## 5 Results
### Additional Issues  
A few issues are raised below from the extensive CFD and experimental studies. Firstly, it is found that the DBD plasma actuator, particularly in the burst mode, promotes a transition to turbulent flows and a laminar separation bubble near the leading edge becomes small due to the early reattachment. The time-averaged boundary layer profile shows that the boundary layer after the reattachment is essentially turbulent [40]. However, the attached turbulent boundary layer maintains a periodicity corresponding to the frequency of the burst mode as described above. The author's understanding from the analysis of the time history is that the role of the DBD plasma actuator is not to attach separated flows but to move the separated shear layer closer to the surface and prevent flow separation by maintaining two-dimensional vortex structure near the airfoil surface. This type of flows can also be observed in the flows at angles of attack slightly lower than the stall without a DBD plasma actuator. The flow structure is also somewhat similar to what is observed in the laminar separation and laminar reattachment flows over an airfoil at lower Reynolds numbers [35]. When we discuss the flow control authority of the plasma actuator, we have to define what is \"good authority\". The definition may be different for different people and for different applications. For example, the lift coefficient \(CL\) shows a similar trend to that for \(L/D\) (lift to drag ratio, where \(D\) is drag) in many cases. Among them, there are cases where \(CD\left(=D/\frac{1}{2}\rho u_{0}^{2}c\right)\) values become large. One example may be the case of \(F^{+}=1.0\) in Table 1. A similar result may be found in the case at higher Reynolds numbers, as shown in Figure 12. The experiment was carried out with an airfoil chord length of 0.2 m (two times longer than the experiment discussed above) and a freestream velocity of 60 m/s (six times higher than the experiment discussed above) in the wind tunnel owned by a private company. In some cases, negative \(C_{p}\) distributions over the upper surface were obtained and the lift was somewhat recovered, as observed in the plots in Figure 12. However, suction peak near the leading edge is not recovered and flow control is not satisfactory. The flow for this type of \(C_{p}\) distributions tends to show a relatively high \(CD\). There are relatively large two-dimensional spanwise vortices (which maintain a negative pressure there) aligned over the upper surface as in Figure 10b. The flow structure in these cases also tend to have strong flow unsteadiness. This type of use is acceptable if the objective in using DBD plasma actuators is simply to increase the lift. However, it is not acceptable if the objective is to keep aircraft flying at more than the stall angle of attack. Flow structures at this Reynolds number commonly include laminar separation near the leading edge, a transition to turbulence, and reattachment. A so-called laminar separation bubble is created. We may evaluate the flow control authority of the plasma actuator by the early reattachment of the laminar separation bubble, or the size reduction of this laminar separation bubble. However, early reattachment does not necessarily mean a high lift or high \(L/D\). Thus, it is important to clarify what the purpose of using the DBD plasma actuator is, and to define a flow control authority consistent with that objective. Another issue determined in this study is the relation between the leading edge and trailing edge \(C_{p}\) values and the lift, drag and \(L/D\). Our data mining result showed strong relation between the \(L/D\) and the \(C_{p}\) value at the trailing edge [49]. Sometimes, data acquisition for a certain region over the airfoil becomes difficult, as the dielectric of the DBD plasma actuator covers the airfoil surface. We may be able to estimate the aerodynamic performance by simply measuring the \(C_{p}\) at the trailing edge.