Abstract
This project is inspired by the remarkable sensing ability observed in harbor seals, which can detect footprints left by upstream objects in water, even long after the object has passed. Harbor seals achieve this through their uniquely structured, flexible whiskers, which interact with wake flows to sense waterborne disturbances. This capability allows them to track and find prey even with their auditory and visual perception suppressed. The undulatory geometry of the whiskers has been identified as the mechanism by which seals suppress any self-generated flow noise in the wake of the whiskers while swimming forward. This unique ability has garnered significant scientific interest, particularly regarding the whiskers’ sensing and drag-reducing properties. While the vortex-induced vibration (VIV) and wake flow dynamics behind harbor seal whisker models are well-studied in the literature, their interaction with the oncoming flow from upstream hydrodynamic objects is not well-understood and sparse in literature. The aim of this research was to study the flow-induced vibration (FIV) response of the whisker when placed in tandem with an upstream object. A series of experiments were conducted in a recirculating water tunnel with the goal of understanding the underlying FIV response of a whisker. The whisker was attached to a one degree-of-freedom (1 DOF) set up, that allows the whisker module to oscillate in the crossflow direction, i.e., perpendicular to the incoming flow, when placed in the wake region of an upstream circular cylinder. Various center-to-center distances between the upstream cylinder and the whisker module were investigated to analyze the system’s sensitivity to the spatial gap between the module and the source of upstream disturbance, thereby comprehending the FIV response’s dependency on this distance. The vortex-induced vibration response of the whisker module attached to the 1DOF setup showed that, due to its undulatory geometry, self-induced vibrations were significantly suppressed, resulting in negligible oscillations. Conversely, when placed downstream of a stationary circular cylinder, the whisker exhibited oscillations at a frequency matching the shedding frequency of the upstream wake. This observation highlights the whisker’s ability to detect wakes and demonstrates the effectiveness of its distinctive geometry in enabling such flow sensing. Further experiments were conducted to understand the impact of various wake patterns or footprints on the whisker’s FIV response. In these experiments, the upstream cylinder underwent prescribed oscillations in the crossflow direction, maintaining a consistent amplitude across three distinct frequencies: half of the natural frequency, equivalent to the natural frequency, and twice the natural frequency. It was found that the whisker module oscillates precisely at the frequency dictated by the prescribed motion of the upstream cylinder. Notably, the responses exhibited substantial oscillation amplitudes at the designated incoming frequency, and this behavior persisted even at the highest reduced velocity evaluated. These initial findings shine light upon the true detectability that is being accomplished by the harbor seal whisker. The sensing capability that the harbor seal achieves through their whiskers has the potential to significantly inspire the design of biomimetic collision avoidance sensors, applicable across diverse domains and applications.