Department of Organismal and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, United States of America.
Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States of America.
Bioinspir Biomim. 2021 May 11;16(4). doi: 10.1088/1748-3190/abe137.
Fish benefit energetically when swimming in groups, which is reflected in lower tail-beat frequencies for maintaining a given speed. Recent studies further show that fish save the most energy when swimming behind their neighbor such that both the leader and the follower benefit. However, the mechanisms underlying such hydrodynamic advantages have thus far not been established conclusively. The long-standing drafting hypothesis-reduction of drag forces by judicious positioning in regions of reduced oncoming flow-fails to explain advantages of in-line schooling described in this work. We present an alternate hypothesis for the hydrodynamic benefits of in-line swimming based on enhancement of propulsive thrust. Specifically, we show that an idealized school consisting of in-line pitching foils gains hydrodynamic benefits via two mechanisms that are rooted in the undulatory jet leaving the leading foil and impinging on the trailing foil: (i) leading-edge suction on the trailer foil, and (ii) added-mass push on the leader foil. Our results demonstrate that the savings in power can reach as high as 70% for a school swimming in a compact arrangement. Informed by these findings, we designed a modification of the tail propulsor that yielded power savings of up to 56% in a self-propelled autonomous swimming robot. Our findings provide insights into hydrodynamic advantages of fish schooling, and also enable bioinspired designs for significantly more efficient propulsion systems that can harvest some of their energy left in the flow.
鱼类在群体中游动时会获得能量上的好处，这反映在维持给定速度时的尾拍频率较低。最近的研究进一步表明，鱼类在跟随邻居游动时可以节省最多的能量，从而使领导者和追随者都受益。然而，到目前为止，还没有确定这些水动力优势的机制。长期存在的拖曳假说——通过在来流减少的区域明智地定位来减少阻力——无法解释本工作中描述的直线编队游泳的优势。我们提出了一种基于推进推力增强的直线游泳水动力优势的替代假说。具体来说，我们表明，由直线俯仰翼片组成的理想学校通过两种机制获得水动力优势，这两种机制根植于离开先导翼片并撞击尾随翼片的波动射流：（i）尾翼上的前缘吸力，以及（ii）对先导翼片的附加质量推动。我们的结果表明，对于紧密排列的游动群体，节省的功率可达 70%。根据这些发现，我们设计了一种对尾推进器的改进，在自主游泳机器人中可节省高达 56%的功率。我们的发现提供了对鱼类编队游泳水动力优势的深入了解，并且还能够实现生物启发设计，以获得更高效的推进系统，从而利用它们在流动中留下的部分能量。
