A single-degree-of-freedom model was established to describe the roll motion of ships under combined effect of wind and wave, then the accuracy of the numerical model was verified by model tests. The time history of roll motion responses in various sea conditions was calculated by the numerical method. The Gumbel method and average conditional exceedance rate (ACER) method were used to predict the extreme value of ship roll motion response. Then based on the extreme value prediction theory, an efficient method for evaluating the ship capsizing probabilities was established. The Monte Carlo method and Level 2 vulnerability criteria for dead ship stability were employed to validate the feasibility of the evaluation method. The research demonstrates the suitability of both the Gumbel method and the average conditional exceedance rate method in predicting the extreme value distribution of ship roll motion in various sea condition, while the average conditional exceedance rate method has a higher accuracy. Compared to the traditional Monte Carlo method, the ship capsizing probability evaluation method proposed in this paper could ensure the accuracy and improve the calculation efficiency. The research in this work could provide references for the dead ship stability evaluation as well as for the safety of ships at seas.

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.

As a maneuverable flexible aircraft, the parafoil has the advantages of superior aerodynamic performance, lightweight and small packing volume. The flexible tethered parafoil on a ship employs strong wind energy as an auxiliary propulsion power during high-altitude hover flights. The dynamic models in the longitudinal plane for the lift-off and the flight-assisting phares were established based on Newton-Euler's law. The simulation calculations for these two processes were carried out using the fourth-order Runge-Kutta method. The findings demonstrate that the tension of the tethered rope is excessive when the rope elongates too slowly during the lift-off phase. Furthermore, it becomes challenging for the parafoil to achieve a smooth take-off when the rope elongates too fast. Therefore, there exists a safe elongation velocity range of 1-2 m/s for the tethered rope during the lift-off phase. When operating within the safe velocity range, the faster the tethered rope elongates, the faster the parafoil lifts off, the smoother the trajectory is, and the smaller the tension and the angle of attack are. In the flight-assisting phase, parafoil-assisted propulsion's effectiveness is found to be positively correlated with the hovering height, the parafoil area, and the angle of attack. Besides, the range of safe operational angles of attack for a propulsion parafoil is considerably broader than that of an airdrop parafoil.

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.

As one of the representative types of high-speed surface craft, stepped planing crafts have strong nonlinear hydrodynamic problem when sailing at high speed. It is a research focus to predict the craft's resistance and motion response accurately. Based on the viscous fluid theory and dynamic mesh techniques, the paper presents the coupling solution of the longitudinal three degrees of freedom motion equations of stepped planing crafts. A set of numerical prediction methods for a stepped planing craft were established. The numerical predictions of the stepped planing craft resistance and navigation attitude at different speeds were carried out, and the variation characteristics of the resistance and navigation attitude of the stepped planing craft with speed were quantitatively analyzed. The distribution law of sliding surface, bottom pressure and flow field was discussed. The results show that when Fr_▽<4.88, the pressure difference resistance coefficient C_p is greater than the friction resistance coefficient C_f, with the pressure difference resistance being a dominant component. When Fr_▽≥4.88, C_p is slightly smaller than C_f, and the friction resistance R_f is equivalent to the pressure difference resistance R_p. The wetted area ratio ζ decreases rapidly with the increase of speed. When Fr_▽≥4.48, ζ tends to be stable at about 0.31. The research results of this paper are of important value to the design of the stepped planing crafts.

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.

Anisotropic complex flows are prevalent in various engineering applications, with transition phenomena occurring at low to moderate Reynolds numbers. Turbulence models based on Reynolds stress anisotropy are employed to handle these anisotropic complex flows. However, such models are based on fully developed turbulence assumption and lack the capability to predict transition phenomena. In recent times, these models are modified with transition models, which still exhibit poor robustness and practical inconvenience. To address this, this paper combines γ transition model with the SST turbulence model, forming the ASST-γ transition model to effectively deal with the transition phenomena in complex flow fields. To comprehensively evaluate the predictive ability of the ASST-γ model for transition, classical transition cases are numerically calculated for three main types of transition: bypass transition, natural transition, and separation-induced transition. The results indicate that the numerical calculations of the ASST-γ model for the three types of transition are in good agreement with experimental results. In particular, it demonstrates better predictive accuracy for the type of separation-induced transition compared to the SST-γ model. ASST-γ model is capable of predicting these three types of transitions, offering a promising solution to transition issues in complex flow fields.

In flow experiments, it is often necessary to measure the flow time history at several locations. However, the number of sensors in the experiment is limited by sensor size and their interference with flow. By optimizing sensor placement, the testing efficiency and accuracy can be improved with more significant time-varying features being captured. Using a time history deep learning method, the study carries out the dimensionality reduction and clustering on the flows of the time-varying features, obtaining distribution of measurement points distributions with similar features. This provides a basis for optimal sensor placement. As an example, the low Reynolds number flow around a square and a circular cylinder was studied respectively. Firstly, dimensionality reduction and feature reconstruction was performed on the flow's time-varying big data. Next, clustering analysis was applied to the low-dimensional latent code, followed by feature judgment across different flow regions, yielding the optimal sensor arrangement for the physical quantities to be measured. The results show that the method in this paper obtains a more refined layout of sensors compared with traditional empirical approaches, providing a useful reference for flow experiments.

Initial defects such as cracks or bubbles will inevitably occur during sea ice condensation, which affect the mechanical properties of ice. To study the crack propagation and fracture characteristics of ice plate with initial defects, based on the peridynamic (PD) theory, the tangential stiffness coefficient for bond is introduced, a two-parameter PD method is proposed, and the two-parameter PD elastic-brittle model of ice material is established. The new model overcomes the limitation of fixing Poisson's ratio and inherits simplicity and stability of the traditional bond-based PD method. The model is verified by simulating the continuous deformation of thin plate and the three-point bending test of sea ice. Furthermore, effects of Poisson’s ratio, initial crack defects, bubble defects, ice grain size, defect shapes and multi-bubble defects on the mechanical properties of sea ice are investigated. The results indicate that the two-parameter PD elastic-brittle model is feasible to predict the continuous to discontinuous failure of sea ice, and the initial defects greatly affect the initiation time and failure mode of the ice structure.

The fatigue crack propagation analysis process of large ocean structures based on fracture mechanics is complex and computationally inefficient. This paper draws on the basic idea of the traditional S-N curve method for calculating fatigue life, constructs a stress intensity factor transfer function, and proposes a spectrum-based fatigue crack propagation analysis method for ocean structures. By combining the Python programming language with SESAM, ABAQUS, and FRANC3D calculation software, an efficient automatic crack propagation program was developed to achieve fatigue crack propagation analysis and residual life prediction of large and complex marine structures based on spectrum analysis. A multi-scale sub-model technology was used to realize the transformation of the model from the shell to the solid, solving the problem of the fusion of large-scale structures and small-sized cracks. Using the self-developed automatic crack propagation program system, the influence of fatigue hotspot (D1, D2, D3) on crack propagation behavior and fatigue life was analyzed and it was found that the crack size increases exponentially during service. The stress intensity factor transfer function has a similar shape for the same node, but varies significantly between differen nodes. The fatigue lives of D1, D2, and D3 are 11.3 years, 17.3 years, and 33.1 years, respectively. The initial crack size has a significant effect on fatigue life, and the crack length-to-depth ratio has a greater impact on the crack propagation process than on fatigue life.

Storm is an important threat to deep-sea mining operations. To assess this risk, asimulation and analysis model of the deep-sea mining riser system was established in this paper and the mechanical performance of mining riser system under a storm environment was analyzed. The safety of storm-resistant and storm-avoidance operations was evaluated. The speed envelope for storm evacuation was calculated and an optimization algorithm for calculating operation envelopes was developed. The results show that the mining riser system is very dangerous under stormy sea conditions, and the weak point is near the rigid pipe's suspension position. The vertical oscillation of the transfer tank is more significant than lateral oscillation in storm environments, and the fluctuation amplitude of the force exerted by the hose on the mining vehicle is not significant. During the storm avoidance evacuation process, the upper rotation is the limiting factor affecting the evacuation speed, and the maximum permitted speed occurs in the downstream direction, while the minimum occurs in the countercurrent direction. In the evacuation process, the horizontal force exerted by the hose on the mining vehicle is greatly affected by the speed. The optimization algorithm proposed in this paper can greatly reduce the calculation time of the operation envelope.

In order to study the influence of mass increase and the corresponding added damping and stiffness on the displacement response of beam members under a uniform blast loading, the elastic-plastic displacement solutions of beam members with mass parameters as variables were derived, using the ductility ratio to control the extent of plastic deformation. Using a rectangular section as representative case for beam members, we designed 17 calculation cases for the parameters of mass, additional damping, and additional stiffness. The effect of these parameters on the vibration displacement of beam members under uniform blast loading was analyzed, particularly the impact on peak elastic and the peak elastic-plastic displacement. The comparison of LS-DYNA numerical simulation verification and theoretical results of the displacement response of the rectangular section steel beam under blast loading was completed, which verified the reliability of the theoretical method proposed in this paper. The results show that the reduction of various peak displacements can be achieved by adding mass and the degree of reduction is lower than that of adding mass. The single effect of additional damping in the mass-damping-stiffness coupling calculation may increase the peak displacement reduction by more than 3 times of that obtained from the mass-damping coupling alone. The additional stiffness effect, when included in the mass-damping-stiffness coupling, reduces peak elastic displacements to similar degree as mass-stiffness coupling alone. The coupling calculation method of mass, additional damping, and additional stiffness should be uesd for the blast-resistant beam design.

In order to explore the regularity of dynamic response of stiffened plates under slamming load and establish static equivalent design method of stiffened plates, which covers the moving characteristics of slamming pressure and the dynamic response characteristics of stiffened plates, the slamming load acting on the flared bow is simplified in this paper into a moving pulse time history which rises linearly and decays exponentially. By using Abaqus finite element software, the elastic dynamic response of multiple stiffened plates under moving slamming load is calculated and the influence of different load parameters on the dynamic response of stiffened plates was analyzed. The Dynamic Load Factor D_LF(Dynamic stress equivalent factor) was introduced to analyze the variation rule and sensitivity of D_LF and the dimensionless parameters of slamming load. The regression and verification of D_LF formula were carried out. A parameter determination method of the simplified slamming pressure model is proposed. The comparison with the results of wedge drop test shows that the proposed slamming stress equivalentizing formula D_LF has higher precision and can effectively improve the efficiency of the preliminary design of ship structure under slamming load.

Aiming at the problem of flow noise reduction in sudden expansion pipe, this paper investigates the flow noise characteristics of viscoelastic fluid in a sudden expansion pipe and reveals the noise reduction mechanism. The flow noise in the sudden expansion pipe could be controlled by viscoelastic fluid which is obtained by mixing the surfactant with water. Based on the finite extensible nonlinear elastic model with Peterlin approximation (FENE-P), the flow noise is calculated by using Fluent UDF, combined with Large Eddy Simulation and the Ffowcs Williams-Hawkings method. The Reynolds number is 8000, the maximum polymer tensile length is 10, and the Weissenberg number (We) ranges from 0.1 to 12. The results show that when We reaches a certain value, the flow field is stabilised and the generation of vortices is delayed. Thus, the formation of large vortex that generates low-frequency noise is inhibited. The fluctuating pressure and fluctuating velocity inside the pipeline and at the pipe wall are reduced significantly, so that flow noise will be suppressed from the perspective of sound source. With the increase of We, the effect of noise reduction is enhanced and the frequency range of noise reduction is extended, but when We exceeds a certain value, the noise reduction effect reaches the threshold.

This paper is to study the principle of line spectrum feature extraction. In view of the deficiency of manual line spectrum feature extraction method, a line spectrum feature extraction method based on machine learning was proposed. The Encoder-Decoder based on convolution neural network was built, and the attention mechanism was introduced between the convolution and pooling layers, so that the important features of the input data could occupy a higher weight to enhance the accuracy of feature extraction. The model was compared with U-Net model and TPSW algorithm in the case of low signal-to-noise ratio, and tested on the actual data. The experimental results show that the improved model achieves a line positioning accuracy of 0.823 at a signal-to-noise ratio of 5 dB. This performance is better than that of the U-Net model and TPSW algorithm with in the 0~5 dB range. Thus the model effectively extracts line spectrum information and improves the accuracy of underwater target detection.

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.