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The main aim of this paper is to analyze the effects of the heave motion of an elastically supported floating oscillating water column device (OWC) on wave energy harvesting efficiency through two-dimensional computational fluid dynamics simulations. After the numerical model is validated using experimental data, it is used to analyze the effect of the natural frequency ratio on the hydrodynamic efficiency of the OWC. The natural frequency ratio is the ratio of the natural frequency to the wave frequency. The numerical results prove that the natural frequency ratio must be greater than 3 for achieving the best hydrodynamic efficiency. The best hydrodynamic efficiency decreases with the decrease in the natural frequency ratio when the latter is 3. If the natural frequency ratio is smaller than 3, both the amplitudes of wave surface elevation and the displacement of the OWC chamber increase but the harvested energy reduces because the relative motion of the wave surface and the OWC device is small. The energy dissipation due to vortices does not have a strong contribution to the reduction of energy at small natural frequencies. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to analyse the hydrodynamic efficiency of offshore stationary OWC devices with different configurations through two-dimensional computational fluid dynamics simulations. The two-dimensional incompressible Reynolds-Averaged Navier-Stokes equations and the Shear Stress Transport (SST) k - ω turbulence model are solved for the simulation of water moetion under regular waves. The airflow through the turbine is calculated using a compressible aerodynamic model. After the numerical model is validated using the experimental data, it is used to analyse the effect of the submerged draught of rear vertical wall of the OWC on the hydrodynamic efficiency, and the improvement of hydrodynamic efficiency by a horizontal bottom plate attached to the back wall. The numerical results prove that increasing the rear wall draught improves the hydrodynamic efficiency if the front wall draught is 0.25 times water depth. If the draughts of the front and rear walls are the same, increasing them simultaneously causes a reduction in the hydrodynamic efficiency. Attaching a horizontal bottom plate on the deeper rear vertical wall improves efficiency. The best efficiency increases with the increase of the length of the bottom plate until the plate length is twice the chamber length. Further increase in the bottom plate length causes a reduction in the best efficiency. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) devices utilise wave-induced oscillation of water column in partially submerged chambers to extract energy of waves in ocean. The principal aim of this paper is to investigate scaling effects of two-dimensional OWC through numerical simulations. The fluid motion under waves is simulated by solving the two-dimensional incompressible Reynolds-Averaged Navier-Stokes equations using finite element method under Arbitrary Lagrangian-Eulerian scheme. To investigate scaling effects, the waves and the OWC are scaled up or down according to Froude number similarity. The turbine is scaled up or down using three different scaling methods: (1) Turbine coefficient remains unchanged, i.e. not scaling the turbine at all; (2) Equivalent pneumatic damping coefficient of the turbine is constant and (3) the two-dimensional turbine coefficient is constant. The numerical results show that method (2) has the smallest effects on the best hydraulic efficiency and method (3) has the smallest effects on the best performance wave number, i.e. the wave number where the OWC receives the largest percentage of the wave energy. The worst scaling method is not scaling the turbine. Because the best performance wave number varies with the model scale in method (1) and (2), it is very likely the scaling effects of the model scale on the hydrodynamic performance at different wave numbers are different from each other. In addition, the effects of the wave height and the chamber volume are studied. Increasing chamber volume causes a reduction of the best performance, however it may improve general performance of OWC because it increases the frequency band of waves with high hydraulic efficiency. Increasing wave height causes a reduction of the hydraulic efficiency. [ABSTRACT FROM AUTHOR]

This paper aims to study the improvement in hydrodynamic efficiency of an OWC device by dividing a single chamber into multiple chambers. The two-dimensional incompressible Reynolds-Averaged Navier-Stokes equations are solved to simulate the water motion due to waves and a compressible aerodynamic model is used to calculate the airflow through the turbine. Simulation of a dual-chamber-two-turbine (2C2T) case was first conducted using a range of turbine coefficients to identify the best turbine coefficient to be used for the rest of the study. The dual-chamber-dual-turbine configuration was found to increase the efficiency more than the case of two chambers sharing a single turbine (2C1T). The triple-chamber-triple-turbine (3C3T) configuration further increases the efficiency when compared with the 2C2T configuration. Despite achieving greater changes in free surface elevation, when compared with multi-turbine multi-chamber configurations, shared turbine configurations achieve lower overall efficiency due to phase differences in surface elevation between chambers. • Dividing single chamber into multiple chambers with total chamber length unchanged improves the hydraulic efficiency. • Using multiple turbines for multiple chambers further improve the hydraulic efficiency compared with using single turbine. • Sharing a single turbine has larger amplitudes of oscillating water surface but smaller efficiency than multiple-turbines. [ABSTRACT FROM AUTHOR]

Improvements to the hydrodynamic efficiency of oscillating water columns (OWC) have the potential to influence commercial OWC adoption. The main goal of this study is to determine whether dual chamber OWCs with either a shared turbine or dual turbines can improve the hydrodynamic efficiency of the OWC through two-dimensional numerical simulations using computational fluid dynamics. In the numerical simulations, water depth is 0.8 m and the total length of the two chambers is 0.6 m. Numerical results show that when the non-dimensional turbine coefficient is 0.192, 1.432, and 3.070, a dual chamber dual turbine configuration can achieve between 7.6% to 26.8% greater hydrodynamic efficiency than both the single chamber single turbine and the dual chamber shared turbine configuration. Dual chamber, dual turbine configurations can increase hydrodynamic efficiency by operating over a broader range of wavelengths due to multiple resonant frequencies and are not affected by phase differences at shorter wavelengths. • OWC with dual chambers are investigated. • Dual turbines improve the efficiency better than shared turbine configuration. • The ratio of the two chamber lengths influences the resonant frequencies. • A front to rear chamber length ratio of 1:2 perform the best for dual turbine case. • Dual chamber dual turbine case exhibit greater synchronicity of the two chambers. [ABSTRACT FROM AUTHOR]