AskNature

“Harbor seals (Phoca vitulina) often live in dark and turbid waters, where their mystacial vibrissae, or whiskers, play an important role in orientation. Besides detecting and discriminating objects by direct touch, harbor seals use their whiskers to analyze water movements, for example those generated by prey fish or by conspecifics…Remarkably, the whiskers of harbor seals possess a specialized undulated surface structure, the function of which was, up to now, unknown. Here, we show that this structure effectively changes the vortex street behind the whiskers and reduces the vibrations that would otherwise be induced by the shedding of vortices from the whiskers (vortex-induced vibrations)…[W]e find that the dynamic forces on harbor seal whiskers are, by at least an order of magnitude, lower than those on sea lion (Zalophus californianus) whiskers, which do not share the undulated structure.” (Hanke et al. 2010:2665)

“Analyzing the 3-D structure of the wake of the vibrissa (Fig.5, right), we find that the separation of primary vortices (Kármán vortices) is replaced by a complex 3-D vortex structure in which various states of vortex separation occur simultaneously on different locations in a spanwise direction. Furthermore, the region of vortex formation is considerably shifted downstream compared with the circular or elliptic cylinder wake, thus opening a gap between the vibrissa surface and the region with fluctuating flow (Figs 5, 6). Therefore, the resulting pressure imposed on the complete vibrissa is more symmetric compared with that on a circular or an elliptic cylinder. The three features together – the reduction of the primary vortex separation, the gap between the vibrissa and the first vortices and the symmetry of the pressure field – prevent large periodic forces on the vibrissa originating from its own wake and thus prevent vortex-induced vibrations.” (Hanke et al. 2010:2669)

“The structure of the harbor seal vibrissa demonstrates a highly efficient mechanism for the suppression of forces owing to vortex shedding from rod-like objects with drag reduction. Numerous applications in biomimetic designs are conceivable, including flow sensors for underwater vehicles or parts of buildings and offshore facilities with reduced vibrations from wind or water flow. Inspiration from nature has so far been lacking in this field of research. Particularly challenging is the suppression of vortex induced vibrations in flexible structures with low mass (Owen et al., 2001), as observed here.” (Hanke et al. 2010:2671)