# Analysis of Vorticity and Velocity Fields of Jets from Gas Injector Using PIV
## 4 Results and Discussion
### Vorticity Intensity
The anticlockwise motion triggered by the vortex brings to the opening of a discontinuity in the vorticity distribution. This lack of fresh charge from the rear area contributes to a significant loss in intensity of the inner ring, whose value is now around 40,000 1/s. Differently, the outer jet start slowing its radial spreading. This phenomenon can be attributed to two causes: the increase in CVC pressure that implies the onset of a \"confinement\" action on the jet and the presence of the low-speed area between the jets produced by the external and inner orifices. To pursue this hypothesis, the plume angle of the jet was observed to reduce as the air backpressures increased. At 5.0 ms aSOE, both jets are surrounded by multiple peaks, with the inner side showing slightly higher values of vorticity intensity. Similarly to the first geometry, rather than significant vortex structures, there are small eddies to drive the mixing efficiency of the investigated geometry when the jet is fully developed. In this phase, the maximum intensity value is around 85,000 1/s. The various stages of flow field concerning the third geometry are illustrated in Figure 10. In the first phase, subsequent to the start of the injection, the three in-plane jets are clearly visualizable showing similar peak values of vorticity intensity, i.e., \(\approx\)75,000 1/s. A similar extension of the central jet is possible due to the shorter path that helium has to follow when passing through the central orifice, thus compensating its smaller size compared to that of the external orifices. Unlike geometries discussed so far, no significant vortexes were noted at 3.4 ms aSOE. In particular, the external jets contribute to the formation of two partial rings, whose intensity is quite reduced compared to the small eddies occurring over the jet edges. The reasons for the absence of relevant vortex structures can be addressed to the relatively small size of the orifices and their orientation which led to the generation of narrow and separate jets, thus able to keep the momentum without losing energy to the surrounding air. At 4.0 ms aSOE, the external jets deviate towards lateral regions, losing their capacity to further penetrate along injector axis. The helium coming from the nozzle tip reaches these areas with a higher velocity, which the vertical component dominates (\(v_{y}\)), thus unable to follow the deviation and leading to the breakup of these structures. After 5.0 ms, because of the constant supply of helium from within the main flow, the two side structures are no longer visible, leaving a single larger jet that is characterized by a higher plume angle compared to the first geometry jet, but with a lower velocity magnitude. In any case, it clearly appears that regardless of the nozzle pattern the large vortex structures move with a speed that is somewhat smaller than that of the jet head, highlighting a tendency of these rings to \"slip\" on jet borders before collapsing. These findings agree with those discussed in [36]. It should be noted that as the angular spacing between individual jets increases, they become unable to merge into a single coherent structure. This reduces their ability to interact with the surrounding air, thereby promoting vortex formation. Among the few studies available, these findings are in line with those reported in [37]. Figure 11 shows the absolute peaks values of the vorticity intensity measured by the three geometry nozzles for the various PRs examined. Specifically, first and second geometries alternate randomly, while third geometry showed an overall lower value of vorticity intensity. Further, unlike velocity, which increases linearly till the achievement of the ROI border, the peaks in vorticity were observed to stabilize at almost constant values after 3.5 ms aSOE, regardless of the nozzle tip. Given the definition of the vorticity intensity, it is therefore possible to suppose that as the average velocity of the jet increases, the instantaneous radius of the vortex structure is reduced.