Effect of free-stream turbulence on the hydrodynamic performance and wake of a 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. Two-dimensional particle image velocimetry (PIV) and direct measurements of the torque applied to the shaft of the turbine were used to obtain phase- and ensemble-averaged flow measurements in the wake of the turbine and to quantify its power extraction performance under varying turbulent intensities on the inflow. The inflow turbulence levels were varied by employing fractal grids upstream of the test section. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques were used to extract coherent structures and dynamic modes in the wake of the turbine. The wake flow characteristics in conjunction with the directly measured torque applied to the shaft of the turbine indicate that an increase in the free-stream turbulence leads to a decrease in turbine performance and provide an insight into the underlying physics. The increase in free-stream turbulence also resulted in the increased turbulent fluctuations in the near-wake of the turbine.