Numerical investigation of passive separation control for an airfoil at low-reynolds-number conditions

Two different passive flow control strategies were investigated for a modified NACA 64₃-618 airfoil at a chord-based Reynolds number of Re = 64,200 and an angle of attack of α = 8.64 deg. For these conditions, the laminar boundary layer separates from the suction side, resulting in a loss of lift and a drag increase. Distributed roughness elements with roughness Reynolds numbers of Re_k = 136 and 446 that were mounted near the leading edge and scalloped leading edges with serration amplitudes of 5 and 0.5% of the chord were considered. The large roughness elements and the scalloped leading edge reduce the flow separation and enhance performance. The flow physics are, however, different. For Re_k = 446, the roughness elements result in high-frequency shedding. The shedding results in an accelerated transition of the separated boundary layer. For the scalloped leading edge with 5% serration amplitude, laminar separation bubbles are situated in the leading-edge troughs. The turbulent wedges that originate from these bubbles coalesce near midchord. For a serration amplitude of 0.5%, the separation line is deformed in the spanwise direction in a manner that is reminiscent of stall cells.