Publication Detail

Passive wake detection using seal whisker-inspired sensing

Heather Beem
2014
177 pp.
MITSG 14-32
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This thesis is motivated by a series of biological experiments, which display the harbor seal's extraordinary ability to track the wake of an object several seconds after it has swam by. They do so despite having auditory and visual cues blocked, pointing to use of their whiskers as sensors of minute water movements. In this work, I elucidate the basic fluid mechanisms that seals may employ to accomplish this detection. Key are the unique flow-induced vibration properties resulting from the geometry of the harbor seal whisker, which is undulatory and elliptical in cross-section. First, the vortex-induced vibration (VIV) characteristics of the whisker geometry are tested. Direct force measurements and flow visualizations on a rigid whisker model undergoing a range of 1-D imposed oscillations show that the geometry passively reduces VIV (factor of > 10), despite contributions from e ffective added mass and damping. This suggests that the harbor seal whisker geometry would detect details of the oncoming flow with reduced background noise".
Next, a biomimetic whisker sensor is designed and fabricated. The rigid whisker model is mounted on a four-armed flexure, allowing it to freely vibrate in both in-line and cross-flow directions. Strain gages on the flexure measure deflections at the base. Finally, this device is tested in a simpli fied version of the fi sh wake - seal whisker interaction scenario. The whisker is towed behind an upstream cylinder with larger diameter. Whereas in open water the whisker exhibits very low vibration when its long axis is aligned with the incoming flow, once it enters the wake it oscillates with large amplitude, while its frequency coincides with the Strouhal frequency of the upstream cylinder. This makes the detection of an upstream wake as well as an estimation of the size of the wake-generating body possible. The energy extraction mechanism that causes the whisker to oscillate in this distinct manner is driven by a slaloming motion among the wake vortices that has been previously observed in energy extracting foils and trout actively swimming behind blu cylinders in a stream. This device holds promise for enabling marine vehicles to passively feel flow features. With minimal power, wake detection and tracking can be used for targeted sensing.

type: Full theses / dissertations

Parent Project

Project No.: 2013-R/RT-2/RCM-34
Title: Marine Center for Development of Biomimetic Underwater Sensors

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