通过模型鸟翼维持附着前缘涡
On the maintenance of an attached leading-edge vortex via model bird alula
摘要：
研究人员假设，鸟的翼形羽毛（或alula）的失速后升力效益源于在其薄轮廓的外手翼上保持附着的前缘涡流（LEV）。在这里，我们通过在风洞中的流动测量来研究翼片和LEV附件之间的联系。我们表明，一个模型alula，其润湿面积是1%的机翼，稳定再循环后倾LEV上的一个非后掠机翼倾斜在失速后的发病率。附着涡是翼片平滑地合并否则分离前缘和侧缘涡流的能力的结果。我们确定了两个促进这种合并的关键过程：i）在机翼前缘产生的展向涡量转向回到机翼平面，ii）位于后部的高量级根尖展向流的壁面射流（>自由速度的80%）。后一个特征引起LEV卷起，而前一个特征使LEV涡量向后倾斜，并通过外侧涡量通量将该流向翼尖排出。我们确定的alula的流向位置（相对于薄翼的前缘）作为重要的涡流转向和alula的倾斜角作为重要的高幅度展向流的产生。这些发现推进了我们对鸟类利用LEV来增强慢速飞行的可能方式的理解。
英文摘要：
Researchers have hypothesized that the post-stall lift benefit of bird's alular feathers, or alula, stems from the maintenance of an attached leading-edge vortex (LEV) over their thin-profiled, outer hand-wing. Here, we investigate the connection between the alula and LEV attachment via flow measurements in a wind tunnel. We show that a model alula, whose wetted area is 1% that of the wing, stabilizes a recirculatory aft-tilted LEV on a steadily-translating unswept wing inclined at post-stall incidences. The attached vortex is the result of the alula's ability to smoothly merge otherwise separate leading- and side-edge vortical flows. We identify two key processes that facilitate this merging: i) the steering of spanwise vorticity generated at the wing's leading edge back to the wing plane and ii) an aft-located wall-jet of high-magnitude root-to-tip spanwise flow (>80% that of the freestream velocity). The latter feature induces LEV roll-up while the former feature tilts LEV vorticity aft and evacuates this flow toward the wing tip via an outboard vorticity flux. We identify the alula's streamwise position (relative to the leading-edge of the thin wing) as important for vortex steering and the alula's cant angle as important for high-magnitude spanwise flow generation. These findings advance our understanding of the likely ways bird's leverage LEVs to augment slow flight.
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