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.