Experimental and theoretical investigations on the effect of a single leading-edge protuberance on airfoil performance

Abstract

Geometrical modification of airfoils by leading-edge protuberances has attracted plenty of attention as a passive separation control method. In this research, the effect of a single leading-edge protuberance with different amplitudes on the performance of a two-dimensional airfoil (NACA 634-021) is investigated through wind tunnel experiment and theoretical analysis. Force measurement experiment shows that all the modified airfoils demonstrate a special two-step stall performance. When the first step of stall occurs, the lift coefficient of the modified airfoils drops to a medium value between the pre-stall and post-stall lift coefficients of the baseline airfoil and keeps nearly constant within a range of AOA (angle of attack). Tuft visualization reveals that during the first step of stall, the flow pattern on different sides of the protuberance is distinctly different, respectively, showing leading-edge separation and trailing-edge separation, which are compartmentalized by the attached flow originated from the peak of the protuberance. The flow control mechanism is contributed to the combined effect of the early occurrence of the leading-edge separation around the protuberance root, the inherent hysteretic performance, and the flow compartmentalization effect. A theoretical model based on the lifting line theory, which takes the amendment of the hysteresis and compartmentalization effects into consideration, is proposed to analyze the performance of the modified airfoils. The sectional and overall performances by theoretical analysis are highly consistent with the tendency of the experiment results. Based on the theoretical model, the spanwise circulation gradient induced by the protuberance is considered to be responsible for the earlier separation around the protuberance roots at pre-stall AOAs. Further investigation on the effect of the geometrical parameters of the protuberance reveals that the amplitude-to-wavelength ratio plays a dominant role on the performance of the modified airfoils. The special flow phenomenon induced by the single leading-edge protuberance discovered in this research and the proposed mechanism provide an essential point of view for fully understanding the combined mechanism of multiple protuberances.