Water-jet pumps are widely used in power fields such as ships, and the internal cavitation flow characteristics have an important impact on propulsion performance. In order to quantitatively evaluate the energy loss in cavitation flow field, the cavitation characteristics of an axial water-jet pump were studied based on entropy production theory. As the tip clearance vortex cavitation is typical in axial-flow propulsion pumps, the local vortical flow features were considered in the present numerical method. The SST-CC turbulence model with rotation correction and a modified cavitation model based on vortex identification were adopted. The numerical calculation method was verified according to the referenced experiment of a model pump. The results show that, under different cavitation conditions, with the deterioration of cavitation, the increase of entropy production value reflects the increase of energy loss of water-jet pump, which is corresponding to the decrease of efficiency curve, and the change law of total entropy production in pump is basically consistent with its power characteristics. Analysis of the energy loss in each geometrical region of the water-jet pump shows that the entropy production in the impeller section is the highest, especially the turbulent dissipation and wall dissipation, which are closely related to the vortex and cavitation flow field in the tip clearance of the impeller. On the different cavitation conditions, study on the flow characteristics at the tip of the impeller shows that the tip leakage vortex region causes cavitation, but significant energy dissipation occurs at the outer edge of leakage vortex and on the nearby wall area, while the cavitating vortex attached on the blade surface is the main source of turbulent dissipation.

For the coupled analysis of tank motion and liquid sloshing, a coupled iteration algorithm was established by involving the Hilber-Hughes-Taylor implicit method for tank motion and the VOF method for liquid sloshing. By application into the experiment designed for a liquid tank of rolling system, the algorithm was validated by comparison of the numerical calculation and experiment results. An effective method was finally presented for analysis of tank motion coupled with liquid sloshing.

For semi-submersible platforms equipped with dynamic positioning (DP) system, which has a small water-plane area and low metacentric height, vertical motion (roll, pitch, heave) will be affected by the normal control force of the thrusters. In light of this, this paper proposes a novel thrust allocation method based on thruster biasing, which aims to suppress the vertical motion and ensure the positioning accuracy of the horizontal motion. Model scale tests are conducted for the dynamic positioning of a semi-submersible platform in the ocean wave basin, to analyze and compare the six-degree-of-freedom motion responses under different environmental loads and thruster biasing configurations. The results show that the thruster biasing strategy can ensure the ability of station-keeping and significantly reduce the response amplitude near the natural frequency of the vertical motion of the platform, the reduction of the power spectrum peak is up to 54.7% under the given test cases, while the vertical motion in the wave-frequency is unaffected. The research results give a novel idea for the control of semi-submersible platform vertical motion.

To study the influence of the pitching and translational coupling motion of underwater structure on its hydrodynamic pressure field, the potential flow theory was used to analyze the hydrodynamic pressure field of the coupling motion of the structure. The overlapping grid technology was used to analyze the hydrodynamic pressure field of the structure in the four motion states of translational motion, translational motion with attack angle, pitching motion, and pitching translation coupling. Taking elliptical structures of different axial length ratios as the research object, the hydrodynamic pressure field under different angular velocities was analyzed. The results show that the pitching motion of the elliptical structure will make the negative pressure peak of the hydrodynamic pressure field curve shift with time. The larger the angle of pitching oscillation is, the greater the negative pressure peak shifts. In addition, the axial length ratio of the elliptical structure will also affect the offset degree of the curve. The spectrum of the hydrodynamic pressure field caused by the pitching oscillation of the elliptical structure has a very low frequency spectrum peak. The frequency corresponding to the peak of the spectrum is consistent with that of the elliptical structure oscillation.

The propulsion mechanism and swimming performance are of great significance to the construction of fish migratory channels. By using computational fluid dynamics method combined with overlapping mesh technology, the two-dimensional fish autonomous swimming was simulated by compiling the UDF program for controlling fish body swing, analyzing the evolution process and parameter changes of fish body pressure field distribution and inverse Carmen vortex street structure, carrying out the changes of fish swimming performance and fish body force under different parameters of tail swing frequency, tail swing amplitude, fish body shape and tail fin size, etc., and revealing the swimming mechanism of fish in the process of autonomous swimming. The results show that: (1) the fish body’s tail fin periodically swings back and forth under the formation of anti-Carmen vortex street, which is the main source of the fish body forward thrust, and with the increase of tail swing frequency and tail swing amplitude, the fish body tail vortex street length and vortex street strength gradually increase, while the effect of tail swing amplitude on the vortex street width is greater; (2) with the increase of swinging frequency and swinging amplitude, the horizontal mean coefficient of synergy and the maximum lateral force coefficient increase, which makes the fish obtain a larger swimming speed, but the increase of mean coefficient of synergy and swimming speed is more obvious when the swinging frequency has been changed, and the increase of maximum lateral force coefficient is more obvious when the swinging amplitude has changed; (3) with the increase of body width index, the horizontal average coefficient of force gradually decreases, and the swimming resistance to be overcome increases, which makes the swimming speed of fish gradually decrease, while the maximum lateral force coefficient gradually increases; (4)and with the increase of caudal fin index, both horizontal mean coefficient of force and maximum lateral force coefficient increase, which leads to the gradual increase of swimming speed of fish. The results of the study can provide a support for fish habitat restoration.

The mooring system is the key structure of a floating platform, which plays an important role in ensuring the safe production of the platform. It is of profound significance to understand and evaluate the safety status and risk level of the mooring system in time. Aiming at the difficulties of real-time in-situ detection of mooring lines under the platform field operation states, a reliability assessment method of catenary mooring system based on prototype monitoring information is proposed in this paper. Compared with the conventional Monte Carlo simulation, the proposed reliability assessment model can improve the calculation efficiency to meet the requirements of real-time reliability analysis. Firstly, based on the catenary equation, a numerical simulation analysis of the forcing behavior of the mooring line was performed considering the influence of the current load. Then, a reliability assessment modelling method for strength analysis and fatigue analysis of mooring system was proposed based on Enhanced Monte Carlo (EMC) method. Finally, based on the prototype monitoring data of a semi-submersible platform in the South China Sea, the reliability assessment of the mooring line was performed by taking into consideration the impact of the corrosion. The simulated results indicate that the present real-time reliability assessment method could provide a superior ability for the guidance of the safety assessment and maintenance of catenary mooring system.

It is inevitable to use various high-strength materials with brittle characteristics in the construction of ship structures. In order to study the fracture and crack propagation behavior of marine brittle materials, a coupling model of FEM and peridynamics was proposed based on the peridynamic theory. Firstly, the long-range force attenuation effect correction was considered on the basis of peridynamics. Then, the sharing node method was used to couple the FEM with the improved peridynamics, and a new fracture criterion was derived. Finally, the accuracy of the coupling model was verified by three examples. The results show that the coupling model improves the computational accuracy of the traditional peridynamic model greatly, and eliminates the“surface effect”, and it overcomes the FEM singularity when dealing with discontinuities. The calculation results of the coupling model are in good agreement with the experimental results, and the present model is feasible to study the fracture of marine brittle materials.

In the course of ship structural strength experiment, it is difficult to obtain the full field deformation data of a structure by the traditional discrete point deformation measuring technique. Therefore, the progressive destruction process of a structure cannot be effectively revealed on the base of the method. In this paper, the deformation field measuring technique of a ship grillage structure was established based on the principle of digital image correlation and 3D laser scanning. The displacement shape function and correlation criterion of subset were proposed for deformation measurement of the ship grillage structure. Moreover, a digital speckle making tool was developed in order to obtain a quantitative and controllable digital speckle field. Based on a portable 3D laser scanning system, a method for measuring the geometry of ship grillage structure was presented. An experiment was carried out to verify the deformation field measuring technique of ship grillage structure. The structural deformation field of the model under longitudinal compression load was obtained by the experiment, and the failure evolution behavior was found before and after the critical state of the model structure. Through the above experiment, the space-time evolution law of deformation field for the ship grillage structure was revealed. A basic measuring technique for failure mode recognition of real ship structures was provided in this paper.

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.

Under the welding process specifications that meet the structural strength requirements, welding energy input and welding sequence result in different welding residual stress and deformation, which significantly impact the typical bidirectional stiffened plate structure’s vibration and acoustic radiation. In order to explore the influence of welding process parameters on the vibro-acoustic characteristics of typical bidirectional stiffened plate structures, the accuracy of the welded structure test method was verified by combining numerical simulation and experiment, several tests of stiffened plate structures were carried out regarding the modal, underwater vibration, and acoustic radiation under different welding energy inputs. The results show that under the same welding sequence, different welding energy inputs have a large effect on the natural frequency of thin plates and a relatively small effect on the natural frequency of thick plates. For stiffened thick plate structures, under the same welding sequence, with the increase in welding energy input, the impact on the natural frequency shows a trend of first decreasing and then increasing, and the overall vibration acceleration level and radiated sound pressure level in the same frequency band decrease first and then increase. Under the symmetrical welding sequence, the optimal welding parameters cover a welding current of 200 A, a welding voltage of 25 V, a welding speed of 3.02-3.06 mm/s, and a welding energy input of 167 J/cm. This study can provide guidance for the design of low-noise processes for acoustic stealth of ships and marine structures.