Reduced frequency effects on combined oscillations, angle of attack and free stream oscillations, for a wind turbine blade element

The dynamic stall phenomenon in horizontal axis wind turbines causes significant energy waste and sometimes wind turbine failure. For modeling a deep dynamic stall phenomenon of a horizontal axis wind turbine blade element, a numerical simulation of an oscillating NREL's S809 airfoil has been performed at Reynolds number of 106 in an unsteady incident velocity; the velocity oscillates with the same frequency as the airfoil oscillation but with different phase difference (ϕ). Since the sliding mesh technique has been applied for the airfoil oscillation, an O-type grid is created resulting in the reduced number of mesh layers. A specific correction improves the quality of the O-type mesh near the sharp trailing edge. For the combined oscillations, the effects of the reduced frequency (k) in the range of 0.05≤k≤0.15 are investigated with the phase differences of ϕ=−π2, +π2,π. The results show their significant dependency on k at specific ϕ values in particular at ϕ=−π2. Combined effects of k and ϕ can change the aerodynamic loads during dynamic stall significantly compared to loads from a case with a steady incident velocity. These significant changes in the flow structure and aerodynamic loads can affect the wind turbine performance during the dynamic stall phenomenon.