Nonlinearities of the gap resonance for a free-heaving moonpool: Higher-order harmonics and natural frequency component

Abstract

The piston-mode gap resonance of a free-heaving moonpool was investigated in a two-dimensional numerical wave tank based on a Constrained Interpolation Profile (CIP) method. The wave elevation at the narrow gap, the moonpool's heave response, and wave loads on the free-heaving moonpool were all specifically examined in the current study, which was primarily focused on the higher-order harmonics of the gap resonance for the free-heaving moonpool against various gap widths and body drafts. The flow pattern around the entrance of the gap of the free-heaving moonpool was identified at the typical resonant frequencies. The frequency range of the beat pattern for the wave elevation at the gap was initially suggested. Furthermore, the natural frequency component (NFC) was observed from the interesting beat patterns, and the NFC-based approach was proposed as a straightforward way to forecast the resonant frequency of the free-heaving moonpool. Numerical studies showed that the quadratic wave excitation (QWE) driven gap resonances were recorded for both the fixed and the free-heaving moonpools versus regular waves. The resonance frequency of the free-heaving moonpool can be directly, precisely, and rapidly estimated using the suggested NFC-based approach. Finally, four frequency zones were proposed to differentiate the various gap resonances for the free-heaving moonpool based on distinct hydrodynamic characteristics of the wave elevation at gap.