Hydrodynamics and flow perception mechanism of side-by-side Harbour seal whiskers in wake flow

Harbour seal whiskers, characterised by their wavy morphology and elliptical cross-section, are capable of perceiving minute flow velocities as low as 10^-4m s^-1. This study investigates the hydrodynamics and flow perception mechanisms of three elastically mounted whiskers arranged side-by-side in the wake of a caudal fin, using direct numerical simulations. The whiskers are spaced at centre-to-centre distances of<i>S</i>/<i>D</i>= 2-4, where<i>S</i>is the inter-whisker spacing and<i>D</i>is the equivalent narrow-face diameter of whisker model. The vibration responses, hydrodynamic forces, wake patterns, energy transfer mechanisms, and flow-sensing performance of the whiskers were analysed. Two distinct wake-whisker interaction patterns are identified, governed by the side-by-side spacing: (Ⅰ) partially-interacted pattern (<i>S</i>/<i>D</i>⩽ 2), in which the upstream caudal fin vortexes bypass the side whiskers, leading to a direct impact on the side whiskers only; (Ⅱ) fully-interacted pattern (<i>S</i>/<i>D</i>> 2), in which the vortexes are able to pass through the gap between the whiskers, allowing all three whiskers to interact with the wake. The energy transfer analysis reveals that the spanwise and chordwise vortexes of caudal fin selectively enhance or suppress whisker vibrations depending on their relative rotation directions, leading to distinct excitation patterns across different spacing configurations. An optimal wake flow perception is achieved at<i>S</i>/<i>D</i>= 3 through lift-vorticity correlation and mutual information analyses under the studied conditions.