The polar environment is cold, complex, and characterized by a lack of data. Aluminum alloy multi-environment boats navigating in polar regions will encounter four types of interface conditions, including waves, water ice, snow, and ice-water interfaces. Therefore, it is necessary to conduct research on the fatigue damage of aluminum alloy multi-environment boats in low-temperature environments. This paper is based on a time-domain analysis method analyzing the stress response of the boats under four types of interface loads. The rainflow counting method was used to statistically analyze the time history of hot spot stresses, and the final damage and fatigue life were calculated based on the Miner linear cumulative damage theory. On this basis, the damage sensitivity to ship speed, interface height, and ice density was analyzed. The results indicate that wave impacts and water ice loads contribute significantly to hull damage, with ice density being the most significant factor affection damage. Fatigue tests were conducted on low-temperature aluminum alloy materials, welded joints, and actual ship nodes. The S-N curve of low-temperature aluminum alloy was fitted, and the reliability of the S-N curve was verified through fatigue tests of actual ship nodes. The research findings can be utilized for the fatigue assessment of 5059 aluminum alloy boats in low-temperature environments.

The identified nonparametric model was used to tune the control parameters of the ship motion. Local Gaussian Process Regression (LGPR) method was utilized to model and predict the ship motion based on the collected ship motion data. The tuning of control parameters was converted to an optimization problem. The fitness function was designed to improve the control performances, and Genetic Algorithm (GA) was applied to tune the control parameters. The simulation results demonstrate that the proposed method has satisfactory control accuracy and dynamic performances in the path following and obstacle avoidance control.

The performance of Autonomous Underwater Vehicles (AUVs) is significantly influenced by their shape design. This study presents a novel AUV shape optimization method that integrates data-driven approaches and machine learning technique to focus on the impact of the head and tail profiles, and the configuration of the fins and rudders on AUV performance. A parameterized hydrodynamic analysis workflow was developed to automatically generate the hydrodynamic parameters required during the optimization process. Based on this workflow, a data-driven framework was constructed for multi-objective optimization of AUV shapes, with the aims of minimizing drag and maximizing maneuverability. Surrogate models for the two optimization objectives above were built using Multilayer Perceptron (MLP) neural networks and ensemble learning methods respectively, and their performances were compared with traditional surrogate models. The optimization problem was solved using the Non-dominated Sorting Genetic Algorithm II. Comparative analysis of the initial and optimized AUV shapes demonstrates significant improvements in hydrodynamic performance, confirming the feasibility and effectiveness of the proposed method.

In order to explore the plastic accumulation characteristics of box girders under cyclic hogging and sagging bending moment loads and to study the attenuation law of ultimate strength of box girders based on incremental plastic failure, this paper creatively designed a test device for continuous cyclic bending moment loading, and carried out a series of four-point bending ultimate strength tests on box girders under different loading conditions based on replaceable box girder specimen. Combined with numerical simulation, this study analyzed the variation characteristics of cumulative plasticity critical point, cumulative plastic deformation and neutral axis height of box girders under cyclic loading, and then the influence of constant and variable amplitudes of rotation angle loads on the ultimate strength attenuation of box girders was evaluated. After these investigations, the fluctuating range for the ultimate bearing capacity of ships encountering random extreme waves bending loads was determined to ensure the safety and reliability of ships sailing in severe sea conditions.

Ship structures can fail due to extreme or cyclic loading, corrosion and erosion, so it is necessary to perform structural health monitoring (SHM) on ships to ensure the safety, reliability and integrity of ship structures to avoid major accidents. Displacement reconstruction is one of the main objectives of structural health monitoring. The inverse finite element method (iFEM) is a structural health monitoring method for real-time reconstruction of full-field displacement in plate and shell structures. In the case of shells with large curvature, existing iFEM algorithm will suffer from the warping problem, which affects the fitting accuracy. In this paper, an improved iFEM algorithm is proposed, which does not change the displacement-strain relationship of the element itself. The new algorithm improves the calculation accuracy of the shell element by estblishing tangent plane coordinate system at the element’s Gaussian integration points and improving the element stiffness integration, without increasing the mesh and node quantities. The result of numerical examples shows that the improved inverse shell element can effectively reduce the displacement error after inverse finite element reconstruction.

Due to the significant differences in material properties between the sandwich panel skins and core, sandwich panels exhibit different vibration characteristics in different frequency bands. This leads to different sound insulation mechanisms and calculation methods in different frequency bands. In this paper, theoretical methods applicable to low-frequency, high-frequency, and full-frequency ranges respectively were adopted to investigate the sound insulation performance and mechanisms of sandwich panels in different frequency bands. The results indicate that the sound insulation of sandwich panels in the low-frequency band is mainly determined by bending vibration mode, while the sound insulation in the high-frequency band is more affected by normal longitudinal waves. In the transition frequency band between the low-frequency and high-frequency bands, the sound insulation is affected by both bending waves and longitudinal waves. The phenomenon of the sound insulation trough caused by bending modes of sandwich skins when the core material behaves as an elastic foundation was analyzed in this paper. The influence of material damping on sound insulation in different frequency bands was studied, and the mechanism of damping weakening the sound insulation trough was analyzed. Finally, the effect of core material parameters and panel areal density on their sound insulation performance were presented.

Due to the difficulty of attenuation of middle and low frequency band sound waves in the process of propagation, the control of middle and low frequency broadband sound waves has become a challenging topic, so it is necessary to develop new materials and structures with low frequency sound absorption and noise reduction functions. The special properties of acoustic metamaterials provide new ideas for the development of sound absorption and insulation. In order to effectively control the noise in the middle and low frequency bands, a new spatial spiral acoustic metamaterial was designed and optimized by using the finite element software COMSOL Multiphysics, and the sound absorption and sound insulation performance in the 100~2500 Hz frequency band were calculated and analyzed. With the help of 3D printing for completing the preparation of metamaterial, the sound absorption and insulation performance of spiral acoustic metamaterial were compared in an experimental study to verify the accuracy of the calculation method.

The stored air mass (i.e. muzzle gas cloud) ahead of a projectile nose in a vertical launch tube has significant influences on the flow field and loads during underwater launching. In this paper the process was simplified to an impulsively started projectile in a stationary vertical tube, which then passied through air mass with constant velocity. The flow was investigated using numerical simulation. The primary conclusions are as follows: The air mass is compressed, pushed out, entrained, and finally forms an annular oscillating bubble, which is accompanied by intense unsteady vortex around the muzzle platform and the projectile. The oscillating pressure induced by the bubble propagates through the flow field and attenuates with time and distance. The oscillating pressure can be considered as being linearly superimposed on original flow pressure, resulting in a periodic adverse pressure gradient along the projectile surface, which affects flow separation and cavitation. The oscillating drag coefficient for different volumes of the air mass can be normalized by dimensionless time defined using the velocity and equivalent spherical diameter of the air mass.

In order to study the influence of multiple damage modes on the ultimate bearing capacity of ship beams, the nonlinear finite element method was used to calculate the ultimate bearing capacity of multiple groups of stiffened plates including pitting, fracture and sag damage modes and their combinations. The influence of multiple damage modes on the ultimate strength of the stiffened plate structure under axial compression is obtained, the ultimate strength calculation formula of the damaged stiffened plate is regressed, and the end shrinkage curves of the stiffener elements under the condition of depression, fracture and corrosion damage modes and their different combinations are constructed. The step-by-step iterative approach of the ultimate strength of the combined damaged hull beam under monotonic load is proposed, and the calculation procedures have been prepared. The actual ship calculation shows that the proposed method is simple to calculate, and the error between the proposed method and the ultimate bending moment calculation result of the finite element method is within 10%, which can be applied to the evaluation of the ultimate strength of old ships.

There still remain many challenging topics for CFD to numerically simulate flow around a bluff body at subcritical Reynolds numbers, such as the high-fidelity resolving and capturing for instability structures in the shear layer, as well as the periodic shrinkage and enlargement of the recirculation region. This paper presents the development of a RANS-based wall-modeled large eddy simulation method (RANS-WMLES) to provide a high-fidelity CFD tool for numerically simulating such complex flow phenomena around a bluff body. Such a new method is different from traditional hybrid RANS/LES models. In particular, for the new method the transition from RANS to LES can be achieved through a filtering parameter which is only related to local grid parameters. Moreover, the transition can be pre-controlled through two customizable parameters and for not only the transition boundary positions between RANS and LES, but also the ability of resolving turbulent kinetic energy. A series of numerical simulations for flow past a sphere at subcritical Reynolds number Re=3700 show that the new method is capable of resolving and capturing with high-fidelity temporally/spatially developed coherent structures for such complex three-dimensional flows around a sphere.