Kinematic signatures of flow-induced resonance and stability limits in bio-prototypes interacting with a Kármán vortex street

The efficient propulsion of flexible swimmers in unsteady flow is a fundamental problem in fluid–structure interaction (FSI), governed by the physical trade-off between maximizing propulsive efficiency and maintaining hydrodynamic stability. This study integrates high-fidelity computational fluid dynamics simulations with high-resolution kinematic tracking to quantify the response mechanisms of four biological prototypes with distinct geometric characteristics in a controlled Kármán vortex street. Our results reveal a morphology-driven spectrum of interaction modes comprising three distinct physical regimes: Vortex synchronization (efficiency-optimization), low-vorticity navigation (stability-prioritization), and hydrodynamic instability (physical failure). In the synchronization regime, we quantify a characteristic non-linear saturation in tail-beat frequency. This kinematic signature is consistent with a flow-induced resonance state, where phase-locking maximizes momentum extraction from the environment. In contrast, laterally compressed bluff-body prototypes are highly susceptible to destabilizing yawing moments, necessitating a spatial avoidance strategy to mitigate the risk of biomechanical failure. This work quantifies how geometric constraints dictate the physical boundary conditions for the FSI mechanism, providing a critical mechanical framework for the multi-mode adaptive control of bio-inspired vehicles in complex turbulent environments.