In this study, an integrated simulation method of floating wind turbines based on wind-wave joint probability model and Modelica coupling simulation framework is constructed for the development of renewable energy in the South China Sea islands and reefs. Taking the island and reef sea area of Sansha City as the research object, the joint probability distribution model of wind and wave is reconstructed by historical meteorological data, and the data of island and reef sea area of Sansha City are established by combining with ocean current data. Based on the Modelica platform, a fully coupled numerical model of aerodynamics-hydrodynamics-control-structure-mooring is developed to study the coupled dynamic response characteristics of Spar-type floating wind turbine in the island reef area of Sansha City. The results show that the peak pitch angle of the platform is 4.74° (<5 ° design threshold) under working conditions. The maximum tension safety factor of mooring system meets the requirements of China Classification Society. Under rated conditions, the turbine speed is stable at 11.98 rpm (rated value 12.1 rpm), and the power output reaches 4.60 MW (rated power 5 MW). The research verifies the engineering applicability of the proposed method system, and provides an innovative technical evaluation framework and scientific decision-making basis for the deployment of floating wind power projects in the South China Sea islands.

With the continuous development of underwater detection technology towards low frequency domain, the sound absorption ability of conventional underwater acoustic coating needs to be significantly enhanced. In order to obtain better low-frequency sound absorption effect, a Topology-Shape-Topology (TST) optimization method is investigated in this paper. A new underwater acoustic coating is designed using the method which optimizes the distribution of materials in the anechoic layer. Compared with the traditional cavity-type acoustic coating, the acoustic structure designed by TST optimization method can achieve excellent sound absorption performance in the range of 200–1000 Hz. The peak value of sound absorption coefficient can come up to 0.9 at 321 Hz, and the average of sound absorption coefficient within the studied frequency range is above 0.8, resulting in an effective improvement of the low frequency sound absorption effect of the traditional underwater acoustic coating. Moreover, the underlying mechanism behind the sound absorption performance is revealed to obtain a further understanding of distribution regularities of materials. The study provides a new idea for the design of underwater acoustic coatings.

Cavity flows are common phenomena for underwater vehicles. For example the drain holes of the submarine are one of the typical cavity flows. There are some complex turbulent phenomena in the cavity, such as shear layer K-H instability, coupling between shear layer and cavity recirculation. So far, many difficulties remain for CFD numerical simulation. Firstly, the setup of an accurate inflow condition is crucial for analyzing turbulent coherent structures inside the cavity with the high fidelity due to the fact that the flow upstream of the cavity is usually in an utterly turbulent state. Secondly, there are frequency components with second-order oscillation modes besides the ones with first-order oscillation modes in the cavity flow. However, the CFD numerical simulation still faces challenges for such second-order oscillation flows. In this paper, a DFSEM-WMHRL method is developed to perform high fidelity CFD simulation for such complex cavity flows, where the WMHRL is a hybrid RANS/LES method with the wall-modelled capability, and the DFSEM is a divergence free synthetic eddy method with the turbulent inlet generation capability. Through a series of numerical simulations on the channel flow with $ {Re}_{\tau }=395 $, the turbulence kinetic energy resolution index r_k and the combined conditions for the RANS/LES hybrid boundary positions are firstly confirmed, indicating the capability of the proposed method for analyzing the second-order statistics of the channel flow with high fidelity. Moreover, numerical simulations were carried out for the cavity flow with $ {Re}_{{\mathrm{D}}}=3360 $ and $ L/D=2 $ based on the proposed method. The results show that the DFSEM-WMHRL method can accurately analyze both second-order statistical characteristics and the fine spectral structures caused by self-sustaining oscillations in the cavity flow.

With the rapid development of offshore wind power industry, the protection of wind turbine foundations has become increasingly important in engineering practice. Based on wave diffraction theory in stratified waters, this paper employs the eigenfunction expansion method to derive analytical solutions for wave interaction with concentric arc-shaped structures. The dimensionless wave loads induced by surface wave and internal wave as well as the corresponding maximum wave elevation distributions are obtained. The results show that the double-layer arc structure exhibits superior wave attenuation performance compared to the single-layer configuration. In stratified fluids, the wave loads induced by surface wave and internal wave are of comparable magnitude, indicating that the influence of internal waves cannot be neglected. Parameters such as wave incident angle, water depth, porosity, arc angle, and radius ratio significantly affect wave loading characteristics. Proper selection of porosity and arc angle can further enhance wave attenuation performance and optimize construction costs.

The complex relationship between irregular corrosion defects and pipeline failure presents substantial challenges for pipeline safety assessments. To investigate the interaction between irregular corrosion defect parameters and pipeline failure bending moments, a theoretical calculation model for the ultimate bending moment of pipelines with irregular corrosion defects was first established under the individual or combined effects of internal pressure, axial force, and bending moment, based on the Net Section Collapse criterion. Then, the accuracy and reliability of the proposed model were verified through experimental data from the literature and finite element analyses. Finally, taking API 5L X80 pipeline steel as an example, the impacts of defect shape, depth ratio, length ratio, and relative position of shallow and deep defects on the ultimate bending moment of pipelines with irregular corrosion defects were analyzed under three different working conditions. The research results indicate that the ultimate bending moment of the pipeline is related to the shape of the corrosion defect. Considering the shape of the corrosion defect in the calculation of the ultimate bending moment can avoid the underestimation of residual strength. The increase in the depth length ratio of irregular corrosion defects reduces the ultimate bending moment of the pipeline. When the depth length ratio remains constant, the larger the depth/length of the corrosion defect, the faster the rate of decrease of the ultimate bending moment. Additionally, adding internal pressure and axial force loads also reduces the ultimate bending moment. When a deep corrosion defect is located in the center of a shallow corrosion defect, the ultimate bending moment reaches its minimum. As the deep corrosion defect moves from the center towards the edge of the shallow corrosion defect, the ultimate bending moment of the pipeline gradually increases but the change is not significant. This paper presents a failure assessment method with the merits of clear physical concept and convenient calculation for pipelines with irregular corrosion defects, which enriches and develops the relevant theories of pipeline safety assessment.

The phenomenon of vibration interference among multiple risers is one of the hot topics in ocean engineering. As a basic model of multi-pipe systems, the dual staggered flexible pipe system is widely used to analyze vortex-induced vibration (VIV) behavior. In this paper, the VIV characteristics associated with a dual staggered flexible pipe system is investigated by a two-way fluid-structure interaction method. The results show that the influence of staggered angles on the in-line and cross-flow vibration responses of upstream and downstream pipes is distinctly different. Owing to the short-range interference between dual pipes, the amplitude of downstream pipe is generally larger than that of upstream pipe, more likely to demonstrate "multi-mode competition" phenomenon. Due to the effect of staggered arrangement and small spacing ratio, the interation between fluid and dual pipes is relatively strong, leading to the failure of forming regular shedding vortex in the wake of the dual pipe system. Through the analysis of vibration response and phase difference at the peaks and valleys of RMS amplitude envelopes, related to the dual staggered flexible-pipe, it is found that when the staggered angle of dual flexible-pipes increases from 15° to 60°, the votex shedding mode changes from "2P" to "2S", the fluid excitation frequency and intensity of the upstream pipe become weakened, and the vibration displacement phase difference and lift force coefficient phase difference of the upstream and downstream pipes are reduced.

At high subcritical Reynolds numbers, the flow past the circular cylinder has two types of complex flow phenomena, namely massive separation flow and shear layer transition. The traditional Improved Detached Eddy Simulation method (IDDES) is based on a fully turbulent assumption and is not suitable for dealing with transition. Meanwhile, the transition models (TRANS) based on the RANS method are incompetent to simulate the three-dimensional turbulent structure caused by massive separation. Therefore, this article integrates the IDDES approach with the TRANS model to develop an IDDES-Tr model capable of simultaneously resolving massive flow separation and shear layer transition. Numerical simulations were conducted for flow around a cylinder at a high subcritical Reynolds number of Re=1.4×10⁵, with results subsequently validated against experimental data. The comparison demonstrates a strong agreement between the simulation and experimental data, indicating that the IDDES-Tr model accurately captures the features of flow around a cylinder at high subcritical Reynolds numbers. In other words, the new model can effectively simulate simultaneously massive separation and shear layer transition.

To solve the trajectory tracking problem of Unmanned Surface Vehicle (USV) when the parameters of model are uncertain, this paper designs a trajectory tracking strategy for USV based on adaptive control and non-dominated fast sorting multi-objective genetic algorithm (NSGA II). Firstly, a three degree of freedom USV kinematic and dynamic model is established. Secondly, based on Lyapunov theory, an online parameter estimation strategy and an adaptive trajectory tracking controller are designed for model parameters with uncertainty, and the convergence of trajectory tracking error is proved based on Lyapunov theory. Subsequently, to obtain the optimal values of a large number of controller parameters that require manual setting in the controller, a multi-objective controller parameter optimization model is established with the objectives of minimizing tracking error and minimizing control input. By solving the controller parameter optimization model through NSGA II, the optimal controller parameters are obtained, thereby enhancing the controller performance. Finally, numerical simulation experiments are conducted, and the experimental results verify the effectiveness of the trajectory tracking control algorithm.

This paper focuses on a numerical algorithm for analyzing flow-induced noise of composite plates. In this study, a method based on the superposition of uncorrelated plane waves was proposed to synthesize turbulent fluctuating pressure and calculate flow-induced noise using the finite element acoustic-vibration coupling module. The detailed theory and procedures of this algorithm were provided for rectangular plates as an example. The computational results were compared with analytical theory to validate the accuracy of the algorithm. Based on the wave number filtering principle, the criteria for selecting the truncation wave number was given, and the algorithm was compared with traditional methods in terms of computational efficiency, demonstrating the advantages of the algorithm in computational efficiency and accuracy. Finally, the mechanism of flow-induced noise generation in composite plate structures with acoustic coating was analyzed in this paper using the proposed algorithm, providing theoretical support for reducing flow-induced noise of underwater vehicles.

Liquid sloshing is prone to occur in liquid cargo ships under the action of wind and waves, and the impact generated by the sloshing will damage the compartment structure, which is one of the important factors inducing safety accidents. Therefore, it is crucial to understand the sloshing phenomenon and propose an efficient anti-sloshing structure. In this study, a new type of floating baffle is proposed, which can provide a better anti-sloshing effect under different filling rates compared with traditional fixed baffle. Sloshing under high-risk filling rate is investigated by numerical simulation based on STAR-CCM+, focusing on floating baffles with different sizes and angles, and the optimal structure of the baffle is selected by analyzing the wave surface displacement, baffle velocity, wave profile, and fluid flow pattern with velocity vector magnitude. The results show that floating baffle can effectively suppress the sloshing by coupling various sloshing mechanisms, its web can cut the free liquid surface in time to suppress the sloshing, and the end of its bottom flange can generate vortex to effectively dissipate the fluid energy, and the heave motion of the baffle can change the trajectory of the fluid mass point under the free surface to reduce the impact on the compartment wall, which further enhances the anti-sloshing effect.