Effect of free-stream turbulence on the hydrodynamic performance and wake structure of an H-Darrieus tidal turbine

The effect of free-stream turbulence on a 1/10^th scale H-Darrieus tidal turbine was investigated through a series of experiments conducted in a water tunnel at a diameter-based Reynolds number (Re_D) = 0.5 x 10⁶. The inflow turbulence level was varied between < 1%, 5%, and 10% by the installation of fractal grids upstream of the test section. The performance of the turbine was characterized across a range of tip-speed ratios (TSR) from 1.0-to-3.4 using continuous measurements of the torque applied to the shaft of the turbine’s rotor. Particle image velocimetry (PIV) was employed to obtain ensemble- and phase-averaged velocity measurements in the wake at the optimal TSR of 2.65 for each turbulence level. Aside from increasing torque transients, the phase-resolved performance data reveals that an increase in free-stream turbulence delays the onset of stall and accelerates the reattachment of the boundary layer, effectively impacting the azimuth angle at which the turbine interacts with the free-stream. The performance data further reveals that turbulence intensity alone is not sufficient in quantifying the power extraction performance of the turbine, while the integral length scale aids in classifying the torque transients and the net power coefficient. The obtained wake flow fields at the optimal TSR show that increased free-stream turbulence reduces the span-wise velocity deficit on the upstream side of the turbine. Additionally, increasing free-stream turbulence decreases the total stream-wise momentum transfer and increases the cross-stream advection. These results offer practical implications in the context of tidal turbine design and implementation. The added non-periodic loading on the turbine blades may require a more robust turbine design depending on the free-stream turbulence characteristics of the tidal environment. Furthermore, the change in momentum transfer rate in the wake of the turbine will require extra consideration for optimal tidal array spacing.