Comparison of wave-structure interaction dynamics of a submerged cylindrical point absorber with three degrees of freedom using potential flow and CFD models.

In this paper we compare the heave, surge, and pitch dynamics of a submerged cylindrical point absorber, simulated using potential flow and fully-resolved computational fluid dynamics (CFD) models. The potential flow model is based on the time-domain Cummins equation, whereas the CFD model uses the fictitious domain Brinkman penalization (FD/BP) technique. The submerged cylinder is tethered to the seabed using a power take off unit which restrains the heave, surge, and pitch motions of the converter, and absorbs energy from all three modes. It is demonstrated that the potential theory over-predicts the amplitudes of heave and surge motions, whereas it results in an insignificant pitch for a fully-submerged axisymmetric converter. It also under-estimates the slow drift of the buoy, which the CFD model is able to capture reliably. We also present experimental wave-structure interaction (WSI) data of a submerged axisymmetric sphere, which further corroborates the finding that the CFD model is closer to reality than the potential flow model. Further, we use fully-resolved CFD simulations to study the performance of a 3-DOF cylindrical buoy under varying PTO coefficients, mass density of the buoy, and incoming wave heights. It is demonstrated that the PTO coefficients predicted by the linear potential theory are sub-optimal for waves of moderate and high steepness. The wave absorption efficiency improves significantly when higher than the predicted value of the PTO damping is selected. Moreover, the mass density also influences the range of resonance periods of the device. Finally, simulations with different wave heights show that at higher heights, the wave absorption efficiency of the converter decreases and a large portion of available wave power remains unabsorbed.