<strong>Gait-Dependent Control Strategies and Wave Propagation Mechanisms of Fish in Complex Flows</strong>

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 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 (2D) Hilbert transform for wave decomposition and time-series analysis, we quantify body wave evolution and intermittent swimming across Uniform Flow (UF), Entrainment (EN), and Kármán Gait (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 dual-mode AUV design.