Ship roll damping estimation based on roll decay curves is very common in engineering. There are many methods to estimate damping from roll decay curves, which lead to difference in damping estimation results. In order to achieve the purpose of obtaining high-precision damping prediction methods, the principle of estimation of roll damping based on roll decay curves in recent years was analyzed in detail first in this paper and then its scope of application was expanded. The methods were divided into two categories according to the principle, namely, one based on piecewise linear assumption and the other based on parameter identification. The data of standard model (DTC) were used to verify and compare the characteristics of different methods, and the error sources of damping coefficient estimation were analyzed. The results show the accuracy of estimation is strongly related to the used method. The Froude energy method with clear physical meaning, few error sources, and insensitivity to measurement noise and error is recommended as the preliminary estimation method. If a damping model with sufficient accuracy can be found, estimation based on the Prony-SS method, which has an inherent noise suppression mechanism and a unique solution, can be further done to improve the accuracy.

Mobile floating offshore wind turbines (MFOWTs) can store the generated wind energy onboard, and navigate to preset working areas due to their mobility. Compared to fixed and moored floating offshore wind turbines, MFOWT can not only harness the abundant far-offshore wind resource but also supply power to offshore energy consumers, because of its superior working ability in deep sea. In this paper, key technical problems in the development of MFOWT are summarized and discussed in several aspects based on a literature review and our research: (1) floating substructure selection; (2) conceptual design and approval; (3) dynamic response analysis for the integrated system; (4) operational strategy decision; (5) safety assessment; (6) model test in ocean wave basin; (7) construction and installation.

An acoustic black hole stiffened plate was designed based on acoustic black hole and stiffener structures. Acoustic radiation characteristics, vibration energy distribution and transmission characteristics were studied by establishing the finite element model of the acoustic black hole stiffened plate. The simulation results show that the radiated sound power of the acoustic black hole stiffened plate is 8~20 dB lower than that of ordinary stiffened plates above the cut-off frequency. Because of the local damping layer, the vibration gathered in the acoustic black hole’s region is effectively dissipated. Therefore, the vibration level is significantly lowered and the acoustic black hole stiffened plate is weakly coupled to the sound field. It is verified that most of the kinetic energy of the acoustic black hole stiffened plate is concentrated in the acoustic black hole region while analysis of the vibration energy distribution characteristics shows that the vibration level of the overall structure is reduced. It is revealed that the acoustic black hole stiffened plate is affected by the superposition effects which include acoustic black hole’s concentrating energy effect and stiffened plate’s blocking vibration effect. Compared with ordinary stiffened plates, acoustic black hole stiffened plates have better vibration and noise’s reduction performance.

As the application of composite materials can improve the cavitation performance and vibration characteristics of propellers, it has been widely concerned in the field of advanced marine propulsion equipment. In this paper, the flow field of a composite material propeller was calculated based on URANS, the dynamic response of the blade structure was solved by FEM, and the hydrodynamic load and structural deformation were transmitted in real time bi-directionally. The simulation of the evolution of tip cavitation in the high wake region shows that the maximum tip deformation increases with the initiation and development of tip cavitation, reaches the maximum at the stage of tip vortex cavitation formation, and then decreases with the collapse of bubbles. The mechanism of the improvement of propeller propulsive efficiency and the suppression of tip cavitation due to the application of composite materials was revealed, indicating that the composite material propeller produces bending torsion coupling deformation under cavitation hydrodynamic load, and adaptively adjusts the angle of attack to suppress cavitation development. Comparison of the cavitation performance between the composite material propeller and the rigid propeller under typical cavitation condition reveals that the composite material propeller has a mild peak pressure fluctuation and a better adaptability to the non-uniform wake.

Based on the immersed boundary-multiple relaxation-lattice Boltzmann method, a numerical simulation of the flow around three cylinders in equilateral triangle arrangement was conducted at Re=100, the influence of two key parameters, the spacing ratio (K_d) and incoming flow angle (α), on fluid force coefficient and flow field characteristics was analyzed, and several flow mechanisms were revealed. The calculation results show that the wake pattern in different flow fields is divided into single vortex, single vortex to double vortex, irregular double vortex, regular double vortex, double vortex to triple vortex, irregular triple vortex and regular triple vortex pattern, and that the downstream vortex street is more regular and the wake pattern changes faster at α=30°. The time-averaged fluid force coefficient of three cylinders is greatly affected by the gap flow velocity, and the changing trend with the spacing ratio (except for the small spacing ratio) at each incoming flow angle is nearly identical. The transition of the wake pattern will change the root mean square value of three cylinders’ fluid force coefficient significantly. When K_d≥3.5, the root mean square value of upstream cylinder’s fluid force coefficient fluctuates greatly. Except for α=0°, the vortex shedding frequency of the midstream cylinder is less affected by α and K_d when K_d≥2.5, which is relatively stable.

Mono-shell submarine's acoustical coating is supposed to possess the multifunctional characteristics of mechanical noise, hydrodynamic noise and acoustic target intensity control. Two difficulties need to be addressed for this purpose: the first one is to improve the mechanical noise low-frequency control effect; the other one is to achieve multifunctional compatibility of acoustical coating. In this paper, from the perspective of reducing the sound radiation of mechanically excited shells, modal superposition method was adopted to establish the vibro-acoustic coupling and sound radiation model of multilayer graded acoustical coating and finite length ribbed cylindrical shell in ideal aqueous medium of infinite size. Based on the acoustic coating modal transfer function characteristics, the acoustic coating gradient distribution characteristics, thickness, the number of layers and other parameters were analyzed to reduce sound radiation of the ribbed cylindrical shell. The characteristic acoustic impedance asymptotic parameter distribution of the layered gradient acoustic cover was given and compared with the sound reduction characteristics of laying a uniform acoustic cover. The results indicate that the acoustic coating with increased acoustic impedance and slow wave velocity from the inner to the outer layer of the ribbed cylindrical shell has the property of reducing the vibration displacement of the outer surface more effectively than the uniform coating, which can significantly increase the sound reduction effect and extend the low frequency range of noise reduction. Optimisation of the acoustic coating should increase the acoustic impedance mismatch of the inner layer and the outer layer adaption effect, in order to help reduce the low frequency acoustic radiation of the ribbed cylindrical shell and to take the reduction of the acoustic target intensity into account.

Based on the concept of ‘casing treatment’ technology in aero-engine, a certain number of axial groove structures were set up in the inner wall of a pumpjet propulsor duct to weaken the tip vortex intensity, so as to reduce the pulsating pressure on the inner wall of the duct. By applying DES method and sliding mesh method, the flow around tip vortex field region of an elliptical hydrofoil was numerically simulated. The numerical calculation results are in good agreement with the experimental values, which verifies the reliability and applicability of the numerical calculation method.On this basis, the numerical simulations of the pumpjet propulsor with and without groove structures were carried out, and the effects of groove structure on the tip vortex core pressure, the pulsating pressure on the inner wall of the duct and hydrodynamic performance were compared and analyzed.The results show that the groove structures can significantly increase the tip vortex core pressure, reduce the amplitude of fluctuating pressure on the inner wall of the duct, and has little effect on the hydrodynamic performance.

Deep draft cylindrical floating nuclear platforms are a new type of multi-functional and efficient platforms, which can effectively solve the problem of energy supply in the development of offshore islands and reefs in the South China Sea. At a certain inflow velocity, the wake area will induce vortex-induced motions (VIMs) of a platform, which will seriously accelerate the fatigue damage of the mooring and riser system, and adversely affect the operation of nuclear reactors inside the platform. Based on the improved delayed detached eddy simulation (IDDES), the sway, surge and yaw motion responses of a platform at different reduced speeds were numerically simulated, and the key characteristics of vortex-induced motions were analyzed from the perspectives of motion trajectory, motion frequency and three-dimensional flow field. The results show that when the reduction speed of U_r ranges between 5.45 and 9.08, the amplitude of sway and yaw increases gradually, the motion trajectory is similar to a“banana”, the frequency of sway and yaw is basically the same, and there is no obvious change of“locking”interval in sway. However, when 7.26 < U_r < 9.08, the amplitude of yaw increases approximately linearly with several peaks appearing near the dominant frequency, and the motion trajectory gradually widens in the downstream direction. In the three-dimensional flow field, it is found that the three-dimensional vortex structures is quite complex, and that the skirt structure at the bottom of the platform interferes with the surface flow separation, which results in a certain vortex reduction.

With the long-term service of a semi-submersible platform at sea, the basic design parameters of the platform will change. If the platform is still analyzed based on the original design parameters, the accuracy of the results will be affected. In this paper, the basic parameters (displacement, center of gravity height and three-degree-of-freedom radius of rotation) that are easy to change during the service of a semi-submersible platform were taken as the updating parameters, the objective function was constructed by the variances, and wave frequency power spectral density of platform roll, pitch and heave, and a Bayesian parameter updating method of semi-submersible platform based on the monitoring data was established. With the monitoring data of a semi-submersible platform in China taken as an example, the parameter updating of the platform was carried out, and based on the updated parameters, the response of the platform under a multi-year return period environmental load was compared and evaluated.

As a typical representative of the propulsion mode of BCF (body-caudal fin), tuna swims fast, which has become one of the important biomimetic research objects.In this paper, based on spring-based smoothing model and local remeshing model, a numerical calculation method for solving RANS equations and analyzing vortex structure extraction established for the flow field of caudal fin were verified by grid independence verification, and the results were compared well with those published in the literature. The investigation on the variation of the hydrodynamic force and vortex structure with the St number shows that the vortex ring structure in the wake presents a staggered distribution in a“zigzag”shape, and that, with the increase of St number, the vortex ring structure in the wake gradually evolves from a single column to a double column, and that the thrust coefficient increases.