Parametric Investigation of Stall Cell Formation on a NACA 0015 Airfoil

An experimental parametric exploration of stall cells on a NACA 0015 airfoil with aspect ratio was conducted. First, flowfield measurements were performed using stereoscopic particle image velocimetry at conditions where a stall cell was present. This enabled some of the properties associated with stall cell(s) and stall cell transition to be defined. For the parametric study, angles of attack from to 21.5 deg and Reynolds numbers from to were observed using oil-flow visualizations, demonstrating the transition from full span separation to fully formed single or dual stall cells. At and , full span separation with recirculation regions was present, whereas at midrange of Reynolds numbers tested, three-dimensional separation was observed with undulations across the span. At , a single stall cell formed when the Reynolds number exceeded a critical Reynolds number. At the highest angles of attack tested, a dual stall cell configuration was present at high Reynolds numbers. In addition, the evolution of a stall cell was observed to be reminiscent of the mechanism proposed by Weihs and Katz ("Cellular Patterns in Poststall Flow over Unswept Wings," AIAA Journal, Vol. 21, No. 12, 1983, pp. 1757–1759). It was also shown that, under specific conditions, a bistable condition was present, where two different patterns were obtained for identical flow conditions. Finally, a less-detailed parametric investigation was also conducted for a NACA 0015 airfoil with , which suggested a relationship between the critical Reynolds number and the aspect ratio.