Unsteady cavitating flow has always been discussed in the field of hydrodynamics. It is a frontier fundamental problem which can be applied in many engineering areas. Cavitation is one of the most important factors that affects the control of high-speed underwater launched vehicles. The phenomenon such as vibration, noise, and erosion caused by cavitation has also attracted attention in recent years. Unlike most simple configurations such as wedges and hydrofoil, which are currently studied in most cavitation flow studies, cavitation usually occurs near high-speed navigation bodies or high-speed hydraulic machinery components in practical engineering problems, including complex geometrical walls as well as moving wall surfaces. Complex problems are studied involves turbulence, multiphase flow, phase change, complex boundary conditions, fluid-solid coupling, etc. The physical phenomena are extremely complicated of unsteady cavitating flow, and we still lack the understanding of the flow mechanism. More in-depth study is needed in the future.

Interaction between the vortex structure and the cavitating flow around the vehicles with complex shape often leads to new flow characteristics, in which the study is not thorough enough. Besides, it is much more difficult for us to considering the vortex structure and the evolution of the cavitating flow around moving body. Both experimental methods and simulations of the problem are more difficult, resulting in a lack of research. Furthermore, the high-frequency excitation caused by the instability of the cavitating flow when considering the cavitating flow around the fluid-solid coupling wall make the problem more complicated. Although research in the area has gradually increased in recent years, related problems still need further in-depth research. The laws and mechanisms of the interaction between cavitating flow and complex structures are not clear, and the research results are relatively scarce. In this paper, we aim to study the mechanism of the cavitating flow around various underwater vehicles under the effect of three types of complex boundary conditions, which are vehicles with complex shape, moving wall and fluid-solid coupling wall. We will study the problem from the shallow to the deep, and summarize the characteristic rules, so as to systematically deepen the understanding of cavitation flow in complex boundary conditions. The study provides the basic basis for solving cavitation-related engineering problems. Specifically include the following problems:

(1) Cavitating flow around axisymmetric projectile near wall is studied in this paper to understand the mechanism of the cavitating flow around the vehicles with complex shape. Based on the Split Hopkinson Pressure Bar (SHPB) launch system, an underwater launched water tank experiment was conducted. Two types of flow patterns of the cavitating flow around axisymmetric bodies near wall were found. The interPhaseChangeFoam solver of the open source software OpenFOAM is used to do the simulation. Numerical methods include large eddy simulation method (LES), fluid volume method (VOF), mass transfer cavitation model for the evaporation and condensation phase transition process and pressure implicit with splitting of operators (PISO) algorithm to solve the incompressible NS equations are used here. By comparing the experimental data and simulated results, the mechanism of the cavity formation and evolution are analyzed. The characteristics of the blockage effect due to the strong confinement effect in the near wall are obtained, and the evolution mechanism of the interaction between the back jet, the vacuole and the vortex motion in the process of local cavitation.

(2) Unsteady cavitating flow around a highly skewed propeller in non-uniform wake is further studied which focusing on the cavitating flow around a moving wall boundary. On the basis of the aforementioned numerical methods, the rotating dynamic mesh method and sliding wall interpolation approach are introduced. A more accurate result than the traditional RANS method are obtained by applying the LES approach, which can capture the initial details of the incepted cavity and tip vortex cavity. Main control parameters of the cavitating flow around the highly skewed propeller in non-uniform wake are obtained by analyzing the simulated flow field results in details when the cavitation number is 2.99. The interactions between the cavity evolution and the vortex structure/ pressure pulsation are discussed.

(3) We combine the cavitating flow with the study of vortex-induced vibrations of a circular cylinder here to understand the mechanism of the cavitating flow around the fluid-solid coupling wall. The dimensionless control parameters of the aforementioned problem are obtained in this paper. Based on the cavitating flow solver PSIO algorithm in OpenFOAM, a high-efficiency fluid-solid coupled numerical simulation program using weak coupling method was developed to calculate the cavitation in a vortex-induced vibrations of circular cylinder problem. Regularity of the non-dimensional main control parameters of the cavitating flow are investigated. Moreover, the effect of cavitation on vortex-induced vibration are obtained by comparing the different simulated results of the vortex-induced vibrations of circular cylinder problem with and without cavitation.