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.

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.

Accurately identifying the far-field radiation hotspots of structures is of great significance for vibration and sound radiation control. In this paper, an open-source boundary element program is used to obtain the radiation impedance matrix of the cylindrical shell, and the normal vibration velocity of the shell is extracted. Based on the singular value decomposition, the non-negative intensity and radiated sound power of the underwater cylindrical shell are obtained, and the far-field acoustic radiation model is acquired, moreover, the method is verified by finite element software. On this basis, the influence of large stiffener at the stucture, where an excitation force is applied, on non-negative intensity is discussed. Furthermore, the mathematical model of active control is established with the aim of minimizing the acoustic radiation power. The influence of control force on structural sound radiation mode is analyzed, and its influence mechanism is revealed using the non-negative intensity distribution. The results show that the radiation mode of the structural surface must be changed to reduce the radiation acoustic power. And the essence of secondary force controlling structural vibration radiation is to change the structural vibration from strong radiation mode to weak mode, thus reducing the radiation efficiency of the structure.

Accurate motion prediction is crucial for the safe navigation of Autonomous Underwater Vehicles (AUVs). A fast Least Squares-Support Vector Machine (LS-SVM) motion prediction approach based on ridge regression algorithm is proposed in this paper. Firstly, the ridge regression analysis was incorporated into the traditional hydrodynamic model and the correlation analysis was conducted on the acceleration time-series input variables to identify the relatively important components. This step effectively reduces computational complexity while maintaining prediction accuracy. Subsequently, for the issue of high dimensionality and computational complexity in the LS-SVM algorithm's kernel function matrix, an improved Lagrange function was designed to eliminate redundant bias terms. This modification lightens the burden of calculating high-dimensional kernel matrices and further enhances the speed of maneuvering prediction. Finally, case studies based on the REMUS model demonstrate that the proposed strategy, compared to the standard LS-SVM prediction method that relies on traditional hydrodynamic models, reduces computational runtime by 29.8% while ensuring prediction accuracy.

Surface piercing propellers are one of the preferred propulsion systems for high-speed crafts due to their favorable features such as no cavitation, low resistance, shallow draft and high efficiency. Since the propellers often operate in oblique flow with blade emergence, there will be vertical forces generated which will have an important impact on the hydrodynamic performance of the planing crafts. In this paper, the effects of vertical force generated by surface piercing propellers on hydrodynamic performance of a stepped planing craft through numerical and experimental methods were studied. And the cases for planing boat with different locations of center of gravity were also involved. The results show that the vertical force produced by surface piercing propellers has a positive effect on the resistance of the planing crafts and that the effect is related to the longitudinal position of the center of gravity of the planing boat. Consequently, the longitudinal position of the center of gravity should be adjusted to the stern for planing craft equipped with surface piercing propellers, and the vertical force of the propellers should not be too large.

The Boussinesq model is a kind of wave model widely used in near-shore engineering, and its computational accuracy mainly depends on the basic performance of the model, while the upper bound of nonlinear application of the model has always been the focus of attention. In recent years, the two-layer Boussinesq model has gained great progress regarding theoretical properties, numerical modeling and applications. However, the value of its nonlinear upper bound has not been given in any literature. So in this study, the stream-function of the two-layer Boussinesq model was solved using a combination of genetic algorithm and Newton's method to determine the upper bound value of the model, considering the highest spatial derivatives of order 3 and 5. In the same way, the stream-function solutions of the corresponding one-layer Boussinesq model were derived. The numerical results show that the nonlinear upper bounds of the two-layer Boussinesq model with the highest derivatives of order 3 and 5 are H/L = 0.137 and 0.138. Compared with the one-layer Boussinesq model, the two-layer model has a greater water depth of applicability regarding strong nonlinear characteristics. The combination of genetic algorithm and Newton's method proposed in this study can provide some references for solving the stream-function waves of the related Boussinesq models.

Aiming at the problem that the ship's course is easily disturbed by the external environment during navigation, an improved non-singular terminal sliding mode control strategy based on Nonlinear Extended State Observer (NESO) is proposed. Firstly, a nonlinear mathematical model of ship's heading system is established, and a nonlinear expanded state observer based on the hyperbolic tangent function is designed to estimate the external disturbances encountered during the ship's navigation and perform feedforward compensation. Then, based on the improved non-singular terminal sliding mode surface and the double power reaching law, an improved non-singular terminal sliding mode control law is designed, which reduces the chattering of the sliding mode control law and improves the convergence speed. The nonlinear extended state observer and the improved non-singular terminal sliding mode control law are combined to form a composite control law. Finally, the stability of the control law is analyzed based on Lyapunov stability theory. Simulation results show the proposed control strategy can effectively improve the tracking performance and robustness of the ship heading control system.

In order to solve the problem of path redundancy and long algorithm execution time, this paper proposed a path planning method that combines the Northern Goshawk Optimization (NGO) algorithm with the improved rapidly-exploring random tree (RRT*). First, a fitness function with obstacle avoidance and goal orientation was designed to optimize the initial NGO population. Additionally, the adaptive sampling step size of the RRT* algorithm was designed according to the fitness function to improve the search efficiency in a large-scale map. Then, the optimal neighbor node sampling mechanism was designed to simulate behavior of the northern goshawk transmitting information to its nearest companions, while also the RRT* node sampling was constrained by considering the USV’s (unmanned surface vehicle) heading angle. Finally, in order to improve path smoothness, the Metropolis criterion was introduced and the smoothness and minimum rudder angle design fitness function were combined to select a more suitable parent node for dynamic rerouting. The experimental results show that compared with RRT*, Informed-RRT* and RRT*-smart algorithms, the improved algorithm reduces the path length by 19.36%, 3.36% and 5.98%, and decreases the search time by 49.33%, 57.01% and 59.16%, respectively. At the same time, the curvature of the path also decreases significantly.

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.

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.