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 aspectratio=4 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 =14.5 to 21.5deg and Reynolds numbers from Rec=2.99105 to 4.47105 were observed using oil-flow visualizations, demonstrating the transition from full span separation to fully formed single or dual stall cells. At Rec<3.02105 and <15.5deg, 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 15.5<<19.5deg, 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 aspectratio=2.67, which suggested a relationship between the critical Reynolds number and the aspect ratio.