# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 4 Discussion
### Viscosity Effect on Attached Cavitation
#### 4.3.2 Attached Cavitation within the Chamfered Groove  
Figure 15 presents the time-mean cavitation inside the chamfered groove with a partial enlarged view of Figure 12. The oil-flow case is featured with an extremely high vapor content within the first half of the chamfered groove, indicating the constant presence of attached cavitation at the immediate downstream of the inlet corner. On the contrary, the water-flow case displays a more dispersive distribution of vapor. Figure 16 offers the transient evolution of cavitation within the chamfered groove. The attached cavity in the oil-flow case, featured with a sheet appearance, adheres constantly to the first half of the chamfered groove regardless of the transient variation. In contrast, the attach cavitation in the water flow case exhibits a more consecutive detachment behavior, instead of a persistent attachment to the inlet corner of the chamfered groove. The increased stability of the attached cavity at the inlet corner in the oil-flow case is associated with the elevated shearing intensity due to high viscosity, which produces a lasting rupture of the fluid and contributes to a high liquid tension even at the rear part. Such a phenomenon is consistent with the theory of shearing-stress-induced cavitation developed by [PERSON] (2018). As reported by [PERSON] et al. (2018), the flow through a rectangle orifice produces stable sheet cavity persistently attached on the wall surface under choking condition, and periodic shedding cavity under non-choking condition, respectively. Either increase in velocity gradient and viscosity contributes to the shearing effect, and thus, the attached cavitation inside the chamfered groove in the oil-flow case appears as a sheet. Figure 14: Time-variation of attached cavitation at poppet trailing edge. (**a**–**f**) water flow case, sharp valve seat, \(\Delta\)p = 45 bar, time increment \(\Delta\)T = 4 \(\times\) 10\({}^{-6}\) s; (**g**–**l**) oil flow case, sharp valve seat, \(\Delta\)p = 45 bar, time increment \(\Delta\)T = 5 \(\times\) 10\({}^{-6}\) s.  
In addition, in water-flow case, the downstream vortex cavitation at the free shearing side extends to the dispersive distributed cavitation within the chamfered groove, as shown in Figure 15a. However, a clear discontinuity in the oil-flow case could be identified between the cavitation within the chamfered groove and the downstream vortex cavitation at the free shearing layer. Such difference suggests a stronger detachment behavior in the water-flow case. As shown in Figure 16a, due to the stronger shedding of cavity, the detachment behavior involves almost the whole attached cavitation in the water-flow case. As shown in Figure 16a, the scattered vortex structure at the rear part is distributed with chaotic orientation, similar to the case of detached cavitation in a rectangle orifice attributed by [PERSON] et al. [26] to vortex dynamics. Thus, the rear part of attached cavitation in water-flow case that consists of several separated cavitating vortex structures, is featured with high incoherence. As a comparison, merely the rear attached cavity is occasionally shed outside the groove into the downstream free shearing layer in the oil-flow case, as shown in Figure 16b, and only noncavitating vortex is shed downstream at the rest time. The corresponding vortex structure reveals the presence of detached vortex in the form of circumferential ring with favorable coherence irrelevant of the presence of detached cavitation, indicating the orderly state of the flow. The shed cavitation inside the chamfered groove from the attached cavitation may be governed by detached vortex dynamics from the groove inlet in the water flow case, which is featured with strong 3-dimensionality. In the oil flow case, the stable sheet cavitation produces a source of instability at the rear part, responsible for the generation of shed vortex ring with preservation of high coherence.