Spanwise Variation in the Unsteady Stalling Flowfields of Two-Dimensional Airfoil Models
Andy P. Broeren and Michael B. Bragg
University of Illinois, Urbana, Illinois, 61801
ABSTRACT
Recent investigations of two-dimensional airfoil stalling characteristics have revealed low-frequency and highly unsteady flow in some cases and large-scale three-dimensional structures in other cases. The latter were referred 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 plates. This paper presents results of detailed investigations of the stalling characteristics of
several airfoils that exhibited both low-frequency unsteadiness 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 lift (static stall) exhibited stall-cell patterns. Conversely, airfoils that had leading-edge separation
bubbles that grew in size as the angle of attack was increased into stall developed the low-frequency, highly unsteady
flow. This unsteadiness was found to be essentially two dimensional.Therefore, the development of either of these
phenomenaappears to be determined by the characteristics of the boundary-layer separation leading up to the stall.