Wave-Assisted Propulsion in Bimodal Sea States: Hydrodynamic Performance and Hydroelastic Tuning

Wave-assisted propulsion (WAP) systems harvest ocean wave energy to generate propulsive thrust, offering a promising approach for improving endurance and energy efficiency of marine vehicles. Previous studies have focused primarily on monochromatic or unimodal wave conditions, leaving WAP performance in realistic ocean environments largely unexplored. This study investigates the hydrodynamic and hydroelastic response of a submerged flapping hydrofoil operating in bimodal sea states generated by the coexistence of swell and wind-sea wave systems. High-fidelity fluid--structure interaction simulations are performed for representative calm, transitional, and storm conditions, with passive pitching provided through a torsional spring. Simulations show that, despite increased complexity of bimodal wave forcing, propulsion performance follows the same effective peak frequency scaling previously established for monochromatic and unimodal waves, demonstrating the robustness of this scaling framework across a broad range of sea states. The findings further reveal that while the optimal normalized tuning ratio remains within a narrow range, dimensional torsional spring stiffness varies with sea state characteristics, highlighting the need for adaptive hydroelastic tuning to maximize thrust. Overall, the results demonstrate that WAP systems provide a robust means of generating wave-powered thrust under realistic ocean conditions while providing practical guidance for improved design of wave-powered marine propulsion systems.