This paper investigates the characteristics of the flow field induced by a supersonic jet at the tail of an underwater axisymmetric vehicles under different pressure ratio conditions, based on the volume of fluid (VOF) multiphase flow model. The study analyzes the evolution of the flow field at the tail of the axisymmetric vehicles and explores the morphological distribution of the induced cavity at various time instances under different pressure ratios. The research findings indicate a close relationship between the jet’s tail cavity morphology and the nozzle pressure ratio. When the pressure ratio is relatively low, the tail cavity exhibits a conical shape similar to the supercavitation. As the pressure ratio increases, the influence of the tail vortex on the jet gradually diminishes, and the high-pressure ratio jet evolves into a pulsating jet under the action of shear entrainment driven by the Kelvin-Helmholtz instability. Within the pulsating jet flow field, phenomena such as “back-attack” and pressure disturbances in the water medium lead to a “positive feedback” effect on the pulsation characteristics of the jet. After the high-pressure ratio jet transforms into a pulsating jet, the degree of jet necking fluctuates significantly with time, and the initial bubble breakup results in a random distribution of the jet necking location with time. With a further increase in pressure ratio, the morphology of the pulsating tail cavity tends to stabilize, and the pressure fluctuation amplitude at the bottom of the axisymmetric vehicles decreases.

The grid fins underwater launch technology scheme can increase the motion stability of underwater vehicles, which is one of the effective ways to improve the adaptability of the vehicles to the launch environment conditions. Aiming at the dynamic deployment process of the grid fins underwater application, the mathematical model of the kinematic parameters of the deployment process was established, and the ground deployment mechanism verification test of the grid fins was designed. The scene of the deployment process of the grid fins in the air medium and the water medium, the variation law of the motion parameters and the deployment time were studied. The influence of the water medium on the parameters of the deployment process of the grid fins was obtained, providing useful experience for the analysis of the dynamic deployment process of the underwater grid fins and the analysis of the motion parameters of the deployment process.

The underwater wall effect has a significant impact on the survival activities of aquatic animals and the task execution of underwater vehicles. Reasonable utilization of underwater wall effects can achieve energy conservation and consumption reduction, while improper control can lead to safety accidents. This paper mainly reviews the achievements of underwater wall effects in various research directions, including research on live observations of aquatic animals swimming near the ground, wall effects on simplified biomimetic models and wall effects on underwater vehicles. It also presents the research progress on the application of underwater wall effects in biomimetic engineering. At the end, the main problems and challenges faced by the research of underwater wall effects are proposed.

Single-layer cylindrical shells are common structural form of underwater vehicles, which have more advantages than double-layer cylindrical shells regarding hydrodynamic noise control. With the increase of speed, however, the hydrodynamic noise of single-layer cylindrical shells cannot be ignored. This paper establishes the vibro-acoustic coupling model of a finite cylindrical shell fully immersed in infinite ideal water medium. On the basis of the comb function, the hydrodynamic noise calculation method for finite cylindrical shells under external turbulent boundary layer (TBL) excitation is established using correlation function and power spectral density function. The comb function method and the direct expansion method are used to establish the TBL wavenumber-frequency spectrum, respectively. The influence of the two methods on the excitations and displacements of the cylindrical shell in the calculation are analyzed. Furthermore, the sound radiation powers determined by the two approaches are compared with that of the statistical energy method. The results indicate that the comb function method produces different power spectrum density functions of TBL excitations and cylindrical shell displacements from the direct expansion method. The sound radiation power of finite cylindrical shells calculated by the comb function method has better agreement with the results of the statistical energy method in the medium and high frequencies, indicating that the hydrodynamic noise computation of finite cylindrical shell based on the comb function is more accurate. The effects of various speeds and shell thicknesses on the sound radiation power of finite cylindrical shell under TBL excitations are also compared. The results comply with the general law of hydrodynamic noise.

Vibration comfort is one of the key technical indicators to evaluate the passenger experience of large cruise ships. Due to the high superstructure and the large difference from ordinary ship types, there is no mature approximate calculation method for global vibration of cruise ships. According to the stiffness and mass distribution of a large cruise ship, this paper proposes a design method for the steel global vibration test model, and designs two schemes: the equiscale model and the non-equiscale abnormal model. Then the abnormal model is used to complete the modal test in the air and the pool, and the test results of the dry/wet modes under the loading and unloading condition are given. The results show that the ratio of the natural frequencies of the wet mode to the corresponding dry mode is basically unchanged under the loading or unloading condition. For the same order mode, the ratio of the natural frequencies under the loading condition to the corresponding unloading condition also remains basically unchanged. The steel-hull global vibration test model can better consider the influence of added water mass, which can provide a valuable reference for the design of the global vibration model and the natural frequency prediction of special ship types.

Marine operations, such as takeoff and landing of carrier-based aircraft, ship fuel supply, and ship lifting operations, have strict requirements for the movement of ships and need to be carried out within the quiescent period window to ensure operational safety. However, the incidence probability of a quiescent period window under high sea conditions is relatively low. Therefore, it is crucial to assess the probability of the target sea area’s quiescent period in advance for the planning and deployment of offshore operations. This article establishes a ship motion probability model based on the statistical characteristics of ship motion to study the short-term statistical characteristics of quiescent periods. Simulation data is used to verify the statistical characteristics of motion, the joint statistical features of motion amplitude-period, proving the accuracy of the model. Subsequently, the probability of continuous small movements below the operational threshold during the quiescent period have been analyzed. The results indicate that the established ship motion probability model and the ship continuous small amplitude motion probability model can accurately reflect the motion characteristics and probability of quiescent periods of ships under corresponding sea conditions. This study can provide the probability of a quiescent period meeting the operational requirements before the ship enters a certain sea area for operation, and also provide the probability of a quiescent period occurring in a certain period of time before the operation, providing auxiliary decision-making for the navigation operation plan of offshore ships.

Tip clearance flow is a complex phenomenon that occurs between the rotor blade tip and the inner surface of the duct of a pump-jet propulsor. The tip clearance size significantly influences both the tip clearance flow and the performance of the pump-jet propulsor. Previous studies on tip clearance flow primarily focused on cases with tip clearance sizes less than 4 mm on model scale. Tip clearance flow of pump-jet propulsors with tip clearance sizes of 1 mm and 16 mm were simulated based on large eddy simulation in this paper. The study focuses on the characteristics of tip clearance flow in the large tip clearance pump-jet propulsor and the effects on cavitation inception, hydrodynamic performance, and duct pressure fluctuation. The results indicate that, compared to smaller tip clearance, the starting position of tip-separation vortex of pump-jet propulsor with large tip clearance is closer to the leading edge of rotor, while the intersection position of tip-separation vortex and tip-leakage vortex is closer to the trailing edge of rotor. Furthermore, the propulsion efficiency of the pump-jet propulsor behind SUBOFF is reduced by approximately 10%. The vorticity and circulation of tip-leakage vortex are larger, and cavitation inception of tip-leakage vortex occurs earlier. The amplitude of fluctuating pressure on duct inner surface is significantly decreased by about 80%. Therefore, the design of the pump-jet propulsor should be made based on comprehensive balance of the above-mentioned performance characteristics to find the optimal tip clearance size.

In order to study the interference characteristics between multiple wingsails on sail-assisted vessels during navigation, a lateral arrangement scheme of a two-element wingsail based on the relative wind direction angle was designed. The Reynolds averaged N-S equation was used for numerical simulation under steady conditions. The aerodynamic interference performance of the two-element wingsail was analyzed, and an optimization scheme for the angle of attack and flap deflection angle was proposed to address the stall problem caused by the interference of multi-sails. Furthermore, the interstage interference characteristics of wingsails were obtained. The results show that, in the single row arrangement scheme, the optimal spacing is 1.5c for relative wind angles of 30°, 90°, and 120°. However, the interstage interference can cause the wingsail to stall at the relative wind angles of 90° and 120°. After optimization, the auxiliary thrust coefficient can be increased by more than 5.2%, and the flow separation on the downstream wingsail disappears.

To investigate the turning maneuverability of polar ships in floating ice area, in this study a combined CFD-DEM approach was adopted to numerically simulate the turning motion process of a medium-sized polar ship in floating ice area. In the simulation the ship’s turning motion at different rudder angles and ice concentrations have been considered, and the parameters of the ship’s turning motion have been predicted. The results show that the existence of floating ice will significantly hinder the ship’s turning motion, and the range of the tactical diameter in floating ice area is 1 to 1.5 times than that in open water conditions. The forces and moments acting on the hull exhibit strong randomness, and the instantaneous fluctuations of the ship’s speed and yaw rate are more pronounced. At the same rudder angle, the ice longitudinal force increases with the increase of ice concentration, while the variation of the fluid longitudinal force is not significant. The average total lateral force and total yawing moment are in the same direction as the turning maneuver, while the average ice yawing moment tends to be in the opposite direction.

Currently, the International Maritime Organization (IMO) has approved and implemented the assessment requirement for Minimum Propulsion Power (MPP) of ships in adverse sea conditions. The assessment method and relevant influence factors will have a vital impact on ship's design and operation. On the other hand, MPP is essentially a criterion for manoeuvring safety at actual seas. However, the practical assessment methods adopted in IMO guidelines do not directly and accurately account for ship's course-keeping ability in severe seas. A time-domain comprehensive method with supplementary course-keeping ability criteria has been proposed in the authors' preliminary research. Based on an updated mathematical model and criteria, this paper presents more detailed elaborations, results and discussions on the time-domain method, including the comparative analyses with a power line method and two steady-state equilibrium methods based on IMO guidelines and draft. Discussions on the influences of key factors, involving criterion conditions and calculation parameters, are also presented. The results indicate that different methods exhibit varying advantages and complexity in MPP assessment, thus constituting a multi-level assessment framework for MPP. In particular, the time-domain comprehensive assessment has a higher accuracy with more realistic description of manoeuvre behaviors, capable of offering a solution for the ships that cannot meet other assessments, or for the assessment requiring additional course-keeping ability. Furthermore, an expanded range of wave direction sets a stricter but potentially necessary requirement, while using the self-propulsion factors at low speeds can eliminate the unnecessary conservation of assessment result caused by those at design speed.