# Analysis of Vorticity and Velocity Fields of Jets from Gas Injector Using PIV
## 4 Results and Discussion
### Velocity Magnitude
The presence of a low-speed region between the two jets was noted to form before the achievement of the steady state conditions. This area can be addressed to the presence of air in between the two jets, resulting in a local speed of around 45-50 m/s, while surrounded by the tail of the two jets at 90-100 m/s. At PR 20 condition, the overall increase in momentum of the jets makes them able to create more  
Figure 3: Velocity distribution at 3.4 ms aSOE, GEO–1. Single pair of frames (**left**) and averaged frames (**right**) PR10. stable vortexes. In addition, opposite the low-ratio condition, the in-plane jet becomes predominant for extension, with the outer jet that rapidly fills the low-speed area, reducing the radial propagation of the entire structure. Figure 4: Velocity distribution, GEO-1, average frames PR4 (**left graphs**) and PR20 (**right graphs**). The higher orientation of the outer orifices of the third geometry, as well as the increased distance between these and the central orifice, make it possible to clearly visualize the three jets cross sectioned by the laser sheet (Figure 6). Due to the small size of nozzle orifices, it was not possible to recognize any relevant vortex structure, regardless of the PR condition. Further, it was noted that the radial propagation grows until the head of the jet, which is the case for the PR condition. Figure 5: Velocity distribution, GEO-2, average frames PR4 (**left graphs**) and PR20 (**right graphs**). the lateral jets deviates towards external regions. At this stage, the velocity magnitude of the lateral jets rapidly decreases to less than half of the maximum value. In this condition, the fresh charge of helium coming from the rear area sweep away the lateral jets head. Among all the injector tips investigated, the third geometry produced in the shortest jet, and the short distance between external orifices did not allow for the presence of low-speed regions, thus demanding the mixing efficiency to the small eddies. Further, the velocity magnitude measured is the lowest among that observed, with an overall gap about of 15-20% compared to the first geometry and 5-10% to the second. Figure 6: Velocity distribution, GEO-3, average frames PR4 (**left graphs**) and PR20 (**right graphs**). In Figure 7 the maximum velocity values for each case are illustrated. Those shown in the graphs are the values calculated by the 20 peaks measured for each individual pair of frames, while the dashed area represents the ROI border. The largest size of first geometry nozzle cap orifices always ensured the achievement of the highest values of velocity magnitude, regardless of the PR condition, which ranged from 108 to 242 m/s. In particular, at early stages the second and third geometries were observed to be slightly faster than the first nozzle pattern, with a gap ranging from 10 to 20%. In this phase, rather than orientation, the size of the orifices on the nozzle tip plays a key role in restricting the jet, giving it a better ability to penetrate through the air. This trend is reversed between 3.4 and 4.0 ms aSOE, when the first geometry becomes faster than other configurations. The gap between the three configurations rises as the PR increases, with the first nozzle leading to a difference in a few percentages at low PR, to slightly more than 20% of PR10 and PR20 cases. Regarding the range of the reported average data, it was observed that velocity peaks exhibited a higher standard deviation in the moments subsequent to the injection trigger, with values ranging between 6 and 11%. This is attributed to the influence of the low-correlation area. As the jet extended, this value rapidly decreased, reaching values between 3 and 6%, with an improved reproducibility of the velocity peaks noted at higher PR values. Figure 7: Average velocity peaks for the three nozzle patterns at different PRs. In particular, the third geometry was found to be the most stable with standard deviation ranging from 3 and 5% under most operative conditions.