Flow direction effects on tidal stream turbines

This thesis investigated the non-dimensional performance characteristics of a tidal stream turbine and how they varied in response to changes in flow direction. The problem was considered from an industrial perspective and used the commercial software package ANSYS CFX and a 1:20th scale experimental turbine. Initial considerations analysed the performance of the turbine in an ‘upstream’ or ‘downstream’ configuration relative to the turbines support structure. The conclusions resulting from this were that up to a point by increasing separation between an upstream turbine and its support structure the greater average nondimensional performance characteristics became. Also, more significantly, it was identified that this orientation and clearance reduced the blade stanchion interaction considerably relative to the downstream orientation. The study made justification for the inclusion of a yaw mechanism to rotate the turbine to face the flow for flood and ebb phases of the tide. In an operational environment this would be expected to enhance the life of the turbine’s blades, thrust bearings, and gearbox - which are known to be prone to fatigue failure, due to highly dynamic loads. The thesis continued to expand into the potential uses of a yaw mechanism to address flow misalignment experienced throughout the tidal cycle. In order to justify this, the non-dimensional performance characteristics of the same turbine were compared for a series of flow misalignment cases. The CFD analysis showed that increased flow misalignment in either the positive and negative direction had the effect of reducing turbine torque and power performance characteristics, and also significantly increases the out-ofplane bending moments. A distinction between the positive yaw angles and negative yaw angles was identified in the turbine’s performance. The negative flow misalignment showed more favourable performance changes than the positive flow misalignment, this was due to the turbines rotational direction. The subsequent recommendations to industry were included making use of the turbines rotational direction and yaw mechanism, to experience lower performance reductions in the case of flow misalignment. Experimental results from tow tank testing at CNR-INSEAN using the 0.5 m diameter turbine validated the nondimensional performance characteristics of the CFD results. It was identified that steady state CFD results did not capture the performance characteristics of flow misalignment cases as well as the transient CFD results. The experimental turbine captured temporal features identified in the CFD analysis. Recommendations to industry include the careful consideration of steady state CFD analysis in non-idealised flow conditions.