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Title: The application of low aspect ratio hydrofoils to the secure positioning of static equipment in tidal streams.
Authors: Owen, Alan
Supervisors: Bryden, Ian
Forehand, David
Issue Date: May-2007
Publisher: The Robert Gordon University
Abstract: The costs of installing tidal energy technology are high, requiring expensive vessels to drill sockets in the sea bed or to handle gravity based structures of substantial mass, and this impacts on the commercial viability of any proposed marine renewables development. This thesis offers a viable alternative to socketed or gravity based installations by proposing that the downwards lift force that can be developed from the flow over a hydrofoil can be used to resist the slip and overturning moments applied to a structure by the flow. The fundamental theory of axial and crossflow energy conversion devices is outlined and the current methods of fixing and supporting tidal stream devices are analysed. The origins of tidal stream flows are discussed and the effects of local topography, bathymetry and system resonance are used to explain the significant differences between real tidal behaviour and the ideal of Newton’s equilibrium theory. The idiosyncratic and localised nature of tidal streams is thereby made clear as well as the need for a solid understanding of the resource prior to device design and installation. The principles of classical hydrodynamics and conformal mapping are used in the context of relating theoretical lift and drag functions to low aspect ratio hydrofoils with endplates, and a numerical model of distributed surface pressures around a hydrofoil is demonstrated. Subsequently, the concept is evaluated using two 1/7th scale test devices, one is field tested in a large stream under real flow conditions, and the second in a tow tank under ideal laboratory conditions. The limitations and challenges of model scaling are shown and the semi-empirical Froude method of scaling using residual forces is applied to the towing model. Analysis of the experimental data shows a correlation with normal distribution and extrapolation of the experimental results shows that the Sea Snail can operate with an average lift coefficient of 0.7 and drag coefficient of 0.18. Application of the experimental data to the full scale device demonstrates that the Sea Snail principle is not only valid, but is a significant advance on existing installation methodologies.
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