## 6 Conclusions  
The research activities of the author's group for the flow control authority of the DBD plasma actuator are summarized and reviewed. The mechanism of flow control by the DBD plasma actuator is discussed based on a series of computations and experiments for low-speed flows over airfoils with these devices attached near the leading edge. The DBD plasma actuator is shown to be very effective for controlling flow separation at a Reynolds number of \(6.3\times 10^{4}\) when it is applied to flows at angles of attack above the stall angle. The mechanism of flow structures induced by the DBD plasma actuator, particularly in burst mode, and different control strategies based on the observation are discussed. Analysis of the phase-average flow structures reveals that there are remarkable structures in the flows near the airfoil surface, which characterize the actuator-oriented flows to reduce flow separation. The study identifies three flow features related to the induced flows for the flow separation control, and these are why the DBD plasma actuator works better than other similar devices. It is shown that some of the flow features can be enhanced and become dominant in the flows by a proper setting of the actuator parameters. Therefore, the parameters should be optimized for specific applications to achieve effective flow control. Based on the analysis, guidelines for the effective use of the DBD plasma actuator are proposed. The DBD plasma actuator is also applied to the flows at cruise condition. The lift-to-drag ratio for a simple airfoil, when the DBD plasma actuator is applied, turns out to be better than that for a well-designed high-performance airfoil. Improvement of the lift-to-drag ratio is achieved not by an increase in lift but by a decrease in pressure drag. In this application, the DBD plasma actuator delays the transition, and flows over an NACA0015 airfoil become similar to the flows over a well-designed Ishii airfoil. The plasma actuator is shown to be a very effective flow control device when applied to airfoil flows both at high angles of attack and at cruise angles of attack. Figure 16: Power spectrum density of the chordwise velocity fluctuation at several locations [50] NACA0015 (\(\alpha=6^{\circ}\)), Ishii airfoil (\(\alpha=4^{\circ}\)), \(F^{+}=6.0\), \(BR=10\%\), \(Dc=0.05\), \(Re=6.3\times 10^{4}\). Figure 17: Instantaneous snap shot of the flow structures (iso-surfaces representing the second invariant of the velocity gradient tensor colored with the chordwise vorticity) NACA0015 (\(\alpha=6^{\circ}\)), Ishii airfoil (\(\alpha=4^{\circ}\)), \(Re=6.3\times 10^{4}\), \(F^{+}=6.0\), \(Dc=0.05\), \(BR=10\). Fluid dynamics is very difficult because of its strong nonlinear nature. A small change in the flow may totally change the global flow structures such as the stall phenomenon. This type of nonlinearity is considered to be a disadvantage but could be considered to be a benefit as well. If we know the underlying flow mechanisms and find the way to enhance some of the important features, we may be able to control overall flow structures with a very small energy input localized in time and space. DBD plasma actuators may be useful devices for such input, although there remains much to understand for achieving better control of flows. The following are available online at [[http://www.mdpi.com/2076-3417/8/4/546/s1](http://www.mdpi.com/2076-3417/8/4/546/s1)]([http://www.mdpi.com/2076-3417/8/4/546/s1](http://www.mdpi.com/2076-3417/8/4/546/s1)), Video S1: Movie animation of Figure 17b (iso-surfaces representing the second invariant of the velocity gradient tensor colored with the chordwise vorticity). The computational results presented in this manuscript were mainly obtained under the support of the Strategic Programs for Innovative Research (SPIRE) of the High-Performance Computing Initiative (HPCI) in Japan.