Numerical study of the nonlinear interaction and energy capture of an OWSC under multi-frequency focused freak waves using Riemann-SPH

To address the survivability and energy capture efficiency of Oscillating Wave Surge Converters (OWSCs) under extreme sea states, this study develops a high-fidelity numerical model coupling Riemann-based Weakly Compressible Smoothed Particle Hydrodynamics (Riemann-based WCSPH) with multi-body dynamics. This model is employed to systematically investigate the nonlinear interaction mechanisms between multi-frequency focused freak waves and an OWSC. The numerical simulations reproduce the target multi-frequency focused waves based on the linear theoretical spectrum, with the simulated crest agreeing with the first-order target within 0.09%. However, due to the inherent limitations of the first-order wave generation boundary, the model underpredicts the extreme nonlinear crest amplification observed in the physical Baldock experiments, though it satisfactorily captures the overall phase synchronization and wave packet envelope. The results reveal that under freak wave impacts, the dynamic response of the OWSC exhibits a prominent “velocity-dominated” characteristic: while the pitch angle response shows displacement saturation, the instantaneous angular velocity surges by 71% compared to regular wave conditions, subjecting the mechanical system to substantial instantaneous power flow shocks. Furthermore, the back-regulation mechanism of Power Take-Off (PTO) damping on the flow field is uncovered. It is found that high damping induces a significant “fluid blockage effect,” converting the flap’s wave-induced dynamic loading into an elevated quasi-static pressure buildup that deteriorates the structural loading environment. Parametric analysis indicates that the capture width ratio follows a unimodal trend with the damping coefficient. An approximate optimal damping coefficient ($$C_{pto} \approx 60~\mathrm {N \cdot m \cdot s/rad}$$) is identified at which the capture width ratio is maximized, representing a favorable impedance-matching point between the hydrodynamics and the mechanical system; however, this damping level also corresponds to a substantially amplified peak structural load relative to the lightly-damped baseline, so that the identified condition reflects a trade-off between energy capture and structural loading rather than a simultaneous optimum for both. To address the survivability and energy capture efficiency of Oscillating Wave Surge Converters (OWSCs) under extreme sea states, this study develops a high-fidelity numerical model coupling Riemann-based Weakly Compressible Smoothed Particle Hydrody... [2053 chars]