Mechanism limiting the speed of wave gliders under high sea states

To elucidate the mechanism limiting wave-glider speed under high sea states, a hydrodynamic model of the semi-passive submerged glider was developed. Using Fluent overset meshes and UDFs, the passive motion of the submerged glider was coupled with tandem-hydrofoil passive rotation. Simulations for Sea States 2–5 were validated by South China Sea trials. The wave glider attained relatively high mean forward speed at wave heights of 1.2–1.3 m. With further sea-state intensification, its speed ceased to increase and gradually declined. When the wave height increased from 1.25 to 2.5 m, the peak pitch angle increased from 16° to 21°, and the peak occurred approximately 0.15 T earlier. The increased pitch amplitude and earlier peak occurrence markedly shortened the durations for which the submerged glider maintained a favorable nose-down attitude of 1°–6° and the hydrofoil effective angle of attack remained within 8°–12°, thereby weakening the sustained conversion of wave input into effective horizontal thrust. Under high sea states, intensified pitch oscillations reduced the coordination among submerged-glider attitude, hydrofoil effective angle of attack, and thrust output, constituting the main cause of the speed limitation. Sea-trial trends in mean forward speed and pitch response agreed with the simulations, confirming the model's validity.