Effect of Angle on Flow-Induced Vibrations of Pinniped Vibrissae

Abstract

Two types of vibrissal surface structures, undulated and smooth, exist among pinnipeds. Most Phocidae have vibrissae with undulated surfaces, while Otariidae, Odobenidae, and a few phocid species possess vibrissae with smooth surfaces. Variations in cross-sectional profile and orientation of the vibrissae also exist between pinniped species. These factors may influence the way that the vibrissae behave when exposed to water flow. This study investigated the effect that vibrissal surface structure and orientation have on flow-induced vibrations of pinniped vibrissae. Laser vibrometry was used to record vibrations along the whisker shaft from the undulated vibrissae of harbor seals (Phoca vitulina) and northern elephant seals (Mirounga angustirostris) and the smooth vibrissae of California sea lions (Zalophus californianus). Vibrations along the whisker shaft were measured in a flume tank, at three orientations (0°, 45°, 90°) to the water flow. The results show that vibration frequency and velocity ranges were similar for both undulated and smooth vibrissae. Angle of orientation, rather than surface structure, had the greatest effect on flow-induced vibrations. Vibration velocity was up to 60 times higher when the wide, flat aspect of the whisker faced into the flow (90°), compared to when the thin edge faced into the flow (0°). Vibration frequency was also dependent on angle of orientation. Peak frequencies were measured up to 270 Hz and were highest at the 0° orientation for all whiskers. Furthermore, CT scanning was used to quantify the three-dimensional structure of pinniped vibrissae that may influence flow interactions. The CT data provide evidence that all vibrissae are flattened in cross-section to some extent and that differences exist in the orientation of this profile with respect to the major curvature of the hair shaft. These data support the hypothesis that a compressed cross-sectional profile may play a key role in reducing self-noise of the vibrissae.

Vibrissal sensitivity in a harbor seal (Phoca vitulina)

“…The sensitivity of the seal also overlaps with the signals produced by the vibrissal structure during flow interactions. Excised seal vibrissae produce a distinct low-frequency vibrational signal (<300 Hz) when exposed to low-velocity (0.5 m s −1 ) water flow under laboratory conditions (Murphy et al, 2013). The spectral characteristics of these vibrissal signals correspond closely to the frequency sensitivity of the sensory system revealed by psychophysical testing.…”

“…The low absolute detection thresholds measured in the present study demonstrate acute sensitivity of the vibrissal system and highlight the importance of fine-scale vibration detection in seals. Especially sensitive detection capabilities may be of particular importance to seals considering the fact that the morphology and orientation (Murphy et al, 2013) of the vibrissae have been shown to suppress the amplitude of vibration. In the context of this reduced amplitude effect, changes in whisker vibrations caused by encountering a hydrodynamic disturbance while moving through the surrounding fluid present a distinct input signal to the vibrissal system.…”

Passive wake detection using seal whisker-inspired sensing

“…Since then, a few other parties have carried out studies on the vibration properties of seal whiskers on both real and model specimens [91,143,95]. The important thing to note is that none of the experiments that provided quantitative results were conducted under conditions that allowed the whisker to vibrate freely in response to the flow.…”

“…Real whiskers are curved along the span, and it would be good to determine how much the VIV response of the harbor seal whisker changes with the addition of curvature. Footage of swimming harbor seals by [95] led those authors to observe that the major curvature of the hair shaft was generally in the downstream (caudal) direction. In that case, the curvature would likely not dramatically increase the VIV response.…”

“…The position of individual whiskers during real prey pursuit is unknown. If the videos provided in [95], which show seals swimming in lab conditions, depict the same whisker positions during wake tracking, the following are a few notable features and corresponding hypotheses about the hydrodynamic implications of this arrangement. Screenshots from the videos are provided in Figure 5-4.…”