# Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study
## 1 Introduction  
Traditionally, mineral oil is used extensively as fluid medium in hydraulic transmission due to its favorable properties. Recently, water hydraulics has aroused persistently growing attention due to its potential in a variety of engineering application, including mining engineering, underwater vehicle, and shipping industry. However, the viscosity of common mineral oil is larger than water with a factor over 10, contributing to a significant difference in the overall performance between water- and oil-hydraulic components. Most findings in previous research on hydraulic transmission were achieved with mineral oil as working fluid, and thus might be inappropriate for water hydraulics. In addition to the deteriorated lubrication and higher Reynolds Number, water hydraulics is subjected to a higher risk of cavitation than oil hydraulics, and the potential cavitation erosion poses a great challenge for life span of water hydraulic components, particularly for poppet valves, which widely serve as pressure regulator and suffer from an extremely huge pressure drop on the order up to \(10^{9}\) Pa. The explicit acknowledgement of cavitation inside water- or oil-poppet valves was achieved for decades. [PERSON] and [PERSON] [1; 2] performed the first systematic investigation on cavitating flow through an oil poppet valve with a half-cut prototype. The experimental results revealed that the occurrence of cavitation leads to remarkable variation in flow performance and flow force. [PERSON] et al. [3] investigated the flow structure inside an oil-poppet valve by means of high-speed images and CFD analysis. An optimization strategy with superior cavitation suppression was proposed based on the obtained findings. In the recent years, the progress in water hydraulic transmission inspired studies on cavitation inside water poppet valves. [PERSON] et al. [4] revealed the cavitation morphology at varied pressure drop based on high speed images, and investigated the difference in flow discharge coefficient for water poppet valves with different structures. [PERSON] et al. [5] proposed a novel poppet valve structure with improved cavitation suppression. [PERSON] et al. [6] conducted a CFD analysis of a water poppet valve with a two-step valve seat structure and established an optimal design with criteria in terms of cavitation suppression based on pressure distribution. [PERSON] et al. [7] examined the influence of inlet pressure fluctuation on cavitation and flow structure inside water poppet valves with varied structures. [PERSON] et al. [8] studied the cavitation effect on flow force for water poppet valves with different valve seat structures. Such non-exhaustive list of literatures reveals the higher cavitation intensity in water poppet valves under the same operating condition. But most of the prior studies were only concerned with cavitation of either water- or oil-poppet valves. To the best knowledge of the authors, [PERSON] et al. [4] contributed to the only study that involved a comprehensive comparison between water- and oil-cavitating flow inside poppet valves. The comparative study quantified the difference in terms of pressure distribution and variation in flow discharge coefficient with pressure drop. Such difference is undeniably correlated with alteration in flow structure due to viscous property. However, the small dimension on the order of \(10^{-3}\) or less, the transient evolving dynamics, and strong 3-dimensionality cause great difficulties in exploring the governing mechanism solely based on experimental methods. With the continual development of computer technology and CFD method, numerical simulation becomes an alternative for analysis of micro-scale flow dynamics and finds wide application in jet flow through small-size orifice, contributing to fundamental examination of viscosity-relevant effects on a series of jet flow cases. [PERSON] et al. [9] investigated the changes of cavitation distribution with viscosity in the pilot stage of deflector servo-valve. [PERSON] et al.