An experimental study of unsteady flow over airfoils near stall

This study has considered the unsteady and three-dimensional stalling characteristics of several airfoils. Previous research has documented the presence of a low-frequency flow oscillation near stall. The objective of this study was to determine if this unsteady flow occurs for other airfoils operating near stall, and if so, to identify the common characteristics including potential three-dimensional variation. These objectives were carried out by measuring the lift and flowfield properties for twelve airfoils having different stalling characteristics at a chord Reynolds number of 300,000.; The results showed that the low-frequency oscillations were a general phenomenon that occurred for six of the twelve airfoils tested. These airfoils all exhibited a leading-edge separation bubble that grew in size on the upper surface as the angle of attack was increased into stall. This feature is typical of the thin-airfoil stall type. Three of these airfoils also exhibited substantial turbulent boundary-layer (or trailing-edge) separation and the stall type of these airfoils was a combination of thin-airfoil and trailing-edge stall. The rms lift coefficients at stall for the three thin-airfoil stall airfoils were high (0.040 to 0.080) and contained distinct low-frequency components which increased with increasing angle of attack. Similar results were obtained for the combination thin-airfoil/trailing-edge stall airfoils except that the lift fluctuations were much more intense (0.080 to 0.180) and were quasi-periodic in the low frequency. Phase-averaged flowfield data revealed that both the leading-edge separation bubble and the trailing-edge separation were uniform along the span of the airfoil model. This uniformity was also observed in the wake. Furthermore, the low-frequency unsteadiness was found to be very similar in character for airfoils which were very different in contour.; The remaining six airfoils did not exhibit low-frequency unsteady behavior near stall and the resulting lift fluctuations were very small (less than 0.040). These airfoils exhibited trailing-edge separation which moved forward on the airfoil as the angle of attack was increased into stall. The separation was not uniform along the span and the surface flowfield was indicative of stall-cell structures observed by others. This spanwise variation was also observed in the wake.