Yingfei Zan

This study investigates the multiphase coupling characteristics and evolution mechanisms of the flow field induced by high-pressure gas release in a confined space. The gas supply and drainage mechanisms that occur during the gas–liquid displacement are systematically clarified through a combined experimental–numerical methodology that accounts for the pressure decay and air compressibility effects of the air tank on the water drainage process. The results demonstrate that the transient pressure characteristics of the gas supply system play a dominant role in determining the drainage dynamics. On this basis, the ascent time of pressure-driven drainage in the water tank can be predicted with a maximum error of 1.4%. Furthermore, the concept of an effective pressure-driven drainage interval is proposed. Theoretical derivation and dimensional analysis reveal that this effective pressure-driven drainage interval is inversely correlated with the drainage duration. The results presented herein allow us to establish a framework for understanding pressure-driven ballast systems, and provide references for pressure-based surfacing design and strategy optimization.

Floating offshore wind turbines (FOWTs) experience six-degree-of-freedom motions and related forces acting on their mooring systems. Under combined wind, wave, and current loading, platform motions and mooring line tensions are dynamically coupled. To support ballast control decisions without repeatedly running high-fidelity coupled simulations, this paper developed a specialized computational framework combining FAST-AQWA time-domain simulation, short-term mooring tension prediction, and ballast control optimization. Five predictive models were trained and evaluated using the same sliding window dataset. After training, the predicted mooring tension data were used for ballast control calculations. In this study, the bidirectional long short-term memory (BiLSTM) model showed the highest prediction accuracy for mooring tension and the Model Prediction Control (MPC) produced a smoother control action and significantly reduced the amplitude of low-frequency roll and pitch. The proposed framework provides a practical approach for combining short-term response prediction with ballast control and demonstrates that explicit mooring tension constraints must be incorporated into the design considerations of subsequent control systems.

The maneuverability of a submarine in the vertical plane is a key indicator of navigation safety. However, existing studies typically evaluate maneuvering performance based on hydrodynamic coefficients, often neglecting the flow-field evolution induced by different steering strategies. In this study, a high-fidelity numerical model for the vertical-plane motion of the DARPA SUBOFF submarine is established using the Reynolds-Averaged Navier–Stokes (RANS) method and validated against benchmark data. Unlike traditional analyses that employ a fixed rudder angle, this work systematically compares three steering strategies with continuously varying rudder angles—trapezoidal, step, and linear steering—examining their motion responses, hydrodynamic performance, and unsteady flow-field evolution. The results show that, although step steering produces the fastest response with the strongest transient characteristics, it also triggers pronounced flow separation and significant unsteady effects. Linear steering yields a smoother but the weakest motion response, with reduced rudder effectiveness and a noticeable lag effect. In contrast, trapezoidal steering maintains a stable flow field around the submarine, with uniformly concentrated vorticity distribution, ensuring smooth and safe motion and achieving a favorable balance between response speed and flow stability. The findings provide theoretical reference for research on submarine vertical-plane steering motion, rudder-angle control, and flow-field stability.

The process of compressed air being blown into a body of water generates a complex gas–liquid two-phase flow field. This paper describes numerical simulations of compressed air blowing water using the volume-of-fluid multiphase flow method. The accuracy and convergence of the numerical method are verified by comparison with experimental data and high-speed imaging results. Based on a visualized symmetric water tank blowing system, the gas–liquid two-phase flow characteristics inside the tank are analyzed. In the initial stage of the blowing process, the gas jet flow at the center is minimally affected by the gas–liquid mixing induced by the initial momentum of the compressed air. Subsequently, the gas jet mixes with the surrounding liquid water, forming an elliptical gas–liquid mixture in the middle of the tank. As the jet develops, it evolves into a turbulent mixing region influenced by the buoyancy of the surrounding environment. Under the effect of gas turbulence and buoyancy, a cavity structure forms inside the tank, causing the gas–liquid interface to separate. The liquid level rapidly drops as gas accumulates in the tank, and the water is eventually expelled. A visualized ballast tank blowing system is designed, and the gas–liquid two-phase flow patterns inside the tank are discussed. The variations in internal pressure and the free liquid surface are determined, and the air supply and drainage mechanisms of the water tank, along with their influencing factors, are clarified. The results verify the correctness of the numerical model.

Water entry is a complex process in marine engineering, marked by highly dynamic and intricate flow behavior. This study investigates the dynamic characteristics of cavity evolution during the water entry of a porous plate through experimental and numerical methods. A numerical model based on the smoothed particle hydrodynamics (SPH) method was developed and validated against experimental data under various velocities. The results indicate that the model accurately captures key stages of cavity evolution, showing good agreement with experiments across different velocity values. Detailed analysis of cavity and jet evolution reveals distinct mechanisms of jets driven by pressure distribution, along with corresponding energy dissipation patterns. Parametric studies indicate that cavity width, pinch-off time, and jet height all exhibit a linear positive correlation with water-entry velocity, with jet height being more sensitive to variations in velocity. The cavity and jet of the porous plate are smaller than those of the flat plate under the same conditions. Furthermore, post-pinch-off flow field evolution exhibits four typical dynamic zones, where interactions between upward and downward flows induce significant velocity fluctuations of the porous plate. These findings provide theoretical insights into the hydrodynamic behavior of porous structures during water entry and offer valuable guidance for modeling and optimizing impact problems in marine engineering applications.

The flow memory effect is a kind of viscous force caused by acceleration between continuous phases and underwater vehicles and the development of the boundary layer near the interfacial surface. This paper establishes a 6‐degrees‐of‐freedom unsteady equation of motion describing the flow memory effect on viscous hydrodynamic loads. Motion simulations based on the unsteady and steady equations of motion for a work‐class open‐frame remotely operated vehicle are studied. Comparative studies show that the flow memory effect decreases the roll amplitude by over 50% at the natural frequency and has a greater impact on the motions in directions without control forces. The flow memory increases the lateral distance of rotation motions by over 10%. Under the flow memory effect, velocity fluctuations that persist over 2–5 times the length of the remotely operated vehicle are induced after the thrust control stops. The application of the unsteady equation of motion as part of a controller is verified through simulation experiments. A sudden motion in the process of rotation is designed to study the improvement of the control ability of the unsteady model. The experimental results suggest a performance improvement of ~30% over the steady‐model‐based controller when the roll angle error reaches the extreme value. The lower sensitivity of the unsteady‐model‐based controller is proved by motion control under multiple gains.

Water entry slamming is a complicated issue in marine engineering, characterized by significant impact loads and complex flow. This paper establishes a 3D numerical model of flat plate water entry slamming based on smoothed particle hydrodynamics (SPH), and the dynamics and flow field evolution are analyzed during water entry. The results indicate that SPH effectively captures the key dynamic characteristics of flat plate water entry. The experimental data validate the model, and the SPH particles reproduce the phenomena of jet formation, cavity development, and fluid splashing. The observed pressure is maximum at the center of the flat plate, and the maximum pressure and vertical force of the flat plate exhibit a quadratic relationship with the water entry velocity. The flow field evolution from initial jet formation at the time of slamming to droplet splashing shows obvious stages. As the water entry depth of the flat plate increases, the growth rates of the cavity width and splash height gradually slow under fluid viscosity and drag. The water entry velocity has the greatest influence on droplet splashing, whereas its influence on the jet separation point and the position of the free liquid surface is less significant.

In marine engineering, the installation of structures inevitably involves the process of water exit. This paper studies the vertical force, the shape of the free surface, and the evolution of the water entrained in a cavity in the process of lifting a structure, so as to provide guidance for practical engineering operations. Using a 1:8 experimental model, this paper derives the governing equations based on the Reynolds-averaged Navier–Stokes approach and uses the volume of fluid method to capture the shape change of the free surface. The vertical forces obtained at different lifting speeds are found to be in good agreement with the results of previous model tests. The results show that the numerical simulation method and mesh generation described in this paper can simulate the changes in the physical quantities associated with the structure in the process of water exit. The vertical force on the structure increases nonlinearly as the lifting speed rises, and the maximum lifting speed is conservatively estimated to be 0.034 m/s using the Det Norske Veritas recommended method. The maximum vertical force occurs as the whole structure leaves the water. The water entrained in the structure is mainly located at the sides and bottom. The lifting velocity plays an important role in the water exit process. The water exit force first increases and then decreases to a stable value as the lifting velocity increases, while the maximum water exit force increases nonlinearly.