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The ringing of a floating barge's roll motion under focused wave groups is studied using a fully nonlinear method in a numerical wave tank based on the potential flow theory. The higher order boundary element method (HOBEM) is used to solve the boundary value problem. The mixed Eulerian–Lagrangian (MEL) technique and the fourth-order Runge–Kutta time stepping scheme is used to simulate the interaction between the floating barge and focused wave groups. As the peak spectra frequency is set as one-third of the natural frequency of the barge's roll motion, the influence of the peak wave amplitude of the incident focused wave groups on the roll response of the barge and wave loads is studied. The third-order components are found to be significant in both roll motion and moment as the incident peak wave amplitude increases. Relationship between the roll response of the barge and the peak spectrum frequency of the incident focused wave groups is also studied. The peak of the third-order roll response of the barge is found to decrease greatly as the triple peak spectral frequency deviates from the roll natural frequency. [ABSTRACT FROM AUTHOR]

A two-dimensional numerical Computational Fluid Dynamics (CFD) model is established on the basis of viscous CFD theory to investigate the motion response and power absorption performance of a bottom-hinged flap-type wave energy converter (WEC) under regular wave conditions. The convergence study of mesh size and time step is performed to ensure that wave height and motion response are sufficiently accurate. Wave height results reveal that the attenuation of wave height along the wave tank is less than 5% only if the suitable mesh size and time step are selected. The model proposed in this work is verified against published experimental and numerical models. The effects of mechanical damping, wave height, wave frequency, and water depth on the motion response, power generation, and energy conversion efficiency of the flap-type WEC are investigated. The selection of the appropriate mechanical damping of the WEC is crucial for the optimal extraction of wave power. The optimal mechanical damping can be readily predicted by using potential flow theory. It can then be verified by applying CFD numerical results. In addition, the motion response and the energy conversion efficiency of the WEC decrease as the incident wave height increases because the strengthened nonlinear effect of waves intensifies energy loss. Moreover, the energy conversion efficiency of the WEC decreases with increasing water depth and remains constant as the water depth reaches a critical value. Therefore, the selection of the optimal parameters during the design process is necessary to ensure that the WEC exhibits the maximum energy conversion efficiency. [ABSTRACT FROM AUTHOR]

In this study, we investigated the hydrodynamic and energy conversion performance of a double-float wave energy converter (WEC) based on the linear theory of water waves. The generator power take-off (PTO) system is modeled as a combination of a linear viscous damping and a linear spring. Using the frequency domain method, the optimal damping coefficient of the generator PTO system is derived to achieve the optimal conversion efficiency (capture width ratio). Based on the potential flow theory and the higher-order boundary element method (HOBEM), we constructed a three-dimensional model of double-float WEC to study its hydrodynamic performance and response in the time domain. Only the heave motion of the two-body system is considered and a virtual function is introduced to decouple the motions of the floats. The energy conversion character of the double-float WEC is also evaluated. The investigation is carried out over a wide range of incident wave frequency. By analyzing the effects of the incident wave frequency, we derive the PTO's damping coefficient for the double-float WEC's capture width ratio and the relationships between the capture width ratio and the natural frequencies of the lower and upper floats. In addition, it is capable to modify the natural frequencies of the two floats by changing the stiffness coefficients of the PTO and mooring systems. We found that the natural frequencies of the device can directly influence the peak frequency of the capture width, which may provide an important reference for the design of WECs. [ABSTRACT FROM AUTHOR]

The integration of wave energy converters (WECs) with floating breakwaters has become common recently due to the benefits of both cost-sharing and providing offshore power supply. In this study, based on viscous computational fluid dynamics (CFD) theory, we investigated the hydrodynamic performances of the floating box and Berkeley Wedge breakwaters, both of which can also serve as WECs. A numerical wave flume model is constructed using Star-CCM+ software and applied to investigate the interaction between waves and wave energy converters while completing the verification of the convergence study of time and space steps. The effects of wave length on motion response and transmission coefficient of the floating box breakwater model are studied. Comparisons of our numerical results and published experimental data indicate that Star-CCM+ is very capable of accurately modeling the nonlinear wave interaction of floating structures, while the analytical potential theory overrates the results especially around the resonant frequency. Optimal damping can be readily predicted using potential flow theory and can then be verified by CFD numerical results. Next, we investigated the relationship between wave frequencies and various coefficients using the CFD model under optimal damping, including the motion response, transmission coefficient, reflection coefficient, dissipation coefficient, and wave energy conversion efficiency. We then compared the power generation efficiencies and wave dissipation performances of the floating box and Berkeley Wedge breakwaters. The results show that the power generation efficiency of the Berkeley Wedge breakwater is always much higher than that of the floating box breakwater. Besides, the wave dissipation performance of the Berkeley Wedge breakwater is much better than that of the floating box breakwater at lower frequency. [ABSTRACT FROM AUTHOR]