Nonlinear Dynamic Feature of the Fluid Resonance in a Recessing-Type Moonpool

Abstract

Abstract Two-dimensional fully nonlinear potential flow numerical analyses on the fluid resonance within moonpool are systemically performed in the time domain with the varied incident wave amplitudes and a wide range of the incident wave frequency covering the natural frequency of the piston mode. The super-harmonic resonances are found around half and one-third of the first sloshing mode natural frequency in the recessing-type moonpool, where the higher-order harmonics rather than first-order harmonic dominate the responses, and the frequencies of higher-order harmonics are almost identical to the first sloshing mode natural frequency. Such a nonlinear phenomenon, however, disappears in the response of fluid oscillation within a traditional clean moonpool. The harmonic analysis illustrates that the introduction of recess leads to the energy transferring to the sloshing mode, thus exciting the sloshing motions. The joint works of the asymmetry due to recess and the free-surface nonlinearity results in the super-harmonic resonances inside the recessing-type moonpool. The evolutions of fluid oscillations in the recessing-type moonpool, being a complex nonlinear dynamic process, are further examined through the phase dynamics by performing Hilbert Transformation on the time series. Phase trapping and phase slipping states are confirmed in the responses of fluid in the recessing-type moonpool, which reflects the nonlinear evolutions of response developing from partial synchronization to de-synchronization.