The passive control of the unsteady flow around the NACA0012 airfoil at very low Reynolds number (Re=1.4×103 ) is studied numerically by using the ANSYS-Workbench software. This airfoil is equipped with a flexible flap attached at the trailing edge. The resolution of the two-way Fluid-Structure Interaction (FSI) problem was approached by the system coupling service. The presence of the trailing edge flexible flap has an effect to increase the lift coefficient of about 23% and to reduce the instability coefficient from 54.6% to 36.72%, for an angle of attack of 20°. These ameliorations are caused by the change of the flow characteristics. During its interaction with the surrounding flow, the flap oscillates around an equilibrium position with a dimensionless frequency f0*=0.458, this decreases the Strouhal number of the flow from 0.495 to 0.463. Moreover, the presence of this flap resists the rotation movements of the contra-rotating vortices and translates the second vortex downstream of the trailing edge in the longitudinal direction. This imposes major modifications in the airfoil wake properties, where it will be more stable and narrower.
