Hydrodynamics of wave-induced fluid resonance in a recessed moonpool

The hydrodynamics of wave-induced fluid resonance in a recessed moonpool are numerically investigated using both a multiphase viscous flow solver and a fully nonlinear potential flow solver. This study aims to investigate the nonlinear resonance phenomena inherent to the recessed moonpool configuration, with particular attention to the associated reflection, transmission, and radiation waves. Emphasis is also placed on assessing the predictive accuracy of the potential flow solver and elucidating the physical mechanisms underlying its deviations from the viscous flow results. The quasi-steady amplitude- and phase-frequency responses of fluid oscillations are first examined to identify the characteristic multi-peak resonance behavior. Frequency-domain harmonic analysis and spatial modal decomposition are then employed to provide physical insight into the observed resonance features. Furthermore, the transient evolution of nonlinear superharmonic resonances induced by sloshing modes is analyzed in terms of phase dynamics, time-varying wavelet energy spectra, and the spatiotemporal development of modal interactions. Radiation damping energy transfer mechanisms associated with piston- and sloshing-mode resonances are elucidated, revealing frequency-dependent reflection and transmission behavior. The role of viscous energy dissipation is investigated through analysis of the vorticity field. The predictive performance of the potential flow model is critically evaluated based on both transient dynamics and quasi-steady hydrodynamic responses. The physical insights obtained in this study contribute to a deeper understanding of nonlinear, transient, multi-frequency, and multi-mode fluid resonance in recessed moonpools, and provide valuable guidance for the application and limitations of potential flow solvers.