# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 1 Introduction
[10] carried out a simulation on cavitating flow through the flapper-nozzle valve and exhibited the viscous effect on transient cavitation distribution and small-scale vortex, which in return enhances the pressure pulsation and noise level. [PERSON] et al. [11] compared the nozzle atomization of several liquids and confirmed the influence of fuel viscosity on the spreading of cloud cavitation. The recent progress in numerical method in simulating the cavitating jet inside an oil-poppet valve by [PERSON] et al. [12; 13] clarified the underlying mechanism of cavitation dynamics, which involves a complicated inter-relationship between vortex and cavitation. Enlightened by such findings, it is reasonable to speculate that the vortex evolution might vary significantly for the two fluids with quite different viscosity property, since the vortex evolution process depends largely on local Reynolds number. Although the difference in the overall behavior, such as flow discharge variation between oil- and water-poppet valves, which was widely reported in previous studies, the physical process behind remains a vital but unclear issue. Undoubtedly, the governing mechanism involves the alteration of both micro- and macro-scale flow structure due to viscous effect, which in combination with cavitation further contributes to the overall influence on the cavitating jet, as widely reviewed in transitional cavitating flow of circular jet [14]. However, the issue associated with the transient cavitating flow is seldom addressed in previous studies, leading to insufficient basis to predict accurately the detailed changes due to such significant difference in viscosity. Considering the strong shearing effect of the jet potential core through the poppet valve due to the viscosity of selected mineral oil #46 with a factor over 40 larger than water, cavitation in oil-poppet valves might be produced as a consequence of the strong shearing stress, which ruptures the liquid straightforward along the maximum principal stress [15]. Besides, according to previous investigations, cavitation could occur at several separated regions inside practical poppet valves, which might be governed by quite varying flow physics. Under condition of high cavitation intensity, it might involve a potential interaction between cavitation initiated at quite distanced regions by varying mechanism, producing a further degree of complexity to the cavitating flow. Particularly, the variation in viscosity effect between oil- and water-hydraulics possibly produces a remarkable influence on such interactive behavior, which is an untouched problem with both scientific and industrial interest. Motivated by such hypotheses, a 3-dimensional and transient simulation is performed for both water- and oil-cavitating flow inside poppet valves with the following investigation strategy. Firstly, the cavitation morphology is compared between the water- and oil-flow case individually for each of the three kinds of cavitation structures that originates from separated regions, to analyze the cavitation triggering mechanism and generalize the difference in cavitation structure. Subsequently, several crucial variations in the associated flow dynamics are established based on the potential core and the vortex structure, which shed some light on the mechanism underlying the generalized difference in cavitation structure. On such a basis, the unsteady cavitation behaviors including the interactive evolution between cavitation structures arising from different regions and the overall periodic performance are examined with the obtained cavitation mechanisms. With the listed investigation strategy, the current study aims at a comprehensive understanding on the mechanism how the variation in viscosity changes the cavitating flow inside a poppet valve.