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 four-bladed H-Darrieus tidal turbine was investigated through a series of experiments conducted in a water tunnel at a diameter-based Reynolds number (Re_D) of 0.5 x 10⁶ and a tip-speed ratio (TSR) of 2.65. Particle image velocimetry (PIV) and continuous measurements of the torque applied to the shaft of the turbine were performed in conjunction with modal decomposition methods to obtain phase- and ensemble-averaged quantitative flow patterns in the wake of the turbine and to quantify its power extraction performance. The inflow turbulence levels were varied between < 1%, 5%, and 10% by installing fractal grids upstream of the test section. The wake flow characteristics and the direct torque measurements indicate that an increase in the free-stream turbulence intensity leads to a higher frequency of non-periodic vortex shedding, which in turn results in an earlier collapse of the outer shear layer in the near-wake of the turbine. These results offer two practical implications in the context of tidal turbines. The improved wake recovery distance in tidal sites with higher turbulence levels allows for closer turbine spacing, and the added non-periodic loading on the turbine blades may require a more robust turbine design in tidal environments with increased turbulence levels.