Direct numerical simulations were employed for investigating two different passive flow control strategies for a modified NACA 643-618 airfoil at a chord based Reynolds number of Re=64,200 and an angle of attack of α = 8.64deg. 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 a roughness Reynolds number of Rek = 446 that were mounted near the leading edge and a scalloped leading edge with serration amplitudes of 5% and 0.5% of the chord were considered. Both strategies reduce flow separation and enhance performance. The flow physics are, however, different. The roughness elements are large enough to induce local flow separation resulting 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 mid-chord. 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.