Shape of retracting foils that model morphing bodies controls shed energy and wake structure
Shape of retracting foils that model morphing bodies controls shed energy and wake structure
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模拟变形体的回缩箔的形状控制着脱落的能量和尾流结构
DOI：
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发表时间：
2016
影响因子：
3.7
通讯作者：
M. Triantafyllou
中科院分区：
文献类型：
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作者：
S. Steele;J. Dahl;G. Weymouth;M. Triantafyllou
通过水翼的简单模型研究了变形运动体的流动机制，水翼以恒定攻角被拖曳时，会在展向距离上快速缩回。实验和模拟表明，通过改变回缩箔尖端的形状，可以再现不同的形状变化条件，对应于：（i）消失体，（ii）收缩体和（iii）熔化体。边缘锋利、“消失”的箔片向流体释放出强烈的能量；然而，它伴随着额外的能量释放，导致在缩回运动期间在箔片的尖锐尖端边缘处形成强烈的环形涡流。这种额外的能量释放引入了复杂且快速演变的涡流结构。相比之下，流线型的“收缩状”箔片可以避免产生环形涡流，从而留下结构更简单的尾流。 “收缩”的箔片还能从流体中回收大部分初始能量，从而导致尾流结构变得更弱。最后，边缘锋利但空心的“熔化状”箔片提供了充满活力的尾流，同时避免了涡环的产生。结果，一个融化般的物体形成了一个简单、高能量和稳定的尾流，它夹带了所有最初增加的质量流体能量。研究的三种条件对应于水生动物和鸟类所采用的不同流量控制模式，以及在消失的物体中遇到的情况，例如上升的气泡经历相变为流体。
The flow mechanisms of shape-changing moving bodies are investigated through the simple model of a foil that is rapidly retracted over a spanwise distance as it is towed at constant angle of attack. It is shown experimentally and through simulation that by altering the shape of the tip of the retracting foil, different shape-changing conditions may be reproduced, corresponding to: (i) a vanishing body, (ii) a deflating body and (iii) a melting body. A sharp-edge, ‘vanishing-like’ foil manifests strong energy release to the fluid; however, it is accompanied by an additional release of energy, resulting in the formation of a strong ring vortex at the sharp tip edges of the foil during the retracting motion. This additional energy release introduces complex and quickly evolving vortex structures. By contrast, a streamlined, ‘shrinking-like’ foil avoids generating the ring vortex, leaving a structurally simpler wake. The ‘shrinking’ foil also recovers a large part of the initial energy from the fluid, resulting in much weaker wake structures. Finally, a sharp edged but hollow, ‘melting-like’ foil provides an energetic wake while avoiding the generation of a vortex ring. As a result, a melting-like body forms a simple and highly energetic and stable wake, that entrains all of the original added mass fluid energy. The three conditions studied correspond to different modes of flow control employed by aquatic animals and birds, and encountered in disappearing bodies, such as rising bubbles undergoing phase change to fluid.