Blindfolded harbor seals are able to use their uniquely shaped whiskers to track vortex wakes left by moving animals and identify objects that passed by 30 seconds earlier, an impressive feat as the flow features have velocities as low as 1 mm/s. 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 harbor 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.

A flow sensor inspired by the harbor seal’s whiskers has been calibrated in a towing tank and tested at sea. The whiskers’ undulatory, asymmetric geometry causes a wide range of self-excited vibrational responses. These vibrations are measured mechanically and used to determine the oncoming flow direction and velocity. Calibration runs show the vibration amplitude in the cross-flow direction to range from ≈0.1 to 0.6 times the whisker diameter, depending on the flow direction. Deflection in the inline-flow direction ranges from ≈0 to 0.08 times the whisker diameter, depending on the flow velocity. Field tests confirm the ability of the sensor to be used autonomously and at depth. Experiments with the sensor trailing a vortex generator show the sensor locking into the oncoming vortex frequency, demonstrating potential for future use as a wake detector.

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