Harbor seal vibrissa morphology suppresses vortex-induced vibrations

Abstract

SUMMARYHarbor 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. Even the weak water movements left behind by objects that have passed by earlier can be sensed and followed accurately (hydrodynamic trail following). While scanning the water for these hydrodynamic signals at a swimming speed in the order of meters per second, the seal keeps its long and flexible whiskers in an abducted position, largely perpendicular to the swimming direction. 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). Using force measurements, flow measurements and numerical simulations, we 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. The results are discussed in the light of pinniped sensory biology and potential biomimetic applications. Supplementary material available online at

Passive wake detection using seal whisker-inspired sensing

“…The three-dimensionality is described in [58], though they report a significantly shorter undulation wavelength than that in [45]. …”

“…When the sub engine was audible, the seal followed a direct path to it (dotted yellow line). Dimensions were taken directly from [58] and enlarged by a factor of 50. …”

“…A key hint was revealed in [58], which showed a potential connection between vibrations and sensing. The following section provides background on vortex-induced vibrations, which are the underlying physics that will help reveal the answer. )…”

“…The study by [58] was the first to attempt to synthesize these two topics. The authors made a claim that the whisker geometry was able to suppress VIV.…”

Vortex Dynamics in Flow Instabilities and Biomimetic Flows (Prof. Jin S. Chung Award Lecture, 2015)

“…The whiskers (vibrissae) of the harbor seal are used to detect unsteadiness in the flow so as to locate prey (Denhardt et al 2001;Hanke et al 2010). They have a highly unusual shape as shown in Figure 5, in the form of an elliptical cross-section whose half axes vary sinusoidally along the span of the whisker, resulting in a striking undulatory geometric configuration with average wavelength about 4 to 6 times the average transverse direction.…”