Feilich Kara L, Lauder George V
The Museum of Comparative Zoology, 26 Oxford St., Harvard University, Cambridge, MA 02138 USA.
Bioinspir Biomim. 2015 Apr 16;10(3):036002. doi: 10.1088/1748-3190/10/3/036002.
Fishes are found in a great variety of body forms with tail shapes that vary from forked tuna-like tails to the square-shaped tails found in some deep-bodied species. Hydrodynamic theory suggests that a fish's body and tail shape affects undulatory swimming performance. For example, a narrow caudal peduncle is believed to reduce drag, and a tuna-like tail to increase thrust. Despite the prevalence of these assertions, there is no experimental verification of the hydrodynamic mechanisms that may confer advantages on specific forms. Here, we use a mechanically-actuated flapping foil model to study how two aspects of shape, caudal peduncle depth and presence or absence of a forked caudal fin, may affect different aspects of swimming performance. Four different foil shapes were each made of plastics of three different flexural stiffnesses, permitting us to study how shape might interact with stiffness to produce swimming performance. For each foil, we measured the self-propelling swimming speed. In addition, we measured the forces, torques, cost of transport and power coefficient of each foil swimming at its self-propelling speed. There was no single 'optimal' foil exhibiting the highest performance in all metrics, and for almost all measures of swimming performance, foil shape and flexural stiffness interacted in complicated ways. Particle image velocimetry of several foils suggested that stiffness might affect the relative phasing of the body trailing edge and the caudal fin leading edge, changing the flow incident to the tail, and affecting hydrodynamics of the entire foil. The results of this study of a simplified model of fish body and tail morphology suggest that considerable caution should be used when inferring a swimming performance advantage from body and tail shape alone.
鱼类具有多种多样的身体形态，尾巴形状各异，从类似金枪鱼的叉状尾到某些体深物种的方形尾。流体动力学理论表明，鱼的身体和尾巴形状会影响其波动式游泳性能。例如，人们认为狭窄的尾柄可减少阻力，而金枪鱼状的尾巴可增加推力。尽管这些说法很普遍，但对于可能赋予特定形态优势的流体动力学机制，尚无实验验证。在此，我们使用一个机械驱动的扑翼模型来研究形状的两个方面，即尾柄深度和是否有叉状尾鳍，如何影响游泳性能的不同方面。四种不同的翼型分别由三种不同弯曲刚度的塑料制成，这使我们能够研究形状如何与刚度相互作用以产生游泳性能。对于每个翼型，我们测量了其自推进游泳速度。此外，我们还测量了每个翼型在自推进速度下游泳时的力、扭矩、运输成本和功率系数。没有单一的“最佳”翼型在所有指标上都表现出最高性能，而且对于几乎所有游泳性能指标，翼型形状和弯曲刚度都以复杂的方式相互作用。对几种翼型的粒子图像测速表明，刚度可能会影响身体后缘和尾鳍前缘的相对相位，改变入射到尾部的水流，并影响整个翼型的流体动力学。这项对简化的鱼体和尾形态模型的研究结果表明，仅从身体和尾巴形状推断游泳性能优势时应格外谨慎。
