If the fluid-structure interaction problem in frequency domain is solved by boundary element method, irregular frequencies will appear at the resonant frequencies of virtual interior fluid domain and exhibit large fluctuation. In order to investigate the underwater acoustic radiation properties of a full-scale submarine, a shaft-hull coupled system was established based on Suboff model. The FE/BE method was adopted to calculate the vibroacoustic radiation of the system in low- and medium-frequency. The closed virtual impedance surface (CVIS) method was respectively used and not used to study the influence of irregular frequencies. It is shown that: (1) for slender submarine hull with complicated shape, irregular frequencies are not only negligible, but even exhibit in continuous irregular bands; (2) enormous errors will occur if irregular frequencies are not eliminated, and convergence zones in far field of deep ocean environment are also“contaminated”; (3) for submarine whose the main body is constructed as slender cylindrical shell, the 1^st irregular frequency can be obtained according to the analytical expression of the irregular frequencies of slender cylindrical shell, and the non-dimensional 1^st irregular frequency is about ka=2.4.

In this paper, numerical simulation of projectile’s underwater motion process with tail-slaps was studied based on CEL (coupled Eulerian-Lagrangian analysis) method. The stability principle of projectile’s motion with tail-slaps was studied. Some parameters, such as initial angle of attack when tail-slaps occurred, maximum angle of attack, and instability critical angle of attack were used to characterize the projectile’s tail-slap. The relationship between stability and angle of attack was revealed. The location change of pressure center caused by projectile’s tail-slap was studied, too. The results showed that the static stability margin of projectile was enough to meet its stability requirements. Finally, three constraint criteria for the motion stability of projectile, including the requirement for the size of cavity, the relative location of center of pressure and center of mass, and the static stability margin of projectile, were obtained. The location and area of tail-slap can be used to control the static stability margin of projectile, some suggestions for the design of projectile’s shape were proposed, too.

A sea-launching ship will have complex nonlinear motion response under the environmental loads and launch impact load, which has a vital effect on the safety of rocket launching process. Based on the three-dimensional potential flow theory, this paper focuses on the motion responses of a sea-launching ship under the action of wind, waves, and impact loads in different sea states. And the effects of the dynamic position system and the launch ignition time on the motion of the launch ship were investigated. The results show that the launch impact load has a great influence on the pitch motion of the ship, especially when the wind/wave direction is parallel to the ship direction. The existence of dynamic position system will increase the roll of the launch ship. And the launch ignition time can be selected at the moment when the pitch motion of the launch ship is about to reach the extreme point, so as to reduce the impact of the launch ship's motion on the rocket attitude during taking-off.

Based on the analysis of the lubrication performance of the eccentric stern bearing, an elasto-hydrodynamic coupling lubrication model for the local wear and stiffness of the stern bearing was established. The joint program of finite difference method and finite element method was compiled to solve the elastic deformation of the bearing, and the mass conservation boundary condition was used to replace the Reynolds boundary condition. The effects of local wear depth, bearing elastic modulus and other factors on the hydrodynamic pressure, liquid film thickness, cavitation area and friction law of the bearing were discussed in detail. The results show that when the local wear depth of rigid body bearing is lower than the threshold value, it is beneficial for bearing lubrication. When the local wear depth exceeds the threshold value, the maximum hydrodynamic pressure, friction force and cavitation area increase significantly. The influence of bearing elastic deformation on the calculation results cannot be ignored. Elastic deformation and local wear exist at the same time, and the change law is basically consistent with the change trend of local wear of rigid body bearings.

Ocean thermal energy conversion is one of the research hotspots of marine renewable energy in recent years. Free-hanging water intake pipes are the key structure to extract deep cold seawater. At present, the vortex-induced vibration (VIV) response characteristics of free-hanging pipes in deep sea currents are not clear yet. In this paper, model tests of a free-hanging pipe under uniform flow were carried out, and the strain response of vortex-induced vibration was measured by the fiber Bragg grating strain sensor. The amplitude and frequency characteristics of the free-hanging pipe were investigated by modal analysis and wavelet transform data processing methods. It is revealed that the maximum amplitude of the VIV displacement response of the free-hanging pipe under the uniform flow mainly occurs at the bottom. The dominant frequencies in inline (IL) direction is basically two times that in cross flow (CF) direction. However, in the conditions where the modal transition occurs, the dominant frequencies in IL and CF directions are the same, accompanied by obvious "traveling wave", "multi-frequency response" and "time-sharing" phenomenon. In addition, the Strouhal number of the overhanging pipe model in CF and IL under uniform sea currents are 0.15 and 0.30, which are slightly smaller than the results of flexible risers hinged at both ends. This value may serve as the parameter input for the vortex-induced vibration prediction of free-hanging pipes.

Sandwich plates, as a new structure, have the advantages of strong design, light weight, high specific stiffness, good performance of vibration reduction and sound insulation, and have been used in various engineering structures. However, there are many types of sandwich structures, and it is difficult to theoretically analyze the sound insulation performance of complex sandwich structures. New approximate analysis methods are urgently needed. In this paper, based on the equivalent method of spring, torsion spring and concentrated mass, the sound-vibration coupling control equation of a C-type sandwich panel was established for the marine C-type sandwich panel. Using the spatial harmonic expansion method, the expression of the sound transmission loss of the C-shaped sandwich structure was derived. The influence of the system parameters of the sandwich panel on the sound insulation performance of the structure was discussed. The results show that the material properties and the thickness of the sandwich panel structure have greater influence on the sound insulation performance of the C-type sandwich panel structure. The finite element simulation was used to verify the validity of the theoretical analysis.

In order to predict the critical buckling load of a filament winding thick composite cylindrical shell under hydrostatic pressure, the buckling governing equation of the thick cylindrical shell under hydrostatic pressure was obtained based on the nonlinear Sander theory, as well as the deformation geometry equation of the cylindrical shell and the constitutive relation of the filament-wound layer. An analytical method for predicting the critical buckling pressure of thick composite cylindrical shells under hydrostatic pressure was proposed by solving the governing equation. Then, critical buckling load of the thick shell with different filament-wound types and angles were calculated with FEM and compared with analytical results for verifying the accuracy and high efficiency of the analytical method. The influence of key parameters such as geometrical and material design on the critical buckling load of thick cylindrical shells was investigated based on the analytical method.

Dolphins and other toothed cetaceans have high-speed swimming ability and are important research objects for underwater biomimetic drag reduction. In this paper, based on the characteristics of dolphin skin structure and adaptive deformation, an engineering-scale bionic study was carried out, a wall deformation motion control equation with the normal velocity of the boundary layer of the wall surface as the input signal was designed, and the drag reduction performance of the deformed wall surface was simulated by the dynamic mesh technology. The results show that the optimal frictional resistance reduction rate of the deformed wall surface is 19.24% and the optimal total resistance reduction rate is 6.4% at a flow rate of 0.5 to 10 m/s. Application of the biomimetic control results in the increase of the thickness of the turbulent boundary layer of the wall surface, reduction of both the surface friction resistance and the turbulence kinetic energy of the flow field. The bionic deformation wall was applied to the surface of the non-attached SUBOFF submarine model to carry out drag reduction design. In the speed range of 3.045-8.231 m/s, the total drag reduction rate greater than 8.0% was obtained.

The elimination of long period waves has always been a difficult problem in coastal engineering that needs to be solved. Plate breakwater has a certain effect on long period waves, due to its convenience in construction, low cost and wide application range. Further research of which is of great significance. A numerical wave making flume is established based on Flow-3D in this paper, the optimal placement scheme and dimensions of the flat plate array breakwater are presented with the relative plate length L₀/L, relative plate spacing j/L, relative plate thickness d/H and number of plates N identified as research parameters. Simulation results show that, when the relative plate thickness d/H=0.1, plate number N=4, relative plate spacing j₁/L=0.3, j₂/L=0.1, relative plate length L₀/L=0.6, the wave suppression effect of non-uniform four-plate array breakwater is the best. It is shown that the arrangement form has good wave elimination for long period waves with period less than T=8 s (prototype period T=25.28 s).

At present, the International Maritime Organization (IMO) has issued the final guidelines of the direct stability assessment of surf-riding/broaching for the second generation intact stability criteria, and how to accurately and efficiently predict the surf-riding/broaching is a key problem to be solved for the direct stability assessment of surf-riding/broaching. So a surge-sway-heave-roll-pitch-yaw coupled mathematical model (6-DOF) is established in this paper. Firstly, the heave and pitch motions are considered in the surge-sway-roll-yaw maneuvering mathematical model, and the amplitudes and phases of heave and pitch motions are calculated by a strip method using an enhanced integrating method, which can solve the problem of divergence resulting from direct seakeeping calculation in time-domain for high speed vessel in stern-quartering waves. Secondly, the Froude-Krylov forces and diffraction forces are calculated by integrating the wave pressure up to the mean wave surface. At the same time, nonlinear hydrodynamic derivations, heel-induced hydrodynamic forces and nonlinear roll damping are considered in the mathematical model. The hydrodynamic lift forces due to the coexistence of wave particle velocity and ship forward velocity are taken into account in the propeller thrust and rudder force model. And the real-time emersion of double rudders in waves is considered in the rudder force model. Finally, a tumblehome ship with twin propellers and double rudders is utilized to study surf-riding/broaching in stern-quartering waves, and the effect of initial relative position of the ship to waves on predicting surf-riding/broaching motion is investigated. The computation results show that the established 6-DOF mathematical model has enough accuracy to be used for the direct stability assessment of the surf-riding/broaching failure mode.