A U-shaped oscillating water column (U-OWC) device has been investigated to enhance power extraction by placing the bottom-mounted vertical barrier in front of a conventional OWC. Then, the optimal design of a U-OWC device has been attempted by using an artificial neural network (ANN) model. First, the analytical model is developed by a matched eigenfunction expansion method (MEEM) based on linear potential theory. Using the developed analytical model, the input and output features for training an ANN model are identified, and then the database containing input and output features is established by a Latin hypercube sampling (LHS) method. With 200 samples, an ANN model is trained with the training data (70%) and validated with the remaining test data (30%). The predictions on output features are made for 4000 random combinations of input features for given significant wave heights and energy periods in irregular waves. From these predictions, the optimal geometric values of a U-OWC are determined by considering both the conversion efficiency and wave force on the barrier. It is found that a well-trained ANN model shows good prediction accuracy and provides the optimal geometric values of a U-OWC suitable for wave conditions at the installation site.

The effect of hydrodynamic performance of the wave energy converter (WEC)–rotor based on linear potential flow theory due to nonlinear viscous damping was investigated. Free decay tests were conducted using computational fluid dynamics (CFD) to obtain the viscous damping moment. The commonly used procedure for obtaining the damping moment is based on peak amplitudes which normally require a long time history records. Such long free decay records may not be possible in nodding WEC rotor due high damping. The energy method proposed by Bass and Haddara requires only the short and full range of the recorded data. This method provides sufficiently good results when the bodies have high damping. The method equates the rate of change of the total energy of a body undergoing free rolling/pitching to the rate of energy dissipated by the damping. The present study adopts a similar methodology for estimating the linear and linear plus quadratic damping. To incorporate the nonlinear viscous damping moment in the linear equation of motion, an equivalent linearization concept is used without neglecting the nonlinear damping effects. The hydrodynamic coefficients obtained from the linear potential flow theory, nonlinear viscous damping moment from the energy method and estimated PTO damping are used to solve the equation of motion of the WEC rotor. The estimated pitch free decay data shows good agreement with the simulated CFD results when compared to the linear viscous damping moment and better agreement is obtained with linear plus quadratic viscous damping moment. The regular and irregular wave analyses show that a considerable effect on the hydrodynamic performance of the WEC rotor is observed when the linear and linear plus quadratic viscous damping are included.

In this study, the hydrodynamic performance of a partially submerged vertical slotted barrier was investigated on the basis of an analytical and numerical approach in regular waves. The matched eigenfunction expansion method (MEEM) was used to solve the boundary value problem including the porous boundary condition. Herein the drag coefficient representing the energy dissipation was estimated by the best possible fitting curve to CFD results for a 2D simplified channel model. A 3D implicit unsteady turbulent model based on incompressible Reynolds-averaged Navier–Stokes (RANS) equations was adopted to assess the analytical MEEM solutions. The analytical and numerical solutions show relatively good agreement with the experimental results. The presented analytical and numerical model provides a complementary approach to estimate the hydrodynamic performance and horizontal forces of the vertical slotted barrier effectively and practically.

In this study, the wave energy dissipation over a horizontal slotted plate with a leeside vertical seawall was extensively investigated by analytical and numerical approaches. The analytical model was developed by using the matched eigenfunction expansion method and the energy dissipation model with a porosity-dependent drag coefficient estimated by computational fluid dynamics (CFD) results from the 2D channel flow with symmetric slats. The energy dissipation of a horizontal slotted plate was assessed by analyzing the variation of the reflection coefficient, wave run-up, vertical wave force on the plate, and horizontal wave force on the vertical wall for different submergence depths, porosities, plate lengths, and wave steepnesses. Results from the analytical model were compared extensively with those from CFD simulations. CFD computations were performed by using an incompressible turbulence model based on Reynolds averaged Navier–Stokes equations and volume of fluid method. Overall, a good agreement was observed between the analytical and CFD results. A qualitative numerical analysis showed that the wave dissipation over a horizontal slotted plate was caused by the generation of higher-harmonic waves, wave breaking, and vortices formation near the slotted plate. The presented analytical and numerical models would provide a mutually complementary tool for the accurate and efficient design of a horizontal slotted plate for reducing the reflection coefficients, wave forces, and wave run-up on seawalls.