## 4 Conclusions  
In this study, parametric experiments for \(F^{+}\) with a wide range of \(Re\) values (\(10^{5}\)-\(10^{6}\)) for NACA0015 airfoil was conducted to identify the multiple flow control mechanisms that exist within the experimental condition. The effects of DBD-PA actuation on the flow field around the wing were investigated by the wind tunnel testing of a 2D wing model (NACA0015) with the DBD-PA mounted on the leading edge. A parametric survey was performed for \(F^{+}\) ranging from 0.1 to 100 and \(Re\) values ranging from \(10^{5}\) to \(10^{6}\). The \(C_{L}\) curve, \(C_{p}\) distribution, and velocity field around the suction side of the wing were obtained using pressure measurements and the PIV visualization of the flow field, with and without DBD-PA actuation. Based on the change in the \(C_{p}\) distribution and \(C_{L}\) curve with the AoA, the characteristics of the increase in \(C_{L}\) with DBD-PA actuation were organized according to the three flow patterns for the flow before actuation, i.e., (1) the completely attached flow; (2) the partially separated flow along the chord length; and (3) the completely separated flow at the leading edge. DBD-PA was concluded to be more effective in controlling stall when the body force was placed closer to the point of boundary layer separation on the suction side of the wing. The flow control effect is diminished when the DBD-PA is mounted on the leading edge of the wing in the high \(Re\) number flow field because the separation point moves downstream due to the laminar-turbulent transition of the boundary layer. Four indices were then evaluated according to the \(C_{L}\) and the velocity field at \(24^{\circ}\) AoA. Comparing these four indices, three phenomena for the increase in \(C_{L}\) were found: (1) increase in TKE due to shedding of high-energy vortices into the suction side of the wing; (2) increase in circulation around the wing due to the momentum transport being promoted in the shear flow on the suction side of the wing and due to the shear layer being attracted to the wing surface; and (3) acceleration of the flow due to shifting of the separation point of the boundary layer to the downstream at the leading edge. For the \(C_{p}\) distribution, three trends in the features of the distribution correspond to the three phenomena for the increase in \(C_{L}\). These three trends were induced independently or in combination by comparing the \(C_{p}\) distribution under each condition. These results indicate that the first and third trends appear to be independent of each other; however, the second trend can depend on these trends and occur simultaneously. These phenomena and trends suggest that the actuation of the DBD-PA mounted on the leading edge of the wing has at least two mechanisms for increasing the \(C_{L}\). The first mechanism includes the shedding of vortices with a very low pressure area into the shear flow on the suction side of the wing. The second mechanism introduces a disturbance into the boundary layer at the leading edge of the wing. However, these two mechanisms cannot explain why the circulation may increase when there is no momentum recovery in the shear flow on the wing; therefore, a third mechanism needs to be identified. In this study, the multiple flow control mechanisms that exist within the experimental condition were identified, especially in high \(Re\) number ranges. As the next phase of this study, by verifying that the same flow control mechanism is obtained for the same flow field regardless of the airfoil, it is possible to estimate the optimum control parameter from the flow field that is known by the numerical analysis for any airfoil. There is a need of the further investigation when the method is applied to DBD-PA systems with other types of actuators. Conceptualization, N.K., M.T. and T.N. ; methodology, N.K., S.B. and T.N. ; software, N.K. ; validation, N.K. and M.T. ; formal analysis, N.K. and S.B. ; investigation, N.K., S.B., M.T. and H.K. ; resources, N.K. and M.T. ; data curation, N.K. and S.B.