Spanwise variation in the unsteady stalling flowfields of two-dimensional airfoil models

Recent investigations of two-dimensional airfoil stalling characteristics h ave revealed low-frequency and highly unsteady flow in some cases and large -scale three-dimensional structures in other cases. The latter were referre d to as "stall cells" and can form on two-dimensional configurations where the ends of the airfoil model are flush with tunnel side walls or end plate s. This paper presents results of detailed investigations of the stalling c haracteristics of several airfoils that exhibited both low-frequency unstea diness and large-scale three-dimensional structures. The airfoils were wind -tunnel tested in a two-dimensional configuration. The primary measurements were spanwise wake velocity and mini-tuft flow visualization. The results showed that airfoils with trailing-edge separations at and above maximum li ft (static stall) exhibited stall-cell patterns. Conversely, airfoils that had leading-edge separation bubbles that grew in size as the angle of attac k was increased into stall developed the low-frequency, highly unsteady flo w. This unsteadiness was found to be essentially two dimensional. Therefore , the development of either of these phenomena appears to be determined by the characteristics of the boundary-layer separation leading up to the stal l.