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A vented Oscillating-Water-Column (OWC) simplifies the pneumatic energy conversion problem by rectifying air flow and enabling a unidirectional-air-turbine to be employed. It shifts power-extraction to the inhalation phase of the cycle and provides equivalent pneumatic power to a full-wave-cycle. Conventional Radial-air-turbines feature a low global-efficiency in OWC applications, however they offer simpler designs and lower thrust loads. The aim of this study is to modify the design of a centripetal-radial-turbine for optimum efficiency in steady-state using CFD methods for application with the pressure/flow profile experienced by the vented-OWC. Nine design variables were used to control the shape of the rotor and its adjustment to the inward-flow direction. The optimized rotor was found to achieve significant efficiency and output power by using asymmetric and non-zero-staggered blades. The downstream section was optimized for an efficient matching with the optimized-inflow-rotor and four parameters were used to control the shape of the downstream section. A diffuser with a 7-degree diffusion-angle was found to be the optimal connection between the turbine and the chamber. The inflow radial turbine obtained 81% peak efficiency in the steady-state, and its average efficiency over the expected flow coefficients is comparable to the axial-turbines used with OWCs. • Optimization studies were used to optimize the rotor of a radial inflow turbine. • Angles of the blades and guide vanes were found to have the highest sensitivity. • The losses in all sections of the optimized inflow turbine are highly reduced. • The optimized inflow turbine was found to obtain 81% peak efficiency in steady-state. [ABSTRACT FROM AUTHOR]

This study analyses the aerodynamic performance of two unidirectional-radial-air-turbine configurations; inflow and outflow. These turbines were studied as the Power-Take-Off unit for application on a vented-OWC and a conventional-bidirectional-OWC with a twin-turbine topology, forming four different turbine-OWC configurations. These configurations were evaluated in terms of full-scale power extraction using extrapolated hydrodynamic experimental data of irregular waves for a King Island test site. The power extraction capacity was evaluated by defining a lower and upper bound of power generation under fixed and controlled-RPM schemes and the energy produced in each configuration was then compared against a state-of-the-art twin-rotor turbine. It was found that the difference between these power extraction bounds was lower in case of the outflow turbine, which shows this turbine is less sensitive to RPM variations than the inflow turbine. In addition, due to its lower resistance to the flow in direct mode, the outflow turbine has a smaller full-scale size than the inflow turbine. It was concluded that the outflow turbine provides better efficiency in a twin-turbine-OWC system, while the inflow turbine yields higher conversion efficiency in a vented-OWC system. The vented OWC equipped with a radial inflow turbine can obtain comparable power to the bidirectional OWC system. • Unidirectional radial turbines are investigated for inflow and outflow directions. • The outflow radial turbine is less affected by backflow in a bidirectional OWC. • Compared to the outflow design, inflow turbine has lower RPM and larger size. • The outflow turbine is less sensitive to RPM variations than the inflow turbine. • The inflow turbine in vented-OWC obtains comparable power with a bidirectional OWC. [ABSTRACT FROM AUTHOR]

Abstract The oscillating-water-column (OWC) is a ubiquitous style of wave-energy-converter for harnessing significant power from ocean waves. A new configuration of OWC described here as a vented-OWC provides opportunity to simplify air-turbine design by limiting the power extraction to inhalation mode. The asymmetric airflow profile of the vented-OWC requires a unidirectional-turbine configuration to be adopted. The focus of this study is to analyse a unidirectional radial air-inflow-turbine design using Computational-Fluid-Dynamics (CFD) suitable for application with a vented-OWC. It is found that downstream of the rotor can cause significant energy losses due to its narrow flow passage. Two configurations of an inward-flow radial turbine were also compared, the first by keeping the casing height constant throughout the turbine domain and the second by increasing the casing height in a way to keep a constant sectional area from inlet to the outlet of the turbine domain. The latter configuration obtained a ten percent gain in peak efficiency compared to the first. The difference is identified as fewer energy losses at the downstream section and comparably higher torque for the same flow-coefficient. Introduction of downstream-guide-vanes was found to retard the chocking phenomenon, which offers a wider operational range for the radial-inflow design. Highlights • A radial inflow turbine for a vented Oscillating-Water-Column is investigated. • Width of the flow-passage at the rotor downstream causes significant energy losses. • Downstream guide vanes can extend the operational range of a radial inflow turbine. • Use of downstream vanes leads to reduced swirl velocity and proper flow guidance. • A constant sectional area can improve the efficiency of a radial inflow turbine. [ABSTRACT FROM AUTHOR]

Abstract Prediction of a ship's roll motion in parametric roll and dead ship condition using CFD and experiments is time consuming and expensive, while a reliable equation based method can drastically decrease the computation time. The commonly used roll motion equation contains roll mass and added mass moment of inertia, damping and restoring terms and associated coefficients. Improving the prediction of these coefficients increases the accuracy of the method. There is a lack of precise calculation for the roll added mass moment of inertia, which is commonly estimated using semi-empirical techniques and as a percentage of the roll mass moment of inertia coefficient. This paper presents a new method to determine the roll added mass moment of inertia using CFD simulations based on a harmonic excitation roll motion technique and the results are compared against Bhattacharyya's method. The influences of excitation frequency, Froude number, roll angle and appendages on the roll motion characteristics and the roll added mass moment of inertia coefficient are investigated. In addition, the roll added mass moment of inertia coefficients on the model scale and full scale ship are computed to investigate the scale effect. [ABSTRACT FROM AUTHOR]

Simulation of a ship's roll motion through equation-based approaches requires restoring moment, added mass moment of inertia and damping coefficients. Accurate estimation of these coefficients enhances the prediction of ship roll response at a resonance condition. However, the magnitude of these coefficients in regular beam seas and close to a resonance condition is unknown. The present study adopts numerical and experimental simulations to investigate the effects of different wave heights and wave frequencies on ship motion characteristics. The motion characteristics results from numerical simulations are compared against experimental measurements, and a good correlation is achieved. The restoring moment in dynamic conditions is calculated and compared to the hydrostatic restoring moment. The dynamic restoring moment is greater than the hydrostatic restoring moment for some wave frequencies and smaller for other wave frequencies. The magnitudes of roll damping and added mass moment of inertia coefficients are calculated at different wave heights and frequencies, and the results yield that there is quite a nonlinear relationship between the roll hydrodynamic coefficients and roll motion characteristics. • Numerical and experimental investigations of ship motions in regular beam waves. • Roll restoring moment in dynamic and static conditions are compared. • Roll added mass moment of inertia is computed in different regular beam waves. • Roll damping is computed in different regular beam waves. [ABSTRACT FROM AUTHOR]