The problem of high-speed transmedium water entry widely exists in the fields such as ocean engineering, aerospace, and more. The mechanism of multiphase flow and fluid structure interaction in high-speed cross medium water entry is a research hotspot in this field, which is of great significance for load prediction, ballistic stability evaluation, structural strength verification, and safety design of transmedium weapons and spacecraft. This paper focuses on the fundamental key mechanical issues involved in high-speed transmedium water entry, focusing on the research status of multiphase flow and cavity evolution, impact load and load reduction methods, motion stability and fluid structure interaction response, and numerical methods of fluid structure interaction. Suggestions and prospects are proposed for the existing problems, aiming to provide fundamental references for related research and design of high-speed transmedium water entry.

This paper starts with the importance and basic physical phenomena of underwater explosion, explaining the background and significance of ship damage and protection research under underwater explosion loads, research progress and status, as well as key challenges. To address these challenges, the paper elaborates on popular underwater explosion theories, models and methods. In terms of theoretical and computational research, a unified theory of bubble dynamics is presented, as well as a transient strong non-linear gas-liquid-solid fully coupled model and numerical method for underwater explosion, which have been used to develop a fundamental industrial software FSLAB capable of solving practical problems in fluid-structure interaction. In terms of experimental research, underwater explosion surrogate experimental methods and model testing methods are elaborated. Based on this, theoretical analysis, computational and experimental results in the field of ship damage and protection under underwater explosion loads are presented and discussed, aiming to provide references for underwater explosion-related research.

High-speed hydrodynamics and its corresponding complex fluid-structure interactions (FSI) are challenging topics associated with naval architecture and ocean engineering, typically characterized by large deformations, moving boundaries, strong convection and multiple fluid media. Since traditional mesh-based numerical methods possess limited ability to accurately simulate such strongly nonlinear problems, it is imperative to develop meshless numerical schemes with high fidelity and robustness to tackle this dilemma. As one of the most promising truly meshless methods, smoothed particle hydrodynamics (SPH) shows apparent advantages in high-speed hydrodynamics problems thanks to its Lagrangian nature. In the present paper, the attention is particularly focused on the latest advances of several SPH techniques with respect to the following high-speed hydrodynamics problems: vessel-induced waves and wakes, the water entry process of projectiles, and underwater explosion and its resulting structural damage; in addition, the future prospects of SPH are provided in the last part of the paper.