## 3 Results and Discussion
### Flow Parameters  
Figure 5 presents the SJ velocity and the efficiency of the actuators as a function of the electric power. The presented graphs had a classic form which can be observed in many papers [47; 48]. However, the differences between cases are slight especially at the power above 1 W. For that reason, the differences between the actuator efficiency in case 0 and other cases were presented in Figure 6. The velocity and the efficiency are in the same way dependent on the time-mean reaction force (see Equations (1) and (4)), but the efficiency includes additionally the electrical power. Therefore, the dependencies presented in Figure 6 are similar in the case of velocity. The impact of the number of chevrons on efficiency is quite complicated. In the case of classic chevron orifice (case a, Figure 6a), the chevron orifice has a generally negative impact on efficiency. Only at the power \(P=1.2\); 1.5; 2; 3 and 4 W the SJA with chevron orifice had higher efficiency than the SJA with circular orifice but only in cases 3a and 4a. Similar results were obtained by [PERSON] and [PERSON] [29] (cases 5 and 6). However, the higher the number of chevrons the less impact of the chevrons on the efficiency. Figure 5: SJ velocity (**a**) and synthetic jet actuator (SJA) efficiency (**b**) as a function of power at resonant frequency \(f_{n}\). Figure 6: Efficiency differences between case 0 and cases a (**a**) and b (**b**). In Figure 6, the absolute uncertainty of the efficiency was additionally marked. It is the uncertainty for one measurement and in the case when two different values are subtracted. The absolute uncertainty should be doubled because considering the worst case possible:  
\[\left(\left(A=A\pm 0.2\right)\ \left(B=B\pm 0.2\right)\ \right)\Rightarrow\ \left(A>B\ \Leftrightarrow\ \left(A-0.2\right)>\left(B+0.2\right)\right) \tag{6}\]  
So, taking into account the measurement errors can be assumed that the chevron orifice has no impact on the efficiency at the power \(P\geq 1.5\,\mathrm{W}\). In the case of chevron orifices with rounded cutouts (case b, Figure 6b), the impact on the efficiency is rather positive. Only at power \(P<1\,\mathrm{W}\) the chevron orifice has a negative impact on the efficiency, and orifices with 4 and 10 chevrons (cases 1b and 3b) decreased the efficiency the most. After considering the measurement uncertainty can be assumed that the chevron orifice has no impact on the efficiency at the power \(P\geq 0.5\,\mathrm{W}\). Although the results differ slightly, it can be assumed that the increase of the chevron numbers with the rounding of the cutouts can increase the actuator efficiency. With the increasing of the chevron's number, the difference of efficiency of the actuator with chevron and circular orifice disappears. This is due to increasing the geometric similarity of these two orifices--the bigger number of chevrons the smaller height of cutouts. [PERSON] et al. [26] showed that the cutouts are the area of counter-rotating vortices formation. These vortices modulated the shape of the vortex ring. During the increase of the number of chevrons, the modified vortex ring will look more and more like a classic vortex ring. Like a polygon with more and more sides, it looks more and more like a circle. The rounding of the cutouts can increase the SJA efficiency (Figure 6b). The chevron orifice contains less volume of air than circular orifice and can cause lower orifice damping than circular (the inside surface of the orifice is smaller). [PERSON] et al. [49] showed that the orifice damping is positively correlated to the SJ velocity, and so to efficiency also. It can be expected that the efficiency of SJA with chevron orifice would be higher than in the case of the SJA with the circular orifice. However, this is not so. It means that the chevrons generate higher pressure losses than the circular outlet.