Gait-dependent control strategies and wave propagation mechanisms of fish in complex flows

Complex unsteady flow environments present substantial challenges to the stability and energy efficiency of autonomous underwater vehicles (AUVs). While fish can utilize mechanisms like the Kármán gait (KG) to conserve energy, a quantitative understanding of the underlying wave mechanics and morphological limitations remains lacking. This study conducts a comparative kinematic analysis of two morphologically distinct species—the streamlined Leuciscus baicalensis and the deep-bodied Carassius auratus gibelio—in the wake of a D-shaped cylinder. Using the two-dimensional Hilbert transform for wave decomposition and time-series analysis, we quantify body wave evolution and intermittent swimming across uniform flow (UF), entrainment (EN), and KG regimes. Results reveal distinct physical modes: UF relies on active propulsion, EN exhibits active damping with recoil noise, whereas KG operates as a passive resonance mode where hydrodynamic loading forms standing waves. Temporal analysis identifies a gait-dependent control shift from stability-driven intermittent bursting in shear layers—where intermittency correlates positively with velocity—to hydrodynamic lock-in with a 100% duty cycle in vortex streets. Critically, morphological compliance dictates the stability boundary: the streamlined species sustains resonance at 0.8 m/s, whereas the deep-bodied species experiences control divergence due to excessive transverse added mass. These findings provide key criteria for a dual-mode AUV design.