# Evaluation and Validation of Viscous Oil Cavitation Model Used in Torque Converter
## 1 Introduction  
Hydraulic torque converter is a closed-loop fluid machinery which transfers power by the conversion between fluid kinetic energy and mechanical energy, and it serves as a core component of automatic transmission and hydraulic transmission (Figure 1). Hydraulic torque converters are widely used in transmission systems of passenger cars, off-road vehicles, commercial vehicles and construction machinery because it is able to provide continuously variable transmission, self-adaption to load, and absorb vibration from the engine [1]. The internal flow state of a hydraulic torque converter is very complicated, the fluid pressure, velocity and temperature vary drastically over different operating conditions. The increasing demand for higher power density leads to higher flow velocity and lower local pressure, which makes the flow inside the torque converter more prone to cavitation. Cavitation will occur when the local pressure is lower than the vapor pressure of the working fluid, and it will not only reduce the performance of torque converter, but also cause vibration, noise, and a series of unfavorable issues [2, 3]. Cavitation is a transient phase change phenomenon, vapor bubbles occur, grow, and collapse in liquid with the variation of local pressure. Hydraulic transmission oil is prone to cavitation because it contains a huge amount of dissolved gas and undissolved micro gas bubbles. The periodic growth and collapse of cavitation will cause vibration and noise of torque converter, and the high local pressure is generated during cavitation collapse, all of these will lead to poor performance, short service life, and cavitation damage or even damage the torque converter in severe cases. As the development of hydraulic torque converter is toward high capacity, high speed, and high power-weight ratio, cavitation in torque converters has become one of the key issues affecting performance and reliability. Therefore, it is of great theoretical and practical significance to study the cavitation flow phenomenon in a torque converter. In recent decades, more and more attention has been paid to the research on the cavitation of hydraulic torque converter and the parameters of cavitation model [4]. [PERSON] made a systematic study on cavitation of torque converter under stall operating condition by using various experimental methods [5]. [PERSON] predicted the cavitation behavior of automotive torque converter under stall, and studied the cavitation, vibration, and the relationship between them through experiments [6]. [PERSON] found cavitation grew near the stator leading edge and expanded with the increase of pump speed by constant enthalpy vaporization model, resulting in severe performance degradation in hydraulic torque converter [7]. [PERSON] studied the cavitation characteristics of a torque converter under different charging pressures and operating conditions by CFD method and Rayleigh-Plesset cavitation model [8; 9]. [PERSON] studied the cavitation characteristics of torque converter under various working conditions by Zwart model, found out cavitation mainly occurred in stator, which leads to serious deterioration of torque capability [10]. [PERSON] studied the mechanism of bubble breakup in the hydraulic torque converter by implanting bubbles in hydraulic torque converter, and calculated the bubble breakdown process by VOF model [11]. [PERSON] studied the influence of stator blade shape on the cavitation process of a torque converter by Zwart model and found out cavitation was directly affected by internal mass flow rate, then he improved the torque converter design considering cavitation [12; 13]. [PERSON] [14] and [PERSON] [15] studied the flow structure and cavitation phenomena in a hydraulic retarder by Zwart model, and found cavitation appeared near the back and root of rotor blade, and the vapor volume fraction increased with the increase of rotor speed.