Position-dependent separated-flow reorganization over an airfoil with leading-edge protuberances

This study used time-resolved particle image velocimetry to examine the separated-flow reorganization over a smooth baseline National Advisory Committee for Aeronautics 634-021 airfoil and an airfoil modified with two leading-edge protuberances. Five representative spanwise positions of the modified airfoil, including root1, peak1, trough, peak2, and root2, were compared under a unified experimental framework. The results show that the downstream separated-flow organization is strongly position-dependent. Relative to the baseline case, the trough and root2 positions exhibit earlier broadening of the low-speed region, earlier establishment of the main reverse-flow region, and earlier activation of the separated shear layer. Peak2 also shows early reverse-flow development, but with a topology distinct from that of the trough case, whereas root1 undergoes substantial flow reorganization without forming a distinct main reverse-flow region. Combined analyses of mean velocity, fluctuation intensity, two-component turbulent kinetic energy, and Reynolds shear stress show that the most dynamically active part of the separated flow is concentrated mainly along the outer side of the mean reverse-flow boundary. Proper orthogonal decomposition of the streamwise velocity fluctuation further reveals differences in modal energy partition and dominant streamwise fluctuation structures, with root2 showing the strongest low-order dominance and the most compact energetic structure.