Passive wake differentiation by seal vibrissae in response to independently oscillating upstream objects

Harbor seals possess a remarkable ability to detect hydrodynamic footprints left by moving objects, even long after the objects have passed, through interactions between wake flows and their uniquely shaped whiskers. While the flow-induced vibration of harbor seal whisker models has been extensively studied, their response to unsteady wakes generated by upstream moving bodies remains poorly understood. This study investigates the wake-induced vibration (WIV) of a flexibly mounted harbor seal-inspired whisker positioned downstream of a forced-oscillating circular cylinder, simulating the hydrodynamic footprint of a moving object. Unlike conventional WIV studies, where the upstream wake is passively formed behind a stationary body and governed solely by its geometry and flow speed, the upstream cylinder in this work undergoes prescribed oscillations. This approach enables independent control over the wake characteristics-such as wake width and shedding frequency-decoupling them from the physical attributes of the upstream source and allowing a more direct assessment of the whisker's sensing response to dynamic wake conditions. Experiments were conducted across a range of reduced velocities (U∗= 3.4-25) and Reynolds numbers (<i>Re</i>= 500-2700), with upstream oscillation frequencies varied from 0.5 to 2 times the natural frequency of the whisker. Volumetric particle tracking velocimetry (PTV) was used to characterize the flow field, complemented by<i>Q</i>-criterion and proper orthogonal decomposition analyses. Results show that while the whisker suppresses its own vortex-induced vibration in open flow, it oscillates strongly at the frequency of the upstream forcing when exposed to wake disturbances, demonstrating its capability to detect and respond to hydrodynamic trails of moving objects. These findings highlight the potential of harbor seal whisker-inspired designs for biomimetic underwater sensing and navigation systems.