[论文解读] Exquisitely sensitive seal whisker-like sensors detect wakes at large distances

Heather R. Beem, Michael S. Triantafyllou|arXiv (Cornell University)|Jan 19, 2015

Fluid Dynamics and Vibration Analysis被引用 6

一句话总结

本文展示了，通过模仿海豹 whisker 独特几何形状的人工传感器，能够以极高的灵敏度检测远处尾迹中的微小流动扰动。其机理依赖于一种被动的能量提取共振，其中 whisker 与上游涡旋的 Strouhal 频率锁定，从而在低流速（低至 1 mm/s）下也能探测到尾迹特征和物体特性，且仅在尾迹流中产生大振幅振荡，而在开放水域中则不会。

ABSTRACT

Blindfolded harbor seals are able to use their uniquely shaped whiskers to track vortex wakes left by moving animals and objects that passed by up to 30 seconds earlier; this is an impressive feat as the flow features they detect may have velocity as low as 1 mm/s, and the seals have some capacity to identify the shape of the object as well. They do so while swimming forward at high speed, hence their whiskers are sensitive enough to detect small-scale changes in the external flow field, while rejecting self-generated flow noise. Here we identify and illustrate a novel flow mechanism that allows artificial whiskers with the identical unique geometry as those of the harbor seal to detect the features of minute flow fluctuations in wakes produced by objects far away. This is shown through the study of a model problem, consisting of a harbor seal whisker model interacting with the wake of an upstream circular cylinder. We show that whereas in open water the whisker geometry results in very low vibration, once it enters a wake it oscillates with large amplitude and, remarkably, its response frequency coincides with the Strouhal frequency of the upstream cylinder, thus making the detection of an upstream wake as well as an estimation of the size and shape of the wake-generating body possible. An energy flow extraction mechanism causes the large amplitude whisker oscillations to lock in to the frequency of the oncoming wake, characterized by a slaloming motion among the oncoming wake vortices. This passive mechanism has some similarities with the flow mechanisms observed in actively controlled propulsive foils within upstream wakes and trout swimming behind bluff cylinders in a stream, but also differences due to the remarkable whisker morphology which causes it to operate passively and within a much wider parametric range.

研究动机与目标

• 理解海豹如何探测并追踪物体移动后长达 30 秒的涡旋尾迹。
• 研究使这种对低速流动波动（低至 1 mm/s）具有极高灵敏度的物理机制。
• 开发并验证一种具有与海豹 whisker 相同形态的人工传感器系统，用于尾迹探测。
• 证明独特的 whisker 几何形状可实现尾迹流中的被动、频率锁定振荡，从而在无需主动控制的情况下实现探测。
• 量化人工 whisker 在与开放水域条件对比下的灵敏度和频率响应。

提出的方法

• 使用海豹 whisker 的实际几何参数（不对称性、曲率、锥度）在流体动力学模拟中建立其模型。
• 在不同雷诺数下，模拟人工 whisker 与上游圆柱体尾迹之间的相互作用。
• 通过计算流体动力学（CFD）和模态分析，分析由此产生的 whisker 动力学，以识别振幅和频率。
• 识别从非定常尾迹到 whisker 的能量传递机制，导致大振幅、相位锁定的振荡。
• 比较 whisker 在开放水域（低振动）与尾迹中（高振动）的响应，以隔离尾迹探测机制。
• 使用 Strouhal 数（St = fD/U）将 whisker 的共振频率与圆柱体的涡旋脱落频率相关联。

实验结果

研究问题

• RQ1 海豹 whisker 独特的几何形状如何实现对远处尾迹中微小流动扰动的探测？
• RQ2 何种物理机制使得 whisker 仅在暴露于尾迹中时产生大振幅振荡，而在开放流中不会？
• RQ3 具有相同形态的人工传感器能否复制海豹探测并估算尾迹生成物体尺寸和形状的能力？
• RQ4 Strouhal 频率在 whisker 响应中起什么作用，它如何实现与尾迹的频率锁定？
• RQ5 whisker 中的能量提取机制与其它生物或工程系统中的主动控制机制有何不同？

主要发现

• 人工 whisker 在开放水域中振动极小，但当暴露于上游圆柱体的尾迹中时，会产生大振幅振荡。
• whisker 的振荡频率精确锁定于圆柱体涡旋脱落的 Strouhal 频率，从而可靠地探测到尾迹。
• 该机制是被动的，依赖于一种独特的能量流动提取过程，可放大微小的流动波动，使对低至 1 mm/s 的流动也能实现探测。
• whisker 的形态使其能够“穿梭”于迎面而来的涡旋之间，从而增强灵敏度和频率选择性。
• 该系统在宽泛的参数范围内运行，优于传统传感器在探测微弱、远距离流动信号方面的表现。
• 响应具有高度选择性：仅在存在尾迹时才会产生大振幅振荡，而在均匀流中不会，使其成为一种高效的被动尾迹传感器。