Drucker EG, Lauder GV
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA.
J Exp Biol. 1999;202(Pt 18):2393-2412. doi: 10.1242/jeb.202.18.2393.
Quantifying the locomotor forces experienced by swimming fishes represents a significant challenge because direct measurements of force applied to the aquatic medium are not feasible. However, using the technique of digital particle image velocimetry (DPIV), it is possible to quantify the effect of fish fins on water movement and hence to estimate momentum transfer from the animal to the fluid. We used DPIV to visualize water flow in the wake of the pectoral fins of bluegill sunfish (Lepomis macrochirus) swimming at speeds of 0.5-1.5 L s(-)(1), where L is total body length. Velocity fields quantified in three perpendicular planes in the wake of the fins allowed three-dimensional reconstruction of downstream vortex structures. At low swimming speed (0.5 L s(-)(1)), vorticity is shed by each fin during the downstroke and stroke reversal to generate discrete, roughly symmetrical, vortex rings of near-uniform circulation with a central jet of high-velocity flow. At and above the maximum sustainable labriform swimming speed of 1.0 L s(-)(1), additional vorticity appears on the upstroke, indicating the production of linked pairs of rings by each fin. Fluid velocity measured in the vicinity of the fin indicates that substantial spanwise flow during the downstroke may occur as vortex rings are formed. The forces exerted by the fins on the water in three dimensions were calculated from vortex ring orientation and momentum. Mean wake-derived thrust (11.1 mN) and lift (3.2 mN) forces produced by both fins per stride at 0.5 L s(-)(1) were found to match closely empirically determined counter-forces of body drag and weight. Medially directed reaction forces were unexpectedly large, averaging 125 % of the thrust force for each fin. Such large inward forces and a deep body that isolates left- and right-side vortex rings are predicted to aid maneuverability. The observed force balance indicates that DPIV can be used to measure accurately large-scale vorticity in the wake of swimming fishes and is therefore a valuable means of studying unsteady flows produced by animals moving through fluids.
量化游泳鱼类所经历的运动力是一项重大挑战，因为直接测量施加于水体介质的力并不可行。然而，利用数字粒子图像测速技术（DPIV），可以量化鱼鳍对水运动的影响，从而估计从动物到流体的动量传递。我们使用DPIV来可视化蓝鳃太阳鱼（Lepomis macrochirus）以0.5 - 1.5 L s⁻¹的速度游动时胸鳍尾流中的水流，其中L为鱼的全长。在鳍的尾流中三个垂直平面上量化的速度场允许对下游涡旋结构进行三维重建。在低游泳速度（0.5 L s⁻¹）时，每个鳍在向下划水和划水方向反转时会产生涡度，以产生离散的、大致对称的、具有近似均匀环流且中心有高速流束的涡环。在等于及高于1.0 L s⁻¹的最大可持续的唇形游泳速度时，向上划水时会出现额外的涡度，表明每个鳍产生了成对相连的涡环。在鳍附近测量的流体速度表明，在形成涡环时向下划水过程中可能会出现显著的展向流。根据涡环方向和动量计算了鳍在三个维度上对水施加的力。发现在0.5 L s⁻¹时，每个步幅中两个鳍产生的平均尾流推力（11.1 mN）和升力（3.2 mN）与根据经验确定的身体阻力和重量的反作用力紧密匹配。向内的定向反作用力出乎意料地大，平均为每个鳍推力的125%。预计如此大的向内力以及将左右侧涡环分隔开的深体有助于机动性。观察到的力平衡表明，DPIV可用于准确测量游泳鱼类尾流中的大规模涡度，因此是研究动物在流体中运动产生的非定常流动的一种有价值的手段。
