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.

In this paper, a new frequency-domain analysis method based on the modified Tovo-Benasciutti (T-B) method was proposed to calculate the fatigue damage under wide-band Gaussian random processes. According to the parametric power spectrum with different spectral shapes, a new nonlinear function model for the key parameter b_TB of T-B method was developed through the time-domain fatigue damage analysis. Compared with the original T-B method, the modified T-B method was proposed by introducing the slope parameter m of S-N curves into the new function model of parameter b_MTB. Through the numerical tests with parametric power spectrum and real power spectrum, the results of time-domain rain-flow counting (RFC) method was used as reference, and the accuracy and robustness of the modified T-B method were verified against several existing frequency-domain methods.

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.

As a new type of structure, there is no mature design and strength analysis method for large-span reticulated shell structures of offshore tourism platforms. In this paper, the relative vertical displacement of the reticulated shell structure column was taken as the wave load control condition, and the wave load was calculated based on the design wave method and the three-dimensional hydroelastic theory.With the once-in-a-century wind speed taken as the wind load working condition, the turbulence model of k-ε was adopted to simulate the wind field for calculation of the wind load, the panel integration method was used to calculate the equivalent node load of the reticulated shell structure, and the calculation method of the external load for large-span shell structures was thus established. On this basis, taking the“Heart of the Sea”tourism platform as an example, the response analysis of the reticulated shell structure was carried out, and the responses of the reticulated shell structure under the action of wind load, wave load and combined loads of both were obtained. The effects of wind load and wave load on the response of the reticulated shell structure were compared and analyzed.

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.

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.

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.

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.

Double-layer gearbox casings can not only provide elastic support for the gear transmission system, but have a good performance in the reduction of vibration noise. To efficiently and accurately predict the control effect of the vibration noise for double-layer gearbox casings, the flexible dynamic condensation model of a marine double-layer gearbox casing with vibration isolators was established based on the modal synthesis method. The condensation model was verified by calculating the natural characteristics of the complete finite element model of the double-layer gearbox casing. The vibration isolation performance of the double-layer gearbox casing was evaluated by analyzing the modal decoupling rate, modal contribution, vibration level difference, and inner casing inclination of the established condensation model. The results show that the modal decoupling rate of the double-layer gearbox casing in the translation direction is above 94.9%. The modal contribution is mainly dominated by the second to sixth mode shapes. The maximum vibration level difference can reach 34.2 dB in the excitation frequency range of 1-6000 Hz. The excitation frequency has a significant effect on the variations of the vibration level difference and inner casing inclination when the excitation frequency is less than 2000 Hz.

Aiming at the fatigue crack growth behavior of marine HTS-A steel at low temperature, the CT sample of HTS-A steel with a thickness of 25 mm was adopted in this paper, and the fatigue crack growth tests at room temperature and different low temperature environments were carried out. On the basis of experimental research, a low-temperature fatigue crack growth prediction method with improved McEvily formula including temperature term was proposed. The results show that the fatigue crack growth rate of HTS-A steel decreases gradually with the temperature decrease. At -60 ℃, the low temperature fatigue crack growth rate of HTS-A steel does not show low temperature brittle fracture, and the test results provide data reference for the low temperature fatigue design of marine HTS-A steel. At the same time, the low temperature test data of HTS-A steel in this paper and the low temperature test data of titanium alloy in the literature were used to verify the rationality and correctness of the low temperature fatigue crack growth rate prediction method. The method can be used to predict the metal fatigue crack growth rate in different low temperature environments.