  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 (PTO) 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.
Further, we use fully-resolved CFD simulations to study the performance of a
three degrees of freedom (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. Simulations with different mass densities of the buoy show that
converters with low mass densities have an increased tension in their PTO and
mooring lines. 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.
