Peabody Museum of Natural History, Yale University, PO Box 208106, New Haven, CT 06511, USA.
J Exp Biol. 2013 Sep 15;216(Pt 18):3404-13. doi: 10.1242/jeb.085993. Epub 2013 Jun 4.
Feathers can produce sound by fluttering in airflow. This flutter is hypothesized to be aeroelastic, arising from the coupling of aerodynamic forces to one or more of the feather's intrinsic structural resonance frequencies. We investigated how mode of flutter varied among a sample of hummingbird tail feathers tested in a wind tunnel. Feather vibration was measured directly at ~100 points across the surface of the feather with a scanning laser Doppler vibrometer (SLDV), as a function of airspeed, Uair. Most feathers exhibited multiple discrete modes of flutter, which we classified into types including tip, trailing vane and torsional modes. Vibratory behavior within a given mode was usually stable, but changes in independent variables such as airspeed or orientation sometimes caused feathers to abruptly 'jump' from one mode to another. We measured structural resonance frequencies and mode shapes directly by measuring the free response of 64 feathers stimulated with a shaker and recorded with the SLDV. As predicted by the aeroelastic flutter hypothesis, the mode shape (spatial distribution) of flutter corresponded to a bending or torsional structural resonance frequency of the feather. However, the match between structural resonance mode and flutter mode was better for tip or torsional mode shapes, and poorer for trailing vane modes. Often, the 3rd bending structural harmonic matched the expressed mode of flutter, rather than the fundamental. We conclude that flutter occurs when airflow excites one or more structural resonance frequencies of a feather, most akin to a vibrating violin string.
羽毛可以通过在气流中飘动产生声音。这种飘动被假设为空气弹性的，它源于空气动力对羽毛固有结构共振频率之一或多个的耦合。我们研究了在风洞中测试的一组蜂鸟尾羽样本中，飘动模式如何变化。通过扫描激光多普勒测振仪（SLDV），在羽毛表面的大约 100 个点上直接测量羽毛的振动，作为空气速度 Uair 的函数。大多数羽毛表现出多种离散的飘动模式，我们将其分类为包括尖端、尾翼和扭转模式。给定模式内的振动行为通常是稳定的，但空气速度或方向等自变量的变化有时会导致羽毛突然从一种模式“跳跃”到另一种模式。我们通过测量由振动器刺激的 64 根羽毛的自由响应并用 SLDV 记录，直接测量结构共振频率和模式形状。正如空气弹性飘动假说所预测的那样，飘动的模式形状（空间分布）与羽毛的弯曲或扭转结构共振频率相对应。然而，结构共振模式与飘动模式之间的匹配对于尖端或扭转模式形状更好，而对于尾翼模式形状较差。通常，第三弯曲结构谐波与表达的飘动模式相匹配，而不是基本模式。我们得出结论，当气流激发羽毛的一个或多个结构共振频率时，就会发生飘动，最类似于振动的小提琴弦。
