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 5809 airfoil has been performed at Reynolds number of 10(6) in an unsteady incident velocity; the velocity oscillates with the same frequency as the airfoil oscillation but with different phase difference (phi). 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 phi = -pi/2, +pi/2, pi. The results show their significant dependency on k at specific phi values in particular at phi = -pi/2. Combined effects of k and 0 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. (C) 2017 Elsevier Ltd. All rights reserved.