Two-Step Stall Characteristic of an Airfoil with a Single Leading-Edge Protuberance

An experimental investigation has been conducted to examine the influence of a single leading-edge protuberance on the performance of a baseline NACA 634-021 airfoil. Static aerodynamic forces of the airfoils have been measured as the angle of attack increased and decreased. A hysteresis loop of the lift coefficient around the stall angle is observed for the baseline airfoil, with a sharp decline of the lift coefficient during angle-of-attack increase and a delayed recovery during angle-of-attack decrease. It was discovered for the first time that both the decline and the recovery of the lift coefficient of the modified airfoil with a single leading-edge protuberance included two steps. When the first step happened, the lift coefficient reached an intermediate value between the maximum value and the poststall value of the baseline airfoil, and it remained almost constant within a range of angles of attack. Surface tuft visualization revealed that, during the first step of stall, one side of the modified airfoil was stalled with leading-edge separation, whereas the other side remained in a nonstall condition. When the second step of stall happened, both sides of the modified airfoil were stalled, except for an attached flow on the protuberance peak. This one-sided stall phenomenon indicated that one important role of the leading-edge protuberance might be confining the local stall region from extending spanwise, which is similar to the effect of a wing fence.