# Analysis of Vorticity and Velocity Fields of Jets from Gas Injector Using PIV
## 6 Conclusions  
In this paper the particle image velocimetry (PIV) technique has been used to analyze the velocity and vorticity characteristics of helium jets delivered by a light-duty gas injector. Three complex geometry nozzles were used to thoroughly examine the effect of orifices size/orientation on velocity magnitude, vorticity and main jet morphological parameters. The constant volume chamber was seeded with vegetable oil droplets to make it possible to track the motion induced by the helium jet to the surrounding air. The image post-processing procedure made it possible to analyze readable images capable of providing global and detailed information on jet morphology and complex flow phenomena, such as  
Figure 13: Helium normalized half plume angle at different PRs. the presence and distribution of vortex rings. The main results of the present investigation are listed below: * The results on vorticity intensity highlight the role of major vortex structures in early and middle stages after injection in promoting mixing phenomena. Then, these structures rapidly collapse, regardless of the nozzle pattern and PR condition, leaving room to the rise in number of small eddies. In particular, these micro-structures were observed to form downstream to the nozzle tip and extend for relatively short distance at low PRs (5-15 mm) and shifted along the injector axis (15-70 mm) at high PRs conditions. * Once the PR condition was established, the velocity magnitude was observed to increase almost linearly with time across all configurations examined. In contrast, after 3.5-4.0 ms aSOE, vorticity intensity kept an almost constant trend for each case considered. These results are in line with the absence of large vortex structures observed during jet development and the parallel increase in the number of small vortices. * The use of nozzle patterns with low orientation orifice paths, like first geometry, leads to the production of jets capable of keeping momentum, thus fastening the resulting jet at expense of a limited radial propagation, i.e., \(\beta/\alpha<1\) at 8.0 ms aSOE. Specifically, the first configuration recorded the highest values of velocity peaks, i.e., from 105 to 309 m/s at 8.0 ms aSOE, regardless of the PR condition examined. * Second and third geometries, both featuring more complex multi-orifice nozzle patterns and a higher degree of orientation of the orifice paths, resulted in an improved radial propagation of the jet, i.e., \(\beta/\alpha\) ranging from 0.5 to 1.5 before achieving the ROI border. On the other hand, the greater area of the jet impacting with the air involved a reduction in jet penetration along the injector axis. Specifically, the gap in velocity was measured to be around of 10-20% compared to first geometry during injection process. The above results could become of paramount importance for the design of injection strategies, i.e., injection duration and interactions with the piston or cylinder walls, to improve the mixing efficiency depending on the application. Experiments consist of a valuable dataset to support 3D-CFD codes. **Author Contributions:** Conceptualization, M.A. and S.S.M. ; methodology, G.C. ; software, G.C. ; validation, G.C. ; formal analysis, G.C. ; investigation, G.C. ; resources, S.S.M. and A.I. ; data curation, G.C. ; writing--original draft preparation, G.C. ; writing--review and editing, M.A. ; visualization, G.C. ; supervision, M.A., S.S.M. and A.I. ; project administration, M.A., S.S.M. and B.M.V. ; funding acquisition, B.M.V. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request. **Acknowledgments:** MA acknowledges financial support from J Gust Richert Stiftelse and Chalmers Area of Advance Transport.