On the maintenance of an attached leading-edge vortex via model bird alula
On the maintenance of an attached leading-edge vortex via model bird alula
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DOI：
10.1017/jfm.2020.364
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发表时间：
2020-08-25
影响因子：
3.7
通讯作者：
Mohseni, Kamran
中科院分区：
文献类型：
--
作者：
Linehan, Thomas;Mohseni, Kamran
研究人员假设，鸟类翼状羽毛在失速后的升力益处来自于在其薄型的外手翼上保持了一个附加的前缘涡旋(LEV)。在这里，我们通过风洞中的流动测量来研究瓣膜和LEV附着物之间的联系。我们证明了一个湿润面积为机翼面积的1%的模型翼面在失速后迎角下稳定了定常平移的非后掠翼上的再循环后倾翼面。附着涡的产生是由于风舌能够平稳地合并分开的前缘和侧缘涡流。我们确定了促进这种合并的两个关键过程：(I)将机翼前缘产生的展向涡量转向机翼平面；(Ii)位于后部的壁面喷流，具有高强度的根尖向展向流动(>；80%的自由流速度)。前一种特性引起翼面卷起，而后一种特性使尾部的涡量倾斜，并通过向外的涡量通量将这种流动疏散到翼尖。我们确定翼面的流向位置(相对于薄翼的前缘)对于涡旋操纵很重要，而翼面的斜角对于大尺度展向流动的产生是重要的。这些发现促进了我们对鸟类利用Levs来增强缓慢飞行的可能方式的理解。
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 at post-stall angles of attack. 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 free-stream velocity). The former feature induces LEV roll-up while the latter 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 birds leverage LEVs to augment slow flight.