# Experimental Study of Coaxial Jets Mixing Enhancement Using Synthetic Jets
## 3 Results and Discussion
### Analysis of Comparative Experiments  
Figure 6a,b show the velocity distribution of nozzle B without and with actuators in case 2. The length of the potential core in the original flow field was 50 mm. The central jet and the annular jet reached the same velocity at approximately x = 90 mm axially, and it can be considered that the two jets were fully mixed. After the actuators were turned on, the potential core length was reduced to 15 mm, and the two jets were fully mixed at x = 45 mm. The outer shear layer was significantly thicker and the velocity gradient was significantly smaller in the radial direction.  
It is apparent that the synthetic jets enhanced the jet mixing of nozzle B more significantly than nozzle A. The two nozzles had different annular outlet areas, and all other conditions are the same. The biggest difference between the experimental conditions of the two nozzles was the momentum coefficient. The momentum coefficient was the ratio of synthetic jets momentum to the coaxial jets momentum, which is represented by C in Table 1.  
Figure 6: Time average velocity distribution of nozzle B without (**a**) and with (**b**) actuators.  
For nozzle A, the streamwise vortices caused the air inside the inner shear layer to move radially outward, and the high velocity air of the central jet fills into the shear layer, so that the velocity at \(\mathrm{y/D_{oa}}\) = 0.07 inside the inner shear layer significantly increased (Figure 3). For nozzle B, the streamwise vortices affected the mainstream center. The center jet air was entrained outward by the streamwise vortices while the annular jet air was entrained inward, and the mixing is greatly enhanced. As a result, the central jet velocity was lowered, and the velocity inside the annular jet was increased. Similar mixing and velocity changes also occurred in the outer shear layer. Therefore, the velocity at \(\mathrm{y/D_{oa}}\) = 0 and \(\mathrm{y/D_{oa}}\) = 0.07 in the central jet range rapidly reduced under larger pulsations, and the velocity at \(\mathrm{y/D_{oa}}\) = 0.27 in the outer shear layer slightly increased (Figure 7).  
According to the above analysis, to further enhance the jets mixing of nozzle A, the method of increasing the momentum coefficient can be used. As an exploration research, two different methods were used to increase momentum coefficient. One was to increase the number of actuators, the combined actuators disk used above were connected in parallel and mounted on nozzle A (case 3), the measurement region moved forward 15 mm in the X axis due to the addition of combined actuators disks. The other was to use four actuators with a higher outlet velocity (case 4). Compared with Figure 2b, the potential core length in case 3 was reduced from 90 mm to 65 mm with C = 1.36% (Figure 8a), and in case 4 it was reduced to 55 mm with C = 1.47% (Figure 8b). The inner and outer shear layers were significantly thickened in case 3 and 4 compared with case1. Taking X = 40 mm as an example, along the negative direction of the Y axis, the width from the central jet velocity of 20 m/s to the annular jet velocity of 10 m/s in the inner shear layer was about 8 mm, the width from the annular jet velocity of 10 m/s to the static ambient in the outer shear layer was about 14 mm, in Figure 2b. The widths of the inner and outer shear layers of case 3 were 10 mm and 18 mm (Figure 8a). In case 4, the widths were 15 mm and 18 mm, respectively (Figure 8b). The higher the momentum coefficient, the better the mixing enhancement. Therefore, it is verified that the momentum coefficient was the most critical factor for coaxial jets mixing enhancement.  
Figure 7: Velocity distribution along axial direction at different sections of nozzle B.