Unsteady separation from the leading edge of a thin airfoil

At high Reynolds numbers, the process leading to dynamic stall on airfoils initiates in the leading-edge region. For thin airfoils, the local motion near rounded leading edges can be represented as flow past a parabola and when the mainstream flow is at an angle of attack to the airfoil, a portion of the boundary layer will be exposed to an adverse pressure gradient. Once the angle of attack exceeds a certain critical value, it is demonstrated that unsteady boundary-layer separation will occur in the leading-edge region in the form of an abrupt focused boundary-layer eruption. This process is believed to initiate the formation of the dynamic stall vortex. For impulsively-started incompressible flow past a parabola, a generic behavior is found to occur over a range of angles of attack, and a limit solution corresponding to relatively large angles is found. The separation in the leading-edge region develops in a zone of relatively limited streamwise extent over a wide range of angles of attack. This suggests that localized control measures (such as suction) may possibly be effective at inhibiting separation.