Body-Bending Kinematics Enable Stable Fish Swimming in High-Energy Periodic Wake

Fish often maintain stable, efficient locomotion in highly energetic, perturbed flows, such as strong currents, shear layers, and the vortex wakes of conspecifics, providing valuable design insights for resilient, bio-inspired underwater vehicles. Inspired by recent flume experiments where trout swam stably within the reverse von Kármán vortex street created by a flapping hydrofoil, exhibiting body-bending kinematics that differ from those used in uniform-flow swimming, we explore how specific body curvature and flexibility contribute to stability in oscillatory vortex wake. We develop a 3D trout-like model with controllable midline curvature that mimics distributed muscle activation and simulate its untethered swimming in a hydrofoil wake using high-fidelity immersed-boundary-method-based flow simulations at biologically relevant Reynolds numbers. The model is free to sway and yaw, ensuring linear and angular momentum are conserved without artificial constraints. We quantify body dynamics, hydrodynamic forces and moments, and wake interactions to identify the kinematic strategies—phase, amplitude, and curvature modulation—that suppress destabilizing moments and enhance propulsive efficiency under vortex forcing. Our results can clarify how fish utilize flexibility and curvature control to mitigate flow perturbations, providing actionable guidelines for designing underwater vehicles that remain stable and efficient in complex, unsteady environments.