# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 4 Discussion
### Viscosity Effect on Vortex Cavitation and Potential Core
#### 4.1.3 Discussion on Viscosity Effect on Vortex Cavitation and Potential Core  
Difference in Circumferential Coherence and 3-Dimensionality  
As a conclusion from the above analysis in terms of vortex cavitation, the evolution of vortex cavitation in the water-flow case generally is accompanied by a fast development process of 3-dimensionality, while vortex cavitation in oil flow retains preferable circumferential coherence, as shown in Figures 5-8. It seems that the viscosity effect tends to retard the evolution of vortex cavitation. Such difference is highly correlated with the vortex dynamics and could be reasonably explained with the corresponding vortex structure. Figures 5-8 also include the 3-D view of the vortex cavitation and vortex structure identified with Q-criterion. Due to the higher Re Number in water flow case, the laminar-turbulent transition process is rather fierce and fast. The coherent vortex structure originating from the Kelvin-Helmholtz instability inside the poppet valve with a sharp valve seat achieves a sufficiently high intensity. In the water-flow case, the fast-evolving vortex triggers cavitation at a more upstream region as a result of the pressure drop at vortex core. Subsequently, the vortex-cavitation interaction also facilitates the development of 3-dimensional vortex structure, conducive to the further break-up of vortex ring into several segments. Consequently, the downstream bubble cloud appears as several separated 3-dimensional bubble clusters. On the contrary, the transition process is delayed by the higher viscosity of mineral oil, which also contributes to the stronger stretching effect imposed on the cavitating vortex. Consequently, the vortex cavitation at the free shearing layer assumes a thin layer feature with favorable coherence. For the poppet valve with a chamfered valve seat, the vortex or cavitation structure at the free shearing layer originates from the chamfered groove. As a consequence of the uniform/concurrent detachment behavior of attached cavitation within  
Figure 7: Simulation results for water cavitating flow inside a poppet valve of a chamfered valve seat with different pressure drop ranging from 25 to 44 bar. Cavitation and vortex structure are identified with iso-surface of \(\mathrm{d}=70\%\) and Q \(\mathrm{=1\,\times\,10^{10}}\). **(a)** 44 bar pressure drop; **(b)** 40 bar pressure drop; (c) 30 bar pressure drop; **(d)** 25 bar pressure drop. the chamfered groove, the water-flow case occasionally exhibits vortex cavitation with an extremely huge volume and a circumferential alignment. Similarly, due to the deviation in viscous effect, the vortex cavitation generally displays stronger 3-dimensionality in the water-flow case, as indicated by the comparison between Figures 7a,b and 8a,b. By comparing the vortex structure evolution between water- and oil-flow case in Figures 5-8, another important fact could be confirmed. The oil-flow case only involves the cross-linking feature with the formation of bubble cloud, which is categorized as an intermediate state between growth and break-down of coherent structure. Meanwhile, it also should be mentioned that either 3-dimensional vortex structure or cavitation structure only appears at the rear region corresponding to the collapse of cavitation. Thus, it could be concluded that the 3-dimensionality mainly comes from the collapse of vortex cavitation. On the contrary, the vortex in the water-flow case exhibits significant 3-dimensionality at the region immediately downstream of valve seat, even at the case of Figure 5d, where cavitation is absent thereby with the development of streamwise vortex. Consequently, the 3-dimensional vortex in the water-flow case primarily comes from the vortex evolution process, indicating the prevalent role of vortex dynamics in the vortex-cavitation interaction.