Effect of free stream turbulence on wind turbine performance
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In this Master Thesis the effect of free stream turbulence has been investigated on a model wind turbine's performance characteristics and the wake development downstream. The experiment took place in the recirculating wind tunnel in the Fluid Mechanics building at NTNU, and the model wind turbine that was used had a diameter of 0,9 meter. A turbulence-generating grid with a mesh size of 0,24 meters produced a turbulence intensity of 5,5 % in front of the wind turbine, which corresponds to atmospheric turbulence levels offshore. The experimental results with free stream turbulence were compared to the results without free stream turbulence. A reference wind speed of approximately 10 m/s were used in all the experiments. The wind turbine is operating most efficiently at TSR=6, and the peak power coefficient without free stream turbulence was Cp=0,461, while it was Cp=0,45 with free stream turbulence. Hence, the power coefficient seemed to be slightly reduced with increased levels of turbulence, except at low tip speed ratios where the effect of stall dominated. The free stream turbulence has two opposite effects on the power extraction of the wind turbine, and this may be the reason why the peak power coefficient was only reduced by 2,4 % with free stream turbulence, which was lower than expected. Increased levels of turbulence increase the drag on the turbine blades, which reduces the power extraction. Simultaneously, the power extraction is proportional to the square of the relative velocity at the blades, which increases with higher levels of turbulence. Wake measurements were performed with a hot wire which measured the velocities and the normal stresses in the streamwise direction. The measurements were done across the wake at the distances 1, 3 and 5 rotor diameters downstream of the wind turbine. The thrust coefficients at TSR=6 are almost identical both with and without free stream turbulence, and the velocity profiles just behind the rotor are therefore also almost identical. However, the effect of the free stream turbulence is clearly seen downstream in the wake, where velocity gradients, inhomogeneities and the kinetic energy in the tip vortices are smoothed and more spread out with free stream turbulence. This is due to increased turbulent diffusion, which is increasing the radial transport of momentum in the wake. The velocity profiles in the wake hence become flatter and broader, and the wake recovers faster with higher levels of turbulence. Even though the power extraction is slightly reduced with free stream turbulence, it seems like the effect on the recovery of the wake is larger, which will lead to higher power extraction and lower fatigue loads on a downwind turbine. Increased levels of ambient turbulence will therefore probably increase the total power output in a wind farm.The length scale of the turbulence produced from the grid was approximately 0,2 meters, which was about 5-6 times smaller than desired compared to a wind turbine in the free atmosphere. The effect of the free stream turbulence on the performance characteristics and wake development of the model wind turbine might therefore be smaller than for a wind turbine in an open environment, and further studies should therefore be performed to investigate this effect.