Leading edge protuberance modifications on airfoils or wings have attracted extensive attentions as a new passive technique for separation control. In this paper, the hydrodynamic performance of a NACA 634-021 foil and a modified foil with leading-edge protuberances were numerically investigated using Spalart-Allmaras turbulence model. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foils was advanced and the decline of lift coefficient became mild, and the post-stall performance was improved. A special bi-periodic flow pattern may occur and stay extremely steady at a wide range of attack angles, accompanied with a relatively steady lift. The transformation from single-periodicity to bi-periodicity occurred within a small range of range of attack angle. A couple of counter-rotating streamwise vortex was formed on the shoulder of each protuberance, altering the vorticity line to share a similar shape as the leading-edge profile. At larger angles of attack, the development of streamwise vortex would be accompanied with transformation to lateral vortex, where strong interaction may happen and give rise to the occurrence of bi-periodic condition. The formation mechanism and control method of the special phenomenon should be investigated more deeply in the future.

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