波动式水下游泳：通过生物仿生鱼连接涡流动力学、推力和尾迹结构
Undulatory underwater swimming: Linking vortex dynamics, thrust, and wake structure with a biorobotic fish
AI总结：
研究通过生物仿生鱼实验，揭示了涡旋动力学、推力生成与尾迹结构之间的关系，建立了基于斯特劳哈尔数的通用模型。
摘要：
基于振荡的推进系统依赖流体-结构相互作用来产生推力。在中等和高雷诺数下，此类系统尾迹中的涡旋形成和组织对于产生推进力至关重要。在本研究中，我们通过实验研究了一条 tethered 机器人鱼在水洞中的尾迹。通过系统地改变鱼尾的振幅和频率以及自由流速度，我们能够观察并表征不同涡旋街，作为斯特劳哈尔数函数。所产生的尾迹是三维的，表现出经典的V形，主要由两个倾斜的涡旋环列车向外传播组成。通过在鱼的中跨平面后使用二维粒子图像测速仪（PIV）以及对速度和涡量场的广泛数据处理，我们展示了涡旋动力学、推力产生和尾迹结构之间的强耦合。我们首先测量了涡旋速度随斯特劳哈尔数的变化，并利用与推力产生直接相关的动量平衡方程进行建模。然后我们关注尾迹结构，如尾迹角度以及涡旋环的取向、直径和涡量。尾迹结构在简单的几何框架中建模，其中涡旋环速度由自由流速度和环自身推进速度组成。该框架通过我们的实验测量以及文献数据的主曲线汇总进行测试和验证，突显了由斯特劳哈尔数主导的普遍行为。这使我们能够全面理解尾迹结构如何随该数变化以及由此产生的推力产生。
英文摘要：
Flapping-based propulsive systems rely on fluid-structure interactions to produce thrust. At intermediate and high Reynolds numbers, vortex formation and organization in the wake of such systems are crucial for the generation of a propulsive force. In this work, we experimentally investigate the wake produced by a tethered robotic fish immersed in a water tunnel. By systematically varying the amplitude and frequency of the fish tail as well as the free-stream speed, we are able to observe and characterize different vortex streets as a function of the Strouhal number. The produced wakes are three-dimensional and exhibit a classical V-shape, mainly with two oblique trains of vortex rings convecting outward. Using two-dimensional Particle Image Velocimetry (PIV) in the mid-span plane behind the fish and through extensive data processing of the velocity and vorticity fields, we demonstrate the strong couplings at place between vortex dynamics, thrust production and wake structure. We first measure the evolution of the vortex velocity with the Strouhal number, and model it using a momentum balance equation directly related to thrust production. We then focus on the wake structure, such as wake angle as well as vortex ring orientation, diameter and vorticity. The wake structure is modelled in a simple geometrical framework where the vortex ring velocity is composed of the free-stream speed and the ring self-advecting speed. This framework is tested and validated by our experimental measurements as well as literature data collapsing on master curves, highlighting a universal behavior dominated by the Strouhal number. This allows us to establish a comprehensive understanding of how the wake structure varies with this number and, thus, thrust production.
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