Numerical investigation of boundary layer separation over an airfoil during dynamic stall

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