Energy concentration-driven hydrodynamic efficiency advantages of three-dimensional over two-dimensional oscillating water columns

Based on the three-dimensional (3D) time-domain higher-order boundary element method, this study systematically investigates the energy concentration characteristics from the perspective of the free-surface scattering flow field in front of an oscillating water column (OWC) device. The aim is to elucidate the physical mechanism underlying the pronounced enhancement of hydrodynamic efficiency under 3D conditions. To move beyond the largely phenomenological descriptions in previous studies, a normalized kinetic energy density (NKED) index is proposed to quantify the level of energy concentration within the streamline convergence region upstream of a single structure. Furthermore, a normalized kinetic energy density difference (NKEDD) index is defined as the NKED in front of a solid cuboidal block of identical size to an OWC minus the NKED in front of the OWC, thus representing the level of energy absorption by the OWC. The results reveal significant streamline convergence and energy concentration in front of the OWC under 3D conditions, with the NKEDD exhibiting a strong correlation with variations in hydrodynamic efficiency. These findings demonstrate that local wave energy redistribution under 3D wave action concentrates additional energy in front of the device, improving its absorption and influencing the hydrodynamic efficiency of the OWCs to exceed the 2D predictions. The proposed index framework provides a quantitative and physically consistent tool for assessing 3D hydrodynamic enhancement mechanisms in OWC structures and to guide the optimization of the OWC chambers under a broader range of operating conditions.