Johansson L C, Hedenström A
Department of Theoretical Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden.
J Exp Biol. 2009 Oct;212(Pt 20):3365-76. doi: 10.1242/jeb.034454.
Reconstructing the vortex wake of freely flying birds is challenging, but in the past few years, direct measurements of the wake circulation have become available for a number of species. Streamwise circulation has been measured at different positions along the span of the birds, but no measurements have been performed in the transverse plane. Recent findings from studies of bat wakes have pointed to the importance of transverse plane data for reconstructing the wake topology because important structures may be missed otherwise. We present results of high-speed DPIV measurements in the transverse plane behind freely flying blackcaps. We found novel wake structures previously not shown in birds, including wing root vortices of opposite as well as the same sign as the wing tip vortices. This suggests a more complex wake structure in birds than previously assumed and calls for more detailed studies of the flow over the wings and body, respectively. Based on measurements on birds with and without a tail we also tested hypotheses regarding the function of the tail during steady flight. We were unable to detect any differences in the wake pattern between birds with and without a tail. We conclude that the birds do not use their tail to exploit vortices shed at the wing root during the downstroke. Neither did we find support for the hypothesis that the tail should reduce the drag of the bird. The function of the tail during steady flight thus remains unclear and calls for further investigation in future studies.
重建自由飞行鸟类的尾涡具有挑战性，但在过去几年中，已经可以对一些物种的尾涡环流进行直接测量。已经在鸟类翼展的不同位置测量了流向环流，但尚未在横向平面上进行测量。蝙蝠尾涡研究的最新发现指出，横向平面数据对于重建尾涡拓扑结构很重要，否则可能会遗漏重要结构。我们展示了自由飞行的黑顶林莺后方横向平面的高速粒子图像测速测量结果。我们发现了鸟类以前未显示的新型尾涡结构，包括与翼尖涡旋符号相反以及相同的翼根涡旋。这表明鸟类的尾涡结构比以前假设的更复杂，需要分别对翅膀和身体上的流动进行更详细的研究。基于对有尾和无尾鸟类的测量，我们还测试了关于稳定飞行过程中尾巴功能的假设。我们无法检测到有尾和无尾鸟类在尾涡模式上的任何差异。我们得出结论，鸟类在向下冲程期间不会利用尾巴利用翼根处脱落的涡旋。我们也没有找到支持尾巴应该减少鸟类阻力这一假设的证据。因此，稳定飞行过程中尾巴的功能仍然不清楚，需要在未来的研究中进一步调查。
