## 5 Conclusions  
This paper describes and analyses the mechanisms that allow for control authority for an asymmetric nanosecond pulsed dielectric barrier discharge (NS-DBD) actuator on the aerodynamic characteristics over a typical wing section, represented by a NACA 0015 airfoil profile. Experiments were performed in a low-speed wind tunnel under ambient pressure conditions at freestream flow speeds of 10 and 20 m/s at angles of attack of 16\({}^{\circ}\) and 24\({}^{\circ}\) for both flow speeds. Pressure taps along the profile chord located at mid-span on both the suction and pressure sides of the airfoil were used also to determine the effectiveness of the actuator at various actuation frequencies. Hot wire anemometry was performed with the aim of identifying dominant frequencies in the flow field over the stalled profile. Furthermore, PIV measurements were taken to identify how different actuation frequencies affect the existing structures in the flow field. Via hot wire measurements without DBD actuation at different measurement lines along the airfoil and in the wake, the dominant characteristic flow frequencies were obtained. These frequencies were used as an actuation frequency for the DBD and were compared to known effective values of reduced frequency \(F_{c}^{+}\). The degree of control authority for each actuation frequency was assessed by examining the pressure distributions along the airfoil. This analysis revealed that actuating the DBD at the low characteristic flow frequencies, the control authority at re-attaching the flow was significant, and in the case of \(\alpha=16^{\circ}\) and \(Re=200,000\) improving the performance of actuation frequencies of \(F_{c}^{+}=1\) and \(2\), which were known to be effective. Furthermore, at this angle of attack, the area under the curve of the pressure distribution was increased at the low characteristic frequencies. Actuating the DBD at the high characteristic flow frequencies produced some effect, which was not as significant. When the most effective frequencies were identified based on pressure distributions, further power spectra were obtained with actuation. This revealed that, when the DBD is actuated at a characteristic flow frequency, the magnitude of the PSD peak is increased. Furthermore, the multiples of the actuation frequency were observed in the power spectra. By comparing the power spectra of different actuation frequencies, it was observed that the most effective frequency also produces a higher PSD peak. By examining the PIV measurements, it was confirmed that the phenomena which occur with DBD actuation are unstable depending on the Reynolds number. Furthermore, it was observed that, depending on the actuation frequency, different flow structures were excited and therefore the actuation frequency is critical to the control authority of a NS-DBD actuator. Figure 27: Swirling strengths at \(Re=200,000\) and \(\alpha=24^{\circ}\): with no actuation **(a)**; at actuation frequency of 69 Hz, **(b)**; and at actuation frequency of 266.6 Hz, **(c)**. In conclusion, nanosecond-pulsed plasma actuators obtain their control authority via several mechanisms, not only limited to an equivalent energy transfer or on the micro-shockwave that emanates from the actuator [15; 17] but particularly on the effect of relating characteristic flow frequencies to the actuation frequencies. This shows that further applications in active flow control can be considered that are not yet suggested in the literature, giving a promising future for these actuators. Conceptualization, C.S. and P.L. ; methodology, all authors; software, C.S. and D.N. ; validation, all authors; formal analysis, C.S. and D.N. ; investigation, C.S. and D.N. and H.B. ; resources, P.L. and P.O. ; writing-original draft preparation, P.L. and C.S. ; writing-review and editing, P.O. and P.L. and C.S. ; visualization, D.N. and H.B. and C.S. ; P.L. performed supervision and project administration. This research received no external funding.