Wake-induced `slaloming' response explains exquisite sensitivity of seal whisker-like sensors

Abstract

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 783 (2015): 306-322, doi:10.1017/jfm.2015.513.Blindfolded harbour seals are able to use their uniquely shaped whiskers to track vortex wakes left by moving animals and identify objects that passed by 30 s earlier, an impressive feat, as the flow features have velocities as low as 1 mm s−1. The seals sense while swimming, hence their whiskers are sensitive enough to detect small-scale changes in the flow, while rejecting self-generated flow noise. Here we identify and illustrate a novel flow mechanism, causing a large-amplitude ‘slaloming’ whisker response, which allows artificial whiskers with the identical unique undulatory geometry as those of the harbour seal to detect the features of minute flow fluctuations when placed within wakes. Whereas in open water the whisker responds with very low-amplitude vibration, once within a wake, it oscillates with large amplitude and, importantly, its response frequency coincides with the Strouhal frequency of the upstream cylinder, making the detection of an upstream wake and an estimation of the size and shape of the wake-generating body possible. This 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 there are also differences caused by the unique whisker morphology, which enables it to respond passively and within a much wider parametric range.The authors acknowledge with gratitude support by ONR, monitored by Dr. Thomas Swean, Jr. under grant N00014-13-1-0059, the William I. Koch Chair in Marine Technology, the MIT Sea Grant Program, and the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology: Center for Environmental Sensing and Modeling (CENSAM).2016-04-1