Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves
Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves
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DOI：
10.1098/rsif.2016.0068
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发表时间：
2016-03-01
影响因子：
3.9
通讯作者：
Liu, Hao
中科院分区：
文献类型：
--
作者：
Li, Gen;Mueller, Ulrike K.;Liu, Hao
硬骨鱼的幼虫在中间雷诺数（Re）范围内游动，利用身体和尾鳍的波动来推动自己。它们共用一条中间鳍，当它们长成幼鱼时，中间鳍会变成独立的。翅片被认为是为了适应中等雷诺数的运动，但其流体动力学作用仍然是谜。利用三维流体力学计算方法，从幼虫在循环游动过程中身体形状的变化来量化游动轨迹。我们预测了鱼鳍上边缘和下边缘附近的不稳定涡，并利用粒子图像测速技术识别了真实幼虫周围的类似涡。我们发现，在靠近翅折上下边缘的体峰上，推力的贡献与边缘涡的周期性产生和脱落相一致，那里出现了巨大的左右压力差。翅片增强有效的流动分离和基于阻力的推力。沿身体，净推力在质心后方的多个区域产生。反事实模拟研究了在一定雷诺数范围内的鳍褶的影响，结果表明，鳍褶有助于幼虫达到较高的游泳速度，但需要较高的功率。我们得出结论，幼鱼的推进部分依赖于沿鳍褶上下边缘的不稳定高强度漩涡，这为鳍褶在幼鱼中无处不在提供了功能解释。
Larvae of bony fish swim in the intermediate Reynolds number (Re) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left-right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.