When a Horizontal axis wind turbine works under yaw condition, each blade element can be considered as an oscillating pitch airfoil while the free stream velocity oscillates horizontally. The unsteady free stream velocity, which is usually ignored, oscillates with the same frequency as the airfoil oscillations and has a great impact on the periodic forces produced by the airfoil oscillation. In order to study the effects of unsteady free stream
velocity on the aerodynamic loads, a 2D NACA0012 oscillating airfoil at Reynolds number of 135000 has been simulated. In this simulation, reduced frequency, reduced amplitude and the phase difference between the free stream velocity oscillation and the airfoil angle of attack oscillation are 0.1≤k≤0.25 ، 0.2≤λ≤0.8 و ϕ=0 ,π, respectively. Results show that free stream oscillations affect the aerodynamic loads, vortex strengths
and dynamic stall characteristics. The lift force can be increased by more than 7 times than that of static case and 3 times compared to the load from steady free stream velocity. Depending on 𝜙 value, the dynamic stall angle of attack can be advanced 1 degree or delayed by more than 7 degrees by increase of reduced amplitude. Also, increase of k always causes delay in leading edge vortex formation and consequently delay in dynamic stall occurrence.