The Whitney Laboratory for Marine Bioscience, Department of Biology, University of Florida, 9505 Ocean Shore Blvd, St Augustine, FL 32080-8610, USA.
J Exp Biol. 2013 Sep 15;216(Pt 18):3442-9. doi: 10.1242/jeb.087502. Epub 2013 Jun 4.
We have little understanding of how fish hold station in unsteady flows. Here, we investigated the effect of flow speed and body size on the kinematics of rainbow trout Kármán gaiting behind a 5 cm diameter cylinder. We established a set of criteria revealing that not all fish positioned in a vortex street are Kármán gaiting. By far the highest probability of Kármán gaiting occurred at intermediate flow speeds between 30 and 70 cm s(-1). We show that trout Kármán gait in a region of the cylinder wake where the velocity deficit is about 40% of the nominal flow. We observed that the relationships between certain kinematic and flow variables are largely preserved across flow speeds. Tail-beat frequency matched the measured vortex shedding frequency, which increased linearly with flow speed. Body wave speed was about 25% faster than the nominal flow velocity. At speeds where fish have a high probability of Kármán gaiting, body wavelength was about 25% longer than the cylinder wake wavelength. Likewise, the lateral (i.e. cross-stream) amplitude of the tail tip was about 50% greater than the expected lateral spacing of the cylinder vortices, while the body center amplitude was about 70% less. Lateral body center acceleration increased quadratically with speed. Head angle decreased with flow speed. While these values are different from those found in fish swimming in uniform flow, the strategy for locomotion is the same; fish adjust to increasing flow by increasing their tail-beat frequency. Body size also played a role in Kármán gaiting kinematics. Tail-beat amplitudes of Kármán gaiting increased with body size, as in freestream swimming, but were almost three times larger in magnitude. Larger fish had a shorter body wavelength and slower body wave speed than smaller fish, which is a surprising result compared with freestream swimming, where body wavelength and wave speed increased with size. In contrast to freestream swimming, tail-beat frequency for Kármán gaiting fish did not depend on body size and was a function of the vortex shedding frequency.
我们对鱼类在不稳定流中如何保持稳定知之甚少。在这里，我们研究了流速和体型对 5 厘米直径圆柱后虹鳟鱼卡门步态运动学的影响。我们建立了一套标准，揭示了并非所有处于尾流涡街中的鱼类都是卡门步态。到目前为止，在 30 到 70 厘米/秒的中等流速下，卡门步态发生的概率最高。我们表明，鳜鱼在圆柱尾迹中速度亏损约为名义流速的 40%的区域内卡门步态。我们观察到，某些运动学和流动变量之间的关系在很大程度上保持不变，跨越流速。尾拍频率与测量的涡旋脱落频率相匹配，而涡旋脱落频率随流速线性增加。体波速度比名义流速快约 25%。在鱼类卡门步态发生概率较高的速度下，体波长比圆柱尾迹波长长约 25%。同样，尾端的横向（即横流）振幅比圆柱涡旋的预期横向间隔大约 50%，而体中心振幅小约 70%。横向体中心加速度随速度平方增加。头部角度随流速降低。虽然这些值与在均匀流中游泳的鱼类不同，但运动策略是相同的；鱼类通过增加尾拍频率来适应增加的流速。体型也对卡门步态运动学起作用。与在均匀流中游泳一样，卡门步态鳜鱼的尾拍幅度随体型增大而增大，但幅度几乎增大了三倍。较大的鱼的体波长和体波速度比较小的鱼短，这与在均匀流中游泳时体波长和波速随体型增大的结果形成鲜明对比。与在均匀流中游泳不同，卡门步态鱼的尾拍频率不依赖于体型，而是涡旋脱落频率的函数。
