The effect of cavity modified stepped geometry on aerodynamic performance of an airfoil at a low Reynolds number

An experimental investigation was conducted to elucidate the effect of modified stepped region on a National Advisory Committee for Aeronautics (NACA) model NACA0012 airfoil surface at Reynolds number (Re) of 1×105. Force measurements as well as titanium dioxide (TiO2)-based flow visualization experiments and velocity measurements were carried out to evaluate aerodynamic performance change, to gain insight into flow structures, and to show the changes in wake region. The angle of attack (α) of the airfoil was chosen to lie between 0° and 30° with 1° intervals for force measurements. The attack angles were chosen as 0°, 6°, and 12° considering pre-stall and post-stall cases for velocity measurements and surface oil visualization. In the light of force measurement experiments, it was concluded that a fully step model and all modified models had a higher lift coefficient (CL) value than the base model (BM). It is observed that the D2 model results in 13.22% improvement in the CL value at α=12° compared to the BM. The drag coefficient of the D3 model shows less increase compared to the other modified models, with a value about 0.113 at α=11°. However, there were no improvements in drag reduction when the base airfoil was modified. Velocity measurements showed that the effect of stepped modifications in the pressure surface provides the wake width changes and the wake width centerline shifts. TiO2-based surface oil visualizations were provided to enlighten the new flow structures as cavity-based induced flow structure, step-based induced flow structure, wavy shape reattachment line and wavy shape recirculation region.