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The aerodynamic performance of wind turbine airfoils is an important foundation for the aerodynamic design and the performance analysis of the wind turbine. The purpose of the study reported herein was to obtain the effect of the free stream turbulence intensity on the aerodynamic performance of wind turbine airfoil NACA 4415, by using a two-dimensional numerical method. The study was performed by varying the turbulence intensity from 0.1% to 10% at fReynolds number of 216,000, and by changing the angle of attack from -4 to 24 degrees. The airfoil has a chord length of 0.153 m. The finite volume method was used in this study, and the aerodynamic performance of the airfoil was analysed. The lift and drag coefficient of the airfoil were discussed and validated. It was found that The free stream turbulence intensity can reduce the values of CD, CD, CP and CL/CD, while The free stream turbulence intensity does not significantly affect the wake area behind the airfoil. The decrease in airfoil efficiency observed was caused by disruption of the boundary layer flow which resulted in a decrease in the total pressure coefficient on the airfoil surface. The intensity of turbuence can attenuate the thickness of the boundary layer, which results in a decrease in shear stress on the surface of the airfoil. It is concuded that the free stream turbulence has a significant effect on airfoil performance, where the effect can increase or decrease the performance