# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 5 Conclusions  
Based on the comparative study, several important conclusions are drawn concerning the viscous effects.  
1. Due to higher Re number, the water-flow case exhibits a stronger laminar-turbulent transition, resulting in a puff pattern of the potential core. Therefore, vortex dynamics plays a dominant role in the vortex-cavitation interaction, and the vortex cavitation at the free shearing side exhibits significant 3-dimensionality. Besides, the thin wall shearing layer contributes to the attached cavitation at the poppet trailing edge.
2. Due to the lower Re number, the oil-flow case has a mild laminar-turbulent transition and thicker wall shearing layer of the waving potential core, producing the paired vortex cavitation structure with remarkable coherence and suppressing the attached cavitation at the poppet trailing edge. The vortex dynamics produce a less significant effect in the vortex-cavitation interaction, compared to that of the water-flow case.
3. The variation in viscosity effect also leads to changes in the interactive evolution between cavitation structures initiated at varied regions. Due to the variation in wall shearing thickness, the cavitation at the poppet trailing edge entrains the shed vortex cavitation in the water-flow case, while the shed vortex cavitation entraps the detached vortex at the poppet trailing edge in the oil-flow case. Such different kinds of interactions between the vortex cavitation and attached cavitation facilitate growth of cavitation located at distinct regions beyond the poppet trailing edge.
4. The stronger vortex dynamics in the water-flow case produces a thorough detachment behavior for attached cavitation within the chamfered groove, leading to consecutive shedding of cavitating vortex with high 3-dimensionality. The higher viscosity of oil-flow produces a considerably larger shearing strength, leading to the stable attached cavitation within the upstream half of the chamfered groove. The rear part attached cavitation within the chamfered groove in the oil-flow case behaves as a source of instability, contributing to persistent shedding of non-cavitating vortex ring and periodic shedding of cavitating vortex ring.
5. Due to the prevalent vortex effect, the cavitation behavior in the oil-flow case suffers from a significant perturbation, while the periodic cavitation behavior sustains principally in the oil-flow case with a dominant cavitation effect in the vortex-cavitation interaction.  
Conceptualization, C.Y. and L.Z.; methodology, C.Y.; software, C.Y.; validation, C.Y. and S.L.; formal analysis, C.Y.; investigation, C.Y. and L.Z.; resources, C.Y. and L.Z.; data curation, C.Y. and L.Z.; writing--original draft preparation, C.Y. and L.Z.; writing--review and editing, C.Y. and L.Z.; visualization, C.Y. and L.Z.; supervision, H.L. and L.Z.; project administration, H.L. and L.Z.; funding acquisition, C.Y. and L.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (No. 51975511), Guangdong Provincial Applied Science and Technology Research and Development Program (No. 2019A1515110844), and Youth Projects of Guangdong Education Department for Foundation Research and Applied Research (No. 2019 KQNCX175).  
Not applicable.  
The authors declare no conflict of interest.