Numerical investigation on vibration mechanisms of harbour seal whiskers in fin wakes during acceleration and cruising

Endowed with extraordinary sensitivity, Harbour seal whiskers can detect and track hydrodynamic wakes from moving bodies. To exploit this biological capability, this study employs the immersed boundary method to numerically investigate the flow-induced vibrations of an elastically mounted whisker model subjected to caudal fin wakes under acceleration (St = 0.65) and cruising (St = 0.3, 0.22) conditions. The analysis encompasses the whisker's vibration responses, wake structures, lift coefficients, and the interaction mechanisms between the whisker and fin wakes. In the accelerating mode, the caudal fin generates three-dimensional hairpin vortices that gradually contract along the span during downstream propagation. These vortices exert localized influence on the whisker's midspan region, predominantly inducing vortex-induced vibrations (VIV). Conversely, in the cruising mode, the caudal fin produces rectangular vortex rings propagating parallel to the flow, which exert extensive effects on the downstream whisker across a broader span. This leads to wake-induced vibrations (WIV) dominating the whisker's dynamic responses, with the dominant frequency synchronized to the caudal fin's pitching and heaving frequencies. Driven by the combined effects of vortex ring spacing and vorticity, the whisker-fin wake interaction modulates the whisker's lift coefficients and energy transfer distributions. These findings deepen the understanding of Harbour seal whiskers' hydrodynamic sensing mechanisms and provide critical theoretical guidance for the development of biomimetic fish-detecting underwater sensors.