Results obtained from numerical modelling of wave and tidal currents and the resulting turbulence parameters at tidal energy sites in the Fall of Warness, which is a region consented by the Crown Estate for deployment of tidal stream devices in the Orkney Islands, Scotland, are reported in this paper. The software suite MIKE 21/3, which is a coupled wave-tidal flow model, has been used for this purpose. The coupled wave-current model is driven by boundary inputs of spatially and temporally varying wind, wave and tidal elevations. Turbulence closure is achieved using a two-equation (k-ε) turbulence model. The coupled model has been calibrated and validated with field measurements of waves and tidal currents acquired by Acoustic Doppler and Current Profilers (ADCPs) deployed in the Fall of Warness. The results indicate that the coupled model works well and the predicted wave-current parameters provide very good match to site measurements at different depths of the water column. The model outputs such as significant wave height, peak wave period, mean wave direction, tidal current speed and its direction, Turbulent Kinetic Energy (TKE) and its dissipation rate and Eddy viscosities are presented and discussed. These parameters will find its use in the design of tidal turbine components and its supporting structures.
Hydrodynamic loads on both fixed and floating tidal stream turbine components, e.g., rotor, supporting structures and moorings etc, need to be carefully determined when the machines are designed to operate in conditions where both waves and tidal currents co-exists. Unsteady flow due to turbulence, wave-current interactions, and variation of flow characteristics with depth can cause unsteady blade loading, resulting in fatigue. High-end computational modelling tools such as CFD software packages may have the ability to simulate wave-current interactions, however, its application to realistic site conditions with complex bathymetry and large computational domains covering several square kilometres, may not be feasible due to computational expenses and it may not even fully reproduce wave-current scenarios, especially when the interaction of directional waves, currents, and turbulences are to be modelled. It is well established that wave loads combined with different flow turbulent intensities, originating from wave-current-turbulent interactions, are the main contributors to fatigue failures of turbine blades. Wave-current induced turbulence affects tidal turbine power production in several ways, specifically through power performance effects, impacts on turbine loads, fatigue and wake effects, and noise propagation; it is therefore important to develop an enhanced understanding of wave-current-turbulence interactions in tidal energy research.
Perforated free surface semicircular breakwater is a good energy dissipater, but its wave suppression ability is relatively low, particularly when subjected to lower breakwater immersion depths and waves of longer period. To overcome the limitations of the breakwater, addition of wave screens of different configurations and porosities below the breakwater caisson is proposed in this study. Experiments have been conducted to investigate the hydraulic performance of the test models and to identify the optimum design for the breakwater. The experimental results show that the breakwater with double screen of 25% porosity offers a reasonably good hydraulic efficiency when immersed in limited water depths.