Exquisitely sensitive seal whisker-like sensors detect wakes at large distances

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.