Improving the aerodynamic performance of low Reynolds number airfoils in oscillating freestream with forced transition method

The Reynolds numbers of current Micro Air Vehicles (MAVs) lie in the range 10⁴-10⁵. At such low Reynolds numbers, airfoils suffer from reduced lift-to-drag ratio and prone to flow separation. To improve the aerodynamic performance of airfoils in such low Reynolds number environment, the method of forced transition is investigated. In this study, transition is fixed ahead of the location at which free transition occurs. To test the effect of this method, E387 airfoil is computed on 4.0 × 10⁴ Reynolds number for different angles of attack. The results show great improvement of lift coefficient and lift-to-drag ratio because of the decreased separation zone. Furthermore, forced transition is applied in low Reynolds number oscillating freestream environment, the performance of airfoils which transition freely deteriorates when Reynolds number decrease. However, the forced-transition airfoils have a much more stable lift coefficient and power index (C₁^3/2/C_d). The results of this paper indicate that the forced transition method can significantly improve the aerodynamic performance of low Reynolds number airfoils.