**1. Introduction**

Waterjet propulsion is largely used in the military and civil fields because of its simple transmission mechanism, low noise, and good maneuverability [1–3]. While compared to propeller propulsion waterjet propulsion can utilize the inflow stamping to increase the anti-cavitation capability at high speeds, cavitation problems may arise under some special conditions such as "low ship speed and high rotating speed" [4,5]. This special working condition usually occurs in the process of ship acceleration or turning. At this time, the net positive suction head of the waterjet propulsion system is obviously lower than the *NPSHr* (Net Positive Suction Head Required) of the pump at this speed, and the propulsion pump is prone to cavitation. Cavitation will cause an increase in shaft power, a reduction in flow rate, head, and efficiency, and a decrease in thrust. At the same time, it will also cause noise and vibration in the pump [6–10].

With the development of technology and the problems encountered in practical applications, there is an increasing demand for pump performance indicators in engineering, and cavitation has gradually become an important factor limiting the further improvement of pump performance [11,12]. Cavitation refers to the process of forming vapor bubbles in the low pressure region of the liquid flow field [13]. Previous studies have shown that the cavitation process occurring in hydraulic machinery is harmful, and its damage is mainly manifested in three aspects. First, cavitation can cause damage to the surface of hydraulic machinery materials [14,15]. Secondly, cavitation will lead to a significant reduction in hydraulic performance of hydraulic machinery [16,17]. Thirdly, cavitation not only affects the steady state fluid flow, but also affects the unsteady flow characteristics or dynamic response of the flow [18,19]. For a long time, scholars at home and abroad have started detailed and in-depth research on the mechanism of hydraulic machinery cavitation, and accumulated a large amount of knowledge and experience through the use of numerical simulations and experiments [20–23]. As early as 1917, Rayleigh [24] conducted a theoretical analysis of the cavitation. Rayleigh solved the flow problem of a spherical cavity infinite flow field with a vacuum or internal pressure constant and gave the Rayleigh bubble athletics equation. This laid the foundation for the modern numerical simulation of cavitation dynamics. Bal et al. [25] described a method for simulating the uniform motion of a two-dimensional or three-dimensional cavitating hydrofoil under a free surface. This method is suitable for 2D and 3D hydrofoils under fully wetted or cavitating flow conditions. Brennen et al. [26] simulated the unsteady cavitation flow in the two-dimensional potential flow and successfully predicted the occurrence of two-dimensional cavitation. Singhal et al. [27] and Schmidt et al. [28] performed full-flow numerical calculation and analysis of the cavitation of the axial flow pump under design conditions, and predicted the development of cavitation flow and cavitation occurrence region in the flow passage. Although numerical simulation technology can predict the occurrence and development of cavitation, the experiment is still one of the most efficient way to study cavitation. Gopalan [29] used the PIV (Particle Image Velocimetry) and high-speed photography to study the flow structure in the closure region of sheet cavitation. The results showed that the collapse of cavitation in the enclosed region is the main cause of the whirlpool disaster. Escaler et al. [30] evaluated the cavitation detection in actual hydraulic turbines based on analysis of structural vibrations, acoustic emissions, and hydrodynamic pressures measured in the machine. They validated the proposed technique by experimenting with real prototypes of different types of cavitation. Through the performance test of centrifugal pumps, Johnson et al. [31] described the hydraulic characteristics of centrifugal pumps in cavitation state more accurately and systematically.

As effective research methods for vibration, noise, and cavitation, numerical simulation and model tests have been adopted by many experts and scholars [32–35]. Experts and scholars have made great progress in the study of cavitation, but the cavitation problem under special conditions in the waterjet propulsion system is still an important factor that restricts the full play of its hydraulic performance. Moreover, the present research on cavitation of waterjet propulsion system mainly focuses on the cavitation characteristics of waterjet propulsion pump. There are few studies on cavitation of waterjet propulsion system with inlet passage. In particular, the influence of inlet passage on the cavitation performance of propulsion pump system and pump is worth further study. Hence, it is of great practical value and academic significance to study the cavitation process of the waterjet propulsion system with and without the inlet passage. In this paper, the cavitation characteristics of the whole flow field of the waterjet propulsion pump system were analyzed by means of a two-phase flow numerical simulation.
