An investigation of the wake recovery of two model horizontal-axis tidal stream turbines measured in a laboratory flume with Particle Image Velocimetry

The uptake of tidal stream-turbine (TST) technology lags other renewable energy sources despite the advantages of predictability, stability and increased power output in comparison to wind turbines of the same dimensions. There remains a need to address environmental concerns about the potential impacts of TSTs including the suspension and deposition of bed sediments if TSTs are to be more widely accepted and deployed. Sediment mobilisation and persistent flow vortices will also adversely affect the performance of other TST devices in an array downstream of the wake. The focus of this work is to improve our understanding of the wake recovery structure of a TST to build on the limited field and laboratory data currently available in order better predict the impact of TSTs on flow and sediment transport. Detailed measurements of the wake flow structures generated by scaled TST devices are presented. These results are the first to be derived from the application of high spatial resolution stereoscopic Particle Image Velocimetry (PIV). Two scale model horizontal-axis TSTs were manufactured and deployed in a laboratory flume (11m long, 1.6m wide and 0.6m deep) at different flow speeds and heights above the bed. The results demonstrate greater wake recovery lengths for the rotor design with wider blade tips, despite the higher wake turbulence generated by the blades. Wake recovery is more rapid at the higher flow speed when greater turbulence from the tips is observed, but wake recovery lengths increase when both rotors are positioned closer to the bed.