Effects of Free-stream Turbulence on Tidal Turbine Blade Performance and Wake

Abstract Tidal turbines operate inflow conditions with elevated levels of free-stream turbulence (FST) that affect the loads acting on components, which in turn affects the performance. The current work focuses on the use of controlled laboratory experiments to investigate the effects of free-stream turbulence on an SG-6043 turbine blade section. Elevated levels of FST are generated using an active grid generator at turbulence intensities (Ti) varying between 1.5-18%. It was observed that elevated levels of FST increased the coefficient of lift and caused a subsequent delay in flow separation. In addition, the coefficient of drag also increases at high angles of attack in elevated levels of FST, leading to a reduction in hydrodynamic performance. The measured standard deviations indicate that elevated FST leads to considerable fluctuations in measured forces, which in turn will accelerate fatigue damage to the blade. We report our findings of experiments conducted with the hydrofoil over a broader range angles of attack at various turbulence intensities. Acoustic Doppler Velocimetry measurements are made at several downstream locations to provide insight into the flow mechanism that causes a delay in separation observed at higher values of Ti (3.74-9.20%). Characteristics of the wake downstream of the blade are also discussed. A Blade Element Momentum analysis was performed to evaluate the performance of a model marine current turbine at different FST levels. It was observed that elevated levels of FST led to higher values of Cp at lower TSR with a shift in TSR corresponding to maximum Cp value, a result of direct contradiction with reported experiments. The results are discussed and provide the capability for a BEM method to capture the effect of free-stream turbulence accurately.