Mechanical power curve measured in the wake of pied flycatchers indicate modulation of parasite power across flight speeds

How aerodynamic power required for animal flight varies with flight speed determines optimal speeds during foraging and migratory flight. Despite its relevance, aerodynamic power provide an elusive quantity to measure directly in animal flight. Here we determine the aerodynamic power from wake velocity fields, measured using tomographical particle image velocimetry, of pied flycatchers flying freely in a wind tunnel. We find a shallow U-shaped power curve, which is flatter than expected by theory. Based on how the birds vary body angle with speed, we speculate that the shallow curve result from increased body drag coefficient and body frontal area at lower flight speeds. Including modulation of body drag in the model results in a more reasonable fit with data than the traditional model. From the wake structure we also find a single starting vortex generated from the two wings during the downstroke across flight speeds (1-9 m/s). This is accomplished by the arm wings interacting at the b...

动物飞行所需的气动功率（aerodynamic power）随飞行速度的变化规律，决定了其觅食与迁徙飞行中的最优飞行速度。尽管该参数具有重要研究价值，但直接测量动物飞行中的气动功率仍是一项难以直接获取的物理量。本研究针对在风洞中自由飞行的斑姬鹟（pied flycatcher），采用层析粒子图像测速法（tomographical particle image velocimetry）测得其尾流速度场（wake velocity fields），并据此计算得到气动功率。研究发现其功率曲线呈平缓的U型，相较于现有理论预期更为扁平。结合鸟类身体倾角随飞行速度的变化规律，我们推测该平缓功率曲线的成因，是低速飞行时鸟类的身体阻力系数与身体迎风面积有所增加。相较于传统模型，在模型中引入身体阻力调制项后，其与实验数据的拟合效果更为合理。通过分析尾流结构，我们还发现在全飞行速度区间（1~9 m/s）内，翅膀下挥过程中由两翼共同产生单个启动涡。这一现象由臂翼在b...处的相互作用所实现。