Seal-vibrissa-inspired wavy cylinders vibrate in the wake of a flapping foil

Seals have been shown to use their vibrissae (facial whiskers) to sense wake flows that are even slower than 1 mm/s (Dehndhardt et al. 1998). This exquisite sensitivity has been associated with the special wavy shape of the vibrissae which reduces cross-stream forces in steady flows (Hanke et al. 2010). We want to know how this wavy shape contributes to vibrissa sensitivity to disturbed flows and what is the role of the mounting stiffness at the root of the vibrissa. Our experiments use scaled-up models of vibrissae, along with comparable circular and elliptical cylinders, in a water flume, where we measure the forces and water flows around them in both steady flows and wake flows. We test both stationary and oscillating vibrissae and determine stability boundaries for self-sustained flow-induced oscillations. We find that the vibrissae models in a steady flow experience reduced cross-stream forces compared to circular cylinders, while vibrissae models positioned in the wake of a flapping hydrofoil feel stronger cross-stream forces which can cause them to vibrate. This effect can help explain how seal vibrissae are able to detect and identify wake flows.