Experimental Study Of Turbulence Influence On Wind Turbine Performance And Wake Recovery

Regarding the issue of unmatched Reynolds numbers for down-scaled wind turbine tests in wind tunnels, a study of the performance characteristics of a single model wind turbine and wind turbine array with two turbines of the same model under laminar and turbulent inflow for both cases was performed. In the experiments with a single turbine, it was found that the maximum wind turbine efficiency was strongly dependent on the inflow turbulence intensity, because turbulence influenced the flow separation in the suction side of the wind turbine blade. This was confirmed by PIV results taken under laminar and turbulent inflow conditions. For the wind turbine array case, it was also found that the efficiency of both turbines was highly related to the turbulence intensity in the inflow. The distance between the two turbines was set at 5, 10, and 15 turbine diameters. In the laminar inflow case, the wake of the front turbine reduced the efficiency of the rear turbine greatly even when the distance was 15 diameters. This was due to slow velocity recovery in the low Reynolds number wake. To solve this issue, turbulent inflow was created using an active grid system installed between the contraction and test-section of the wind tunnel; the maximum turbulence intensity can reach 20%. Velocity fields upstream and in the wake of the turbine were measured using a 2D-PIV system; 1000 pairs of images were acquired for each location to achieve statistical convergence. It was found that by using turbulent inflow the efficiency of both the upstream and the downstream turbine was highly improved. At a constant tip speed ratio for the upstream turbine, the efficiency for the downstream turbine was 4.1 times higher than in laminar case when the two turbines are separated by 5 diameters; for 10 and 15 diameters with the same conditions it was 2.71, and 2.48 times higher respectively. The maximum efficiencies reached for the downstream turbine were 31.6%, 38.8%, and 30% for 15, 10 and 5 diameters of distance respectively. These results are close to the design power coefficient. Therefore, despite the low Reynolds number, a realistic flow similar to the field can be reached using turbulent flow created by an active grid system.