# 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  
#### 4.1.1 Difference in Vortex Cavitation and Potential Core in the Sharp Valve Seat Configuration  
As shown in Figure 5, cavitation morphology is mainly located at two positions for water-flow inside the poppet valve with a sharp valve seat, categorized as vortex cavitation in the shearing region of the potential core and attached cavitation at the poppet trailing edge, respectively. The vortex cavitation, according to simulation results of Figure 5, follows a discontinuous distribution, while the continuous cavitation morphology in experimental images of Figure 5b might be misguiding due to the background blurring of super-imposed cavitation structure at varied azimuthal angles. In the oil-flow case, attached cavitation at the poppet trailing edge becomes almost negligible even at pressure drop of 45 bar, as shown in Figure 6. The vortex cavitation is closely correlated with vortex dynamics, and specifically with the coherent structure in the case of jet flow [14]. In contrast, the attached cavitation is mainly governed by inertial factor or flow separation, as in the typical case of  
Figure 4: Comparison between simulation and experimental data in terms of time-average pressure distribution, indicating consistence between experimental measurement and simulation. circular nozzle cavitation and hydrofoil cavitation. The potential core undergoes a more aggressive break-up behavior prior to the poppet trailing edge in the water-flow case, contributing to the puff structure, in comparison to the waving pattern in the oil flow case, as indicated by the sectional velocity contour in Figures 5 and 6. With a careful examination of the two cases, several discrepancies regarding vortex cavitation could be generalized as follows. Firstly, vortex cavitation is incepted earlier in the water flow at approximately 0.2 mm (quite close to the value 0.2-0.3 mm in experimental images as in Figure 2) after the inlet clearance at 45-bar pressure drop, while vortex cavitation is incepted at approximately 1.3 mm in oil flow case. The difference is mainly due to the higher Reynolds number in water flow, which leads to a stronger laminar-turbulent transition process of the jet flow. Consequently, vortex structure is developed quicker for water jet flow, and the induced pressure drop at the vortex core with a sufficient level to trigger cavitation occurs closer to the clearance, causing an earlier cavitation inception for the water-flow case. Secondly, the vortex cavitation is primarily limited to the free shearing side of the potential core for the water-flow case, whereas the oil-flow case also exhibits cavitation structure with a comparable size at the wall shearing side, and such delicately arranged vortex cavitation rings constitute a distinctive paired structure. Furthermore, the vortex cavitation in the oil-flow case has stronger axis-symmetry, and thus enhanced circumferential coherence. Thirdly, the vortex cavitation at the free shearing side in oil-flow case tends to assume a thin layer feature as shown in Figure 6a, which might be a result of the higher shearing intensity from the larger liquid viscosity. Fourthly, through multiple pairing mechanism of vortex cavitation, vortex cavitation in the form of a large bubble cluster is created at the rear part close to the poppet trailing edge at  
Figure 5: Simulation results for water cavitating flow inside a poppet valve with a sharp valve seat. Cavitation and vortex structure are identified with iso-surface of \(\alpha=70\%\) and Q = 1 \(\times\) 10\({}^{10}\). Common puff pattern in each operating condition is produced at downstream region mainly due to severe transition process. (**a**) 45 bar pressure drop; (**b**) 42 bar pressure drop; (**c**) 40 bar pressure drop; (**d**) 38 bar pressure drop. pressure drop of 45 bar in both water- and oil-flow case.