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On the maintenance of an attached leading-edge vortex via model bird alula
Journal of Fluid Mechanics ( IF 3.6 ) Pub Date : 2020-06-11 , DOI: 10.1017/jfm.2020.364
Thomas Linehan , Kamran Mohseni

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.

中文翻译:

通过模型鸟alula维持附着的前缘涡

研究人员假设,鸟类的 alular 羽毛或 alula 的后失速提升效果源于在其薄型外手翼上保持附着的前缘涡 (LEV)。在这里,我们通过风洞中的流量测量研究了 alula 和 LEV 附件之间的连接。我们展示了一个模型 alula,其润湿面积是机翼的 1%,在稳定平移的未后掠机翼上稳定了一个循环后倾 LEV,该机翼在后失速发生角倾斜。附加的涡流是阿鲁拉能够平滑合并其他分离的前缘和侧缘涡流的结果。我们确定了促进这种合并的两个关键过程:i)机翼产生的展向涡度的转向 s 前缘返回机翼平面和 ii) 位于尾部的高幅度根部到尖端展向流的壁射流(> 80% 的自由流速度)。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气流排向翼尖。我们确定 alula 的流向位置(相对于薄机翼的前缘)对于涡流转向很重要,而 alula 的倾斜角对于产生高幅度的展向流很重要。这些发现促进了我们对鸟类利用 LEV 增加慢速飞行的可能方式的理解。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气流排向翼尖。我们确定 alula 的流向位置(相对于薄机翼的前缘)对于涡流转向很重要,而 alula 的倾斜角对于产生高幅度的展向流很重要。这些发现促进了我们对鸟类利用 LEV 增加慢速飞行的可能方式的理解。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气流排向翼尖。我们确定 alula 的流向位置(相对于薄机翼的前缘)对于涡流转向很重要,而 alula 的倾斜角对于产生高幅度的展向流很重要。这些发现促进了我们对鸟类利用 LEV 增加慢速飞行的可能方式的理解。
更新日期:2020-06-11
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