# Acoustic and Flow Aspects of Synthetic Jet Actuators with Chevron Orifices
## 3 Results and Discussion
### The Axial Distribution of Centerline SJ Velocity  
The axial distribution of centerline SJ velocity and turbulence intensity is presented inFigure 7. In Figure 7a, the centerline time-averaged velocity is presented. In Case 0, the velocity value tends to zero at \(x/d=0\). It is caused by almost-equal velocity in the injection and expulsion stages (Figure 4a). In the case of an actuator with a chevron orifice, the centerline time-averaged velocity is bigger than 4 m/s at \(x/d=0\). Then, the velocity increases, and, reaching the maximum value at \(0.4<x/d<1.3\), the velocity decreases. Case 3 is an exception. The centerline time-averaged velocity reaches the maximum value at \(x/d=0\), and then it decreases.  
The centerline time-averaged velocity still decreases with ratio \(x/d\)[30,33,34]; such a tendency is observed in Figure 7a. A similar course to centerline time-averaged velocity has been observed in [30], but the authors did not comment on that. [PERSON] and [PERSON] [35] postulated that it was associated with a change of SJ form and called this region a transition region. They tested an SJ with a rectangular orifice, and, for that reason, the investigation of this phenomenon cannot be transferred to the present results. However, it can be associated with a change of vortex ring shape [15].  
The centerline momentum velocity is similar for each case (Figure 7b). The highest value of momentum velocity was obtained at \(x/d=0\) and then it decreased. Although the initial values differ significantly (centerline momentum velocity in Case 0 is about 22% higher than in Case 2 at \(x/d=0\)), the velocity values are already equalized at \(x/d>0.5\).  
The maximum velocity of the SJ is presented in Figure 7c. The velocity is highest at \(x/d=0\), and then it decreases in Case 0 and Case 1. The maximal value of SJ increases in Case 2 and Case 3. However, in Case 3, it increases for about 0.2 m/s (less than 2% of the value) and it is a value below measurement uncertainty (Table 2); therefore, this cannot be considered significant. In Case 3, the maximal velocity increases at about 7% in the range of \(0<x/d<0.9\), then it decreases. The maximal velocity value is similar for all cases at \(x/d>7\).  
Figure 7: \(Cont.\)  
Figure 7: Centerline time-averaged velocity (**a**), centerline momentum velocity (**b**), centerline maximum velocity (\(\xi\)), and centerline turbulence intensity (**d**) as functions of ratio \(x/d\) at real power \(p=3\) W.  
The centerline turbulence intensity of SJ is presented in Figure 7d. The turbulence intensity for circular orifice SJAs is the highest at \(x/d=0\), and then it decreases; it could have a local maximum [30]. The local maximum of all the measured cases is obtained at \(4<x/d<5\). For a similar ratio \(x/d\), the local maximum appears in [30] (\(x/d=4.4\)). Centerline turbulence intensity trends to 0.4 and reaches a value close to zero at \(x/d>12\); in [30], intensive turbulence reached a value close to zero at \(x/d>15\).  
This is quite a distance from the nozzle (93-116 cm). [PERSON] et al. [36] showed that the heat transfer coefficient was highest at the distance of 50 mm and was not dependent on the orifice diameter in the case of the use of SJAs as an impinging jet. For the investigated orifices, it is \(x/d=2.5\). At that distance, centerline maximum velocity is higher for about 11-30% of cases with chevron orifices than in Case 0, and centerline turbulence intensity is higher for about 34% of cases with chevron orifices than in Case 0. For these parameters of SJ velocity, the thermal boundary layer should be better disturbed, and probably, for this reason, the chevron nozzle is more useful in cooling applications [37].