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In this paper we compare the heave, surge, and pitch dynamics of a submergedcylindrical point absorber, simulated using potential flow and fully-resolvedcomputational fluid dynamics (CFD) models. The potential flow model is based onthe time-domain Cummins equation, whereas the CFD model uses the fictitiousdomain Brinkman penalization (FD/BP) technique. The submerged cylinder istethered to the seabed using a power take-off (PTO) unit which restrains theheave, surge, and pitch motions of the converter, and absorbs energy from allthree modes. It is demonstrated that the potential theory over-predicts theamplitudes of heave and surge motions, whereas it results in an insignificantpitch for a fully-submerged axisymmetric converter. It also under-estimates theslow drift of the buoy, which the CFD model is able to capture reliably.Further, we use fully-resolved CFD simulations to study the performance of athree degrees of freedom (DOF) cylindrical buoy under varying PTO coefficients,mass density of the buoy, and incoming wave heights. It is demonstrated thatthe PTO coefficients predicted by the linear potential theory are sub-optimalfor waves of moderate and high steepness. The wave absorption efficiencyimproves significantly when higher than the predicted value of the PTO dampingis selected. Simulations with different mass densities of the buoy show thatconverters with low mass densities have an increased tension in their PTO andmooring lines. Moreover, the mass density also influences the range ofresonance periods of the device. Finally, simulations with different waveheights show that at higher heights, the wave absorption efficiency of theconverter decreases and a large portion of available wave power remainsunabsorbed.

physics of fluids

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