梳状模型，用于实验和数值流动研究，将梳状装置安装在平板的 LE 上。由于猫头鹰在滑翔和拍打过程中翅膀呈后掠状，我们的研究还在不同的后掠角下进行。结果表明，边界层内部的流动会发生转向，这种转向会在弦向方向上延伸，距离可达倒钩长度的数倍。这里描述的向内流动转向效应，抵消了通常出现在后掠翼上的向外跨距方向的横向流动。这种流动转向行为也在 SD7003 翼型上通过预先导的 LES 研究得到了证实。最近对后掠翼的理论研究表明，这种在边界层中转向流动的方式可以减弱横向流动不稳定性并延迟转捩。将梳状装置引起的横向速度分布与理论上被证明能延迟层流转湍流的速度分布进行比较，结果非常吻合，这支持了层流控制假说。因此，观察到的效应有望延迟猫头鹰飞行中的转捩，从而实现更安静的飞行。
