Kurt Melike, Eslam Panah Azar, Moored Keith W
Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015, USA.
Mechanical Engineering, Division of Engineering, Business and Computing, Pennsylvania State University at Berks, Reading, PA 19610, USA.
Biomimetics (Basel). 2020 Mar 31;5(2):13. doi: 10.3390/biomimetics5020013.
Many species of fish gather in dense collectives or schools where there are significant flow interactions from their shed wakes. Commonly, these swimmers shed a classic reverse von Kármán wake, however, schooling eels produce a bifurcated wake topology with two vortex rings shed per oscillation cycle. To examine the schooling interactions of a hydrofoil with a bifurcated wake topology, we present tomographic particle image velocimetry (tomo PIV) measurements of the flow interactions and direct force measurements of the performance of two low-aspect-ratio hydrofoils ( = 0 . 5 ) in an in-line and a staggered arrangement. Surprisingly, when the leader and follower are interacting in either arrangement there are only minor alterations to the flowfields beyond the superposition of the flowfields produced by the isolated leader and follower. Motivated by this finding, Garrick's linear theory, a linear unsteady hydrofoil theory based on a potential flow assumption, was adapted to predict the lift and thrust performance of the follower. Here, the follower hydrofoil interacting with the leader's wake is considered as the superposition of an isolated pitching foil with a time-varying cross-stream velocity derived from the wake flow measurements of the isolated leader. Linear theory predictions accurately capture the time-averaged lift force and some of the major peaks in thrust derived from the follower interacting with the leader's wake in a staggered arrangement. The thrust peaks that are not predicted by linear theory are likely driven by spatial variations in the flowfield acting on the follower or nonlinear flow interactions; neither of which are accounted for in the simple theory. This suggests that unsteady potential flow theory that account for spatial variations in the flowfield acting on a hydrofoil can provide a relatively simple framework to understand and model the flow interactions that occur in schooling fish. Additionally, schooling eels can derive thrust and efficiency increases of 63-80% in either a in-line or a staggered arrangement where the follower is between two branched momentum jets or with one momentum jet branch directly impinging on it, respectively.
许多鱼类会聚集形成密集的群体或鱼群，在那里它们脱落的尾流会产生显著的流动相互作用。通常，这些游动者会产生经典的反向冯·卡门尾流，然而，成群游动的鳗鱼会产生一种分叉的尾流拓扑结构，每个振荡周期会脱落两个涡环。为了研究具有分叉尾流拓扑结构的水翼的鱼群相互作用，我们展示了层析粒子图像测速技术（tomo PIV）对流动相互作用的测量结果，以及对两个低展弦比水翼（= 0.5）在串联和交错排列时性能的直接力测量结果。令人惊讶的是，当领头者和跟随者以任何一种排列方式相互作用时，除了孤立的领头者和跟随者产生的流场叠加之外，流场只有微小的变化。受这一发现的启发，基于势流假设的线性非定常水翼理论——加里克线性理论，被用于预测跟随者的升力和推力性能。在这里，与领头者尾流相互作用的跟随者水翼被视为一个孤立的俯仰水翼与一个从孤立领头者的尾流测量中得出的随时间变化的横向流速的叠加。线性理论预测准确地捕捉到了在交错排列中跟随者与领头者尾流相互作用时的时间平均升力以及推力中的一些主要峰值。线性理论未预测到的推力峰值可能是由作用在跟随者上的流场的空间变化或非线性流动相互作用驱动的；而这两者在简单理论中都没有考虑到。这表明考虑作用在水翼上的流场空间变化的非定常势流理论可以提供一个相对简单的框架来理解和模拟成群鱼类中发生的流动相互作用。此外，成群游动的鳗鱼在串联或交错排列中，当跟随者分别处于两个分支动量射流之间或有一个动量射流分支直接撞击它时，可以获得63 - 80%的推力增加和效率提高。
