Liu Xuechao, Liu Kui, Huang Haibo
Department of Modern Mechanics, University of Science and Technology of China, Hefei 230026, China.
Phys Rev E. 2022 Jun;105(6-2):065105. doi: 10.1103/PhysRevE.105.065105.
Fish schools and their potential hydrodynamic advantages are intriguing problems and many underlying mechanisms are unclear due to the complexity of the system, especially for large schools. Here large schools containing four, six, and eight self-propelled foils in a side-by-side configuration are numerically studied. The effect of different combinations out of phase and in phase between two neighboring foils is studied. The results show that the multiple abreast self-propelled foils driven by synchronized harmonic flapping motions can spontaneously form stable side-by-side configurations. When compared with a single foil flapping alone, for cases in which any two neighboring foils are in an out-of-phase state, foils consume more energy with a specific cruising speed. For cases where any two neighboring foils are in an in-phase state, foils propel at a lower speed for a specific flapping frequency. Interestingly, the foils in hybrid states in which both out of phase and in phase coexist are preferred to enhance speed and save power. Further analysis indicates that the stability of the configuration and the lower cost of transport are attributed to the synchronized collaborative wake vortex structure and bow configuration formed by any three neighboring foils in a hybrid state. The collaborative vortices in the wake help the foils move forward alternatively during one flapping cycle. The bow configuration prevents the wake from spreading laterally and enhances the performance. Our paper sheds some light on understanding the self-organized collective behavior and hydrodynamic advantages of large schools.
鱼群及其潜在的水动力优势是有趣的问题，由于系统的复杂性，许多潜在机制尚不清楚，尤其是对于大型鱼群。本文对由四个、六个和八个并列排列的自推进水翼组成的大型鱼群进行了数值研究。研究了相邻两个水翼之间不同的异相和同相组合的影响。结果表明，由同步谐波摆动运动驱动的多个并列自推进水翼可以自发形成稳定的并列结构。与单个水翼单独摆动相比，对于任意两个相邻水翼处于异相状态的情况，水翼在特定巡航速度下消耗更多能量。对于任意两个相邻水翼处于同相状态的情况，水翼在特定摆动频率下以较低速度推进。有趣的是，异相和同相共存的混合状态的水翼更有利于提高速度和节省功率。进一步分析表明，结构的稳定性和较低的运输成本归因于处于混合状态的任意三个相邻水翼形成的同步协同尾流涡结构和弓形结构。尾流中的协同涡有助于水翼在一个摆动周期内交替向前移动。弓形结构可防止尾流向侧面扩散并提高性能。我们的论文为理解大型鱼群的自组织集体行为和水动力优势提供了一些启示。
