Experimental Zoology Group, Wageningen University, 6709 PG Wageningen, The Netherlands.
Biol Lett. 2010 Jun 23;6(3):394-7. doi: 10.1098/rsbl.2009.0806. Epub 2010 Feb 3.
As they fly or swim, many animals generate a wake of vortices with their flapping fins and wings that reveals the dynamics of their locomotion. Previous studies have shown that the dynamic interaction of vortices in the wake with fins and wings can increase propulsive force. Here, we explore whether the dynamics of the vortex interactions could affect the predictability of propulsive forces. We studied the dynamics of the interactions between a symmetrically and periodically pitching and heaving foil and the vortices in its wake, in a soap-film tunnel. The phase-locked movie sequences reveal that abundant chaotic vortex-wake interactions occur at high Strouhal numbers. These high numbers are representative for the fins and wings of near-hovering animals. The chaotic wake limits the forecast horizon of the corresponding force and moment integrals. By contrast, we find periodic vortex wakes with an unlimited forecast horizon for the lower Strouhal numbers (0.2-0.4) at which many animals cruise. These findings suggest that swimming and flying animals could control the predictability of vortex-wake interactions, and the corresponding propulsive forces with their fins and wings.
当动物飞行或游动时，它们拍打鳍和翅膀会产生尾迹涡，揭示了它们运动的动力学。以前的研究表明，尾迹中涡与鳍和翅膀的动态相互作用可以增加推进力。在这里，我们探讨了涡相互作用的动力学是否会影响推进力的可预测性。我们在肥皂膜隧道中研究了对称周期性俯仰和升沉翼片及其尾迹中涡的相互作用动力学。锁定相位的电影序列揭示了在高斯特劳哈尔数下大量混沌的涡尾迹相互作用。这些高数值代表了近乎悬停的动物的鳍和翅膀。混沌尾迹限制了相应力和力矩积分的预测范围。相比之下，我们发现周期性的涡尾迹在较低的斯特劳哈尔数（0.2-0.4）下具有无限的预测范围，许多动物在这个速度下巡航。这些发现表明，游泳和飞行的动物可以通过它们的鳍和翅膀来控制涡尾迹相互作用及其相应的推进力的可预测性。
