# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 4 Discussion
### Viscosity Effect on Attached Cavitation  
#### 4.3.1 Attached Cavitation at the Poppet Trailing Edge  
Concerning the attached cavitation at the poppet trailing edge, there are also two discrepancies between the water- and oil-flow case. Firstly, in oil cavitating flow, the attached cavitation at the poppet trailing edge is essentially negligible even at pressure drop over 40 bar, where vortex cavitation assumes a profound volume, as shown in Figures 6 and 8. In contrast, the attached cavitation at the poppet trailing edge displays a comparable size to either vortex cavitation or attached cavitation within the chamfered groove. Such difference is demonstrated by the time-mean vapor distribution in Figure 11 and could be interpreted according to the flow velocity contour. As shown in Figures 5-8, the potential core of oil jet flow has a wall shearing layer with larger thickness due to the higher viscous effect. The time-mean velocity distribution presented by Figure 12 indicates the oil-flow case has a smaller gradient at the poppet trailing edge, conducive to the suppression of the flow separation. Furthermore, the oil jet potential core  
Figure 10: Cavitation inception in four considered study cases. Cavitation inception in oil-flow case appears as ring structure, while water-flow case is featured with dispersive bubbles. follows a waving pattern at the poppet trailing edge, while the water potential core assumes a puff pattern, as analyzed above incorporating the summary of jet flow by [PERSON] and [PERSON] [25]. The intermittently-passing puff structure leads to stronger alternating velocity pulsation than the waving potential core, as shown in Figures 5-8. Both factors in oil flow case, including the thick wall shearing layer thickness and the waving pattern, tend to suppress the flow separation beyond the trailing edge as well as the induced attached cavitation. Secondly, due to the involvement of vortex cavitation from upstream, the attached cavitation in either water- or oil-flow case could not be fully developed independently to display the self-resonant unsteady behavior as widely observed in foil cavitation. The whole process involves an interaction between the two kinds of cavitation. The evolution between the attached cavitation and vortex cavitation follows a quite different pattern for the two cases. In water jet flow, when passing across the trailing edge, the created cavitating vortex lumps also involves vortex cavities almost neighboring to the poppet wall surface. Consequently, the wall side vortex cavity gets in direct contact with the attached cavitation, and produces a mergence with the attached cavitation straightforward, facilitating detachment of the cavity initiated at the poppet trailing edge. Meanwhile, the detached cavitation tends to draw the shed cavitating vortex towards to the poppet wall, further contributing to the growth of the amalgamated cavitation. As shown in Figure 13a, the attached cavitation at the poppet trailing edge with only an originally small size undergoes an aggressive expansion with the involvement of the shed cavitating vortex, producing a detached cavity with a massive volume. Comparing Figures 5 and 7, the shed vortex cavitation has a larger volume for the poppet with a chamfered groove, and the larger entrained cavitating vortex produces a higher contribution to the growth of the detached cavitation at the poppet trailing edge. Therefore, the attached cavitation at the poppet trailing edge has larger size in Figure 3a than in Figure 2. In the oil-flow case, only non-cavitating vortex is created at the poppet trailing edge, most of the time. And the shed vortex cavitation from upstream favors the detachment of the vortex due to flow separation at the poppet trailing edge. In return, the detached vortex gets entrained by the shed cavitating vortex. Consequently, the shed cavitating vortex experiences a secondary growth beyond the poppet trailing edge, as shown in Figure 14b.