Nature2014-06-26 5:41 PM

鸟儿为什么以V-字队形飞行? Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight

论文摘要 

本期封面所示为在一次由人类引导的迁徙飞行过程中呈V-字队形飞行的欧洲秃鹃。一些鸟类已将自己的飞行技能优化到了能够让它们以近乎完美的V-字队形飞行的程度,但关于它们为什么采用这种方式却一直只有猜测。一种观点是,通过以V-字队形飞行,鸟儿能够最大限度降低能量消耗。而现在,对从自由飞行的候鸟“秃鹃”获得的数据所做分析表明,这种方式从能量角度讲是有好处的。这些数据还显示了鸟儿在飞行中施加控制的一个复杂的、动态的过程。以V-字队形飞行的鸟儿使其翅膀拍打(wing-beat)与路径匹配(path-match)同步,让后面的鸟儿能够利用前面的鸟儿所产生的空气动力“上洗流”(upwash)。然而,紧跟在后面飞行的鸟儿则会以相反相位拍打翅膀,以便尽量减小领头鸟的翅膀所产生的有害“下洗流”(downwash)。所有这一切都要求鸟儿必须具有一系列定相策略(phasing strategy),来应对拍打的翅膀所产生的动态尾流。

Abstract 

Many species travel in highly organized groups. The most quoted function of these configurations is to reduce energy expenditure and enhance locomotor performance of individuals in the assemblage. The distinctive V formation of bird flocks has long intrigued researchers and continues to attract both scientific and popular attention. The well-held belief is that such aggregations give an energetic benefit for those birds that are flying behind and to one side of another bird through using the regions of upwash generated by the wings of the preceding bird, although a definitive account of the aerodynamic implications of these formations has remained elusive. Here we show that individuals of northern bald ibises (Geronticus eremita) flying in a V flock position themselves in aerodynamically optimum positions, in that they agree with theoretical aerodynamic predictions. Furthermore, we demonstrate that birds show wingtip path coherence when flying in V positions, flapping spatially in phase and thus enabling upwash capture to be maximized throughout the entire flap cycle. In contrast, when birds fly immediately behind another bird—in a streamwise position—there is no wingtip path coherence; the wing-beats are in spatial anti-phase. This could potentially reduce the adverse effects of downwash for the following bird. These aerodynamic accomplishments were previously not thought possible for birds because of the complex flight dynamics and sensory feedback that would be required to perform such a feat. We conclude that the intricate mechanisms involved in V formation flight indicate awareness of the spatial wake structures of nearby flock-mates, and remarkable ability either to sense or predict it. We suggest that birds in V formation have phasing strategies to cope with the dynamic wakes produced by flapping wings.

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