Chang Yu-Hung, Ting Shang-Chieh, Su Jian-Yuan, Soong Chyi-Yeou, Yang Jing-Tang
Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Feb;87(2):022707. doi: 10.1103/PhysRevE.87.022707. Epub 2013 Feb 13.
Some small birds typically clap their wings ventrally, particularly during hovering. To investigate this phenomenon, we analyzed the kinematic motion and wake flow field of two passerine species that hover with the same flapping frequency. For these two birds, the ventral clap is classified as direct and cupping. Japanese White-eyes undertake a direct clap via their hand wings, whereas Gouldian Finches undertake a cupping clap with one wing overlaying the other. As a result of their morphological limitation, birds of both greater size and wing span cup their wings to increase the wing speed during a ventral clap because of the larger wing loading. This morphological limitation leads also to a structural discrepancy of the wake flow fields between these two passerine species. At the instant of clapping, the direct clap induces a downward air velocity 1.68 times and generates a weight-normalized lift force 1.14 times that for the cupping clap. The direct clap produces a small upward jet and a pair of counter-rotating vortices, both of which abate the transient lift at the instant of clapping, but they are not engendered by the cupping clap. The aerodynamic mechanisms generated with a ventral clap help the small birds to avoid abrupt body swinging at the instant of clapping so as to maintain their visual stability during hovering.
一些小型鸟类通常会在腹部拍打翅膀，尤其是在悬停时。为了研究这一现象，我们分析了两种以相同拍翅频率悬停的雀形目鸟类的运动学动作和尾流场。对于这两种鸟，腹部拍翅可分为直接拍翅和杯状拍翅。日本绣眼鸟通过其手部翅膀进行直接拍翅，而胡锦鸟则是一只翅膀覆盖在另一只翅膀上进行杯状拍翅。由于它们的形态限制，体型和翼展较大的鸟类在腹部拍翅时会将翅膀呈杯状，以增加翅膀速度，因为翼载荷较大。这种形态限制也导致了这两种雀形目鸟类尾流场的结构差异。在拍翅瞬间，直接拍翅产生的向下气流速度是杯状拍翅的1.68倍，产生的重量归一化升力是杯状拍翅的1.14倍。直接拍翅产生一个小的向上射流和一对反向旋转的涡流，这两者都会在拍翅瞬间减弱瞬态升力，但杯状拍翅不会产生这些。腹部拍翅产生的空气动力学机制有助于小型鸟类在拍翅瞬间避免身体突然摆动，从而在悬停时保持视觉稳定性。
