Experimental and numerical study of a fixed multi-chamber oscillating water column device (MC-OWC)

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This thesis focuses on preliminary investigating the hydrodynamic performance of a fixed Multi–Chamber OWC (MC–OWC) wave energy converter, which consists of a linear array of four OWC chambers aligned in the same direction of the incident wave propagation. These investigations address the gaps found in previous works by putting forward detailed explanations of the effect of wave height, wave period, device draught and power take–off (PTO) damping on MC–OWC device performance using a combined numerical and experimental approach. The research methodology was based on two series of experimental sessions and two numerical models. The first experimental campaign was conducted in a small wave flume in the University of Technology Sydney (UTS) for a MC–OWC device at a model–scale of 1:25. This experiment was performed mainly to validate the numerical models and initially observe device response when subjected to limited regular wave conditions. The second experimental session was carried out in the wave flume at the Manly Hydraulic Laboratory (MHL) in New South Wales, Australia for a MC–OWC devices at a model–scale of 1:16. This experiment was designed to 1) assess the device performance over a wide range of regular and irregular wave conditions, 2) study the impact of wave height, wave period and device draught on the performance of a MC–OWC device, and 3) investigate the effect of the pneumatic damping induced by the power take–off (PTO) system on device performance. The first validated numerical model was a MATLAB time–domain model that was based on a coupling between the rigid piston model and the thermodynamic forces on a MC–OWC device to get a preliminary understanding of device performance. The second numerical model was a fully nonlinear 3D Computational Fluid Dynamics (CFD) model that was constructed using the commercial code STAR–CCM+. After being validated in good agreement against the physical scale model tests, the CFD model was utilised to study the influence of the power take–off (PTO) damping on the water surface elevation inside the chamber, the differential air pressure, the airflow rate and the device capture width ratio under different incident regular wave conditions. The extensive analysis of 198 physical tests and 84 CFD simulations revealed that the water surface elevation, differential air pressure, and airflow rate had a similar response in all chambers to the wave conditions, device draught and PTO damping. However, the first chamber always played the primary role in wave energy extraction, and the performance gradually decreased down to the fourth chamber where the lowest performance was found. The maximum capture width ratio of the whole MC–OWC device was found to be 2.1 under regular wave conditions and 0.95 under irregular wave conditions. These ratios were the highest among all similar concepts that have been reported in previous research.
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