## 1 Introduction  
The hydrodynamic torque converter is a typical coupled turbomachine that transfers power via the interaction between the blades and transmission medium. A hydrodynamic torque converter can provide a high output torque at a speed ratio (_SR_) of zero. Thus, the working machine has good launching performance and the advantages of fluid coupling and vibration damping. A hydrodynamic torque converter typically comprises three elements: a turbine driving the transmission, a pump driven by an engine, and a stator mounted on an overrunning clutch between the pump and turbine [1, 2, 3]. In recent years, hydrodynamic torque converters have been designed with increased launching performance, power density, and rotation speed, which can increase the risk of cavitation. Cavitation reduces the performance of a hydrodynamic torque converter, and its inception and development destabilize the rotation speed. Cavitation has been observed in many types of turbomachines, as shown in Figure 1. [PERSON] studied the relationship between the head drop of the inducer and cavitation under the different flow rates conditions. They found that under a specific range of the flow rate, the computational fluid dynamics (CFD) results agreed well with the test [4]. [PERSON] studied the cavitation behavior inside a scaled pump. A large number of cavitation bubbles at the trailing edge of the pump were observed in their research [5]. [PERSON] tested the fully developed cavitation form of the propeller. The sheet cavitation and tip vortex cavitation were found in their study [6]. [PERSON] employed a transparent torque converter model to observe the cavitation form and location at low rotational speeds. A large number of attached cavitation were found at the leading edge of the stator [7]. Other researchers have studied cavitation in axial flow pump, water pump, etc. [8; 9; 10]. Most of their research focuses on experimental observation and performance testing, but only a few reports are available on the cavitation mechanism and transient cavitation behavior of hydrodynamic torque converters. Microwave telemetry, nearfield acoustic measurement, and particle image velocimetry can be employed to predict the cavitation formation and process in a hydrodynamic torque converter [11; 12; 13; 14; 15; 16; 17]. Nearfield acoustic measurement has been employed to directly detect the cavitation effect of the flow field in a hydrodynamic torque converter with different blade geometries [18; 19; 20]. [PERSON] et al. performed numerical simulations to predict the cavitation behavior of a hydrodynamic torque converter at \(SR=0\) and established a relationship between cavitation and vibration [21]. [PERSON] et al. studied the relationship between the cavitation and performance of a hydrodynamic torque converter. They reported that cavitation occurs mainly near the stator nose when \(SR=0\). Cavitation can cause the stall capacity constant (\(CC_{0}\)) and stall torque ratio (\(TR_{0}\)) to drop drastically with advanced flow separation near the stator [22]. [PERSON] et al. proposed removing the interface between the  
Figure 1: Cavitation in typical turbomachines. (**a**) cavitation generated in inducer, (**b**) cavitation in tip vortices and marine propeller blades, (**c**) cavitation damage of a marine propeller blade, and (**d**) cavitation generated in hydrodynamic torque converter. turbine and stator when simulating the cavitation of a hydrodynamic torque converter and setting a reference pressure level at the pump-stator interface near the shell, which can increase the convergence and accuracy of a two-phase flow cavitation simulation [23]. Previous research mainly employed a periodic steady-state single-flow passage or a series of experimental tests to predict the cavitation flow field of a hydrodynamic torque converter. However, the strong interaction between the blades and flow field means that the cavitation in a hydrodynamic torque converter cannot be in a stable state, and the cavitation bubbles must exhibit dynamic behavior.