均匀流下两柔性梁纵列行为变化研究
COLLECTIVE BEHAVIOR CHANGES OF TWO TANDEM FLEXIBLE BEAMS IN UNIFORM FLOWS
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摘要: 随着水下仿生柔性航行器的蓬勃发展, 如何有效地开展集群组织、提升效能成为研究热点之一, 特别是当柔性航行器编队遭遇外界环境变化时. 基于此, 本文开展流场作用下两个自推进柔性梁纵列行为变化的数值研究, 探讨外部流场的影响规律, 揭示集群纵列行为适应性和多变性机理. 结果表明: 横向流下柔性梁速度曲线变化周期增加1倍, 幅值增大超过3倍, 且出现了波峰、波谷幅值不等的非线性现象, 新的纵列结构形成; 纵向流对柔性梁运动具有线性影响, 其推进速度在顺流/逆流时线性地增加/减少, 变化值与流速大小一致, 两柔性梁纵列结构与静水中保持一致. 对流场细节进行分析可知, 外部流场影响了尾流中涡的运动及融合, 这是导致纵列聚集结构发生变化的原因. 横向流下涡发生融合, 对称涡结构被破坏, 而纵向流下所有尾涡产生相同水平位移, 其对称结构并不发生改变. 此外, 对比了流场下纵列两柔性梁的推进速度、功率、效率数据, 发现最小功率、最大速度和效率并不出现在顺流工况, 更大范围流场方向上柔性梁性能较静水中得到提升. 以上得到的流场下纵列行为变化研究数据和机理可为水下仿生柔性航行器集群遭遇外部流场时主动选取航行策略提供支撑.Abstract: With the upsurge of the bionic underwater vehicles, how to rationally organize those vehicles and maximize their efficiencies has become one of research focuses, especially in the case of encountering the disturbances from external flows. In this paper, the hydrodynamic schooling behavior of two tandem self-propelled beams in different flow circumstances is studied numerically. The influences of uniform flow are discussed, and the mechanism of adaptability and variability of the collective behavior of two flexible beams is revealed. The results indicate that the transverse flow component has nonlinear influences, under which the doubled period and three times large amplitude of velocity curves are obtained. The unequal crest and peak values of velocity curve are observed. The new collective structure is obtained accordingly. The longitudinal flow component linearly affects the cruising velocities of beams without destroying their original hydrodynamic collective structure. Flow details are illustrated and discussed to reveal the mechanism of those hydrodynamic collective behavior changes. Flow-induced vortex motion and mixture take the dominant roles. Transversal flow promote the mixture of vortex, their symmetric structure is broken accordingly. The same motions of vortex caused by longitudinal flow do not change their structure, and then the collective structure of the same longitudinal gap distance keeps. In addition, the propulsion performance data (including velocity, power and efficiency) of two tandem flexible beams in the collective behavior in different uniform flows are compared. It is found that except for the case of swimming following the flow, the propulsion velocity and the efficiency of two flexible beams are promoted in wide range of flow directions. The maximum values of the velocity and the efficient are not obtained in the case of following the flow. The obtained mechanism and data would help the decision of sailing strategy for bionic underwater vehicles in case of encountering external flows.