Boundary-layer separations appearing on an oscillating airfoil are investigated to understand the onset process of dynamic stall phenomena. Dynamic stall predictions are performed using a time-accurate Navier-Stokes solver coupled with the transition-transport equations of Langtry and Menter. The effects of the reduced frequency and the Mach number on the generation of separation bubbles are compared based on a parametric study. Four types of boundary-layer separations are observed over the OA209 airfoil: the laminar separation bubble, shock-induced separation, trailing edge separation, and an unsteady separation related to the dynamic stall vortex. Numerical results show that the appearance of the dynamic stall vortex is not directly related to the existence of the leading edge separations. The presence of the dynamic stall vortex and the resulting lift overshoot are strongly correlated with the reduced frequency of the oscillation.
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