Combining the resonance method and the direct wave extracting method, the dynamic mechanical parameters of viscoelastic materials in a continuous and wide frequency range under controllable water pressure were measured. A pressure chamber test system capable of realizing underwater pressurization was built, a short time broadband impulse generated by an electromagnetic shaker was employed to excite a bar-like sample attached inside the pressure vessel. On one hand, the resonance method was used to calculate the mechanical parameters at resonance frequencies while on the other hand, the direct wave signal was extracted, and the mechanical parameters in a wide frequency range were calculated by the wave velocity method. The broadband mechanical parameters (including the storage modulus and loss factor) under variable water pressure conditions (0.1-6 MPa) for two types of viscoelastic materials were obtained. The experimental results agree well with each other, proving the test method in this paper could directly determine the Young’s modulus at frequencies ranging from 500 Hz to 5000 Hz under variable water pressure. This method provides simplified processing for underwater dynamic mechanical parameter measurements.

In order to accurately calculate the stress intensity factor at the deepest point during the crack propagation process of high-pressure vessels, a method was proposed for fitting the stress data with high-order polynomials and then calculating the stress intensity factor. Taking the crack at the opening of a high-pressure vessel as an example, based on stress data collected with varying data volumes, polynomial fitting of varying degrees was employed to calculate the stress intensity factor at the deepest point of each crack depth. The influence of polynomial degree and data collection volume on the calculation results was analyzed, and the calculation results of this method were compared and validated against the linear interpolation method in the literature. The research results indicate that as the increase of polynomial degree, the characterization accuracy of the fitted curve improves, and the calculation results gradually converge and stablize. The relative error between the calculation results using low-order (third-order) and high-order polynomial fitting shows an "inverted S" trend, with a minimum relative error of about −20%; As the amount of data increases, the calculation results gradually converge, and the relative error of the calculation results under lower and higher data volumes shows a trend of oscillation attenuation, with a maximum relative error of about 7.1%; The calculation results based on high-order polynomial fitting and piecewise linear interpolation are basically consistent, indicating that this method has certain reliability and is suitable for calculating the stress intensity factor at the deepest point during crack propagation.

The welding displacement of marine medium plate caused by welding seriously affects the structural integrity. Butt-welded joint of AH36 steel with the thickness of 14 mm was fabricated using CO₂ welding process and the out-of-plane displacement was straightened using self-developed electromagnetic induction back-heating equipment. A series of experiments were conducted to measure the microstructure, mechanical performance, transverse residual stress and out-of-plane displacement. A welding-back-heating numerical simulation method was proposed, and applied to predict the heating conduction, stress, strain and out-of-plane displacement during the processes of butt welding and back heating. Based on the deformation theory, the transverse bending moment after butt welding and back heating was computed. Results show that the microstructure is mainly cementite and back heating almost has no influence on either the microstructure or mechanical performance. The peak value of transverse residual stress is mainly at the weld seam and becomes larger due to back heating. The back heating generates a larger transverse compressive plastic strain near the surface of AH36 plate, thereby obviously straightening the out-of-plane deformation through the transverse bending moment.

The use of acoustic holography technology for identifying moving sound sources and predicting their sound field in limited water areas is of great significance for studying the sound source and radiation characteristics of underwater vehicles. However, the acquisition of sound pressure radiated by moving sound sources and the influence of interface effects in limited water areas have brought difficulties to the application of acoustic holography technology in limited water areas. In view of this, the motion of the sound source was considered and the linear array measurement and moving frame technology were used to obtain holographic pressure radiated by moving sound sources. Considering the influence of interface effects in limited water areas, three holographic inversion models were constructed based on the equivalent source method in three different interface situations. Numerical simulations were conducted on cylindrical shells in limited water areas, and the results showed that the three inversion methods can achieve better sound source identification and sound field prediction results compared to the free field inversion method without considering interface reflection. A moving standard sound source experiment was conducted in a lake, and the experimental results showed that the proposed method can effectively locate the sound source and accurately predict the sound field.

The flexible boundary constraints have a significant effect on the vibration characteristics of rectangular stiffened plates excited by turbulent boundary layer. In this paper, springs were used to simulate flexible boundary constraints. The response function of an underwater rectangular plate was derived based on the energy principle, combined with the power spectra density expression caused by turbulent boundary layer, and the power spectral density of the plate velocity was obtained. It is shown that the boundary spring stiffness has an effect on the response of the plate. As the spring stiffness of the boundary displacement increases, the vibration response of the plate at low frequencies decreases. The effect of the boundary spring stiffness on the vibration response of plate excited by TBL converges when the boundary displacement spring stiffness k_d is larger than 10¹⁰ N/m². The effect of stiffened rib direction and the number of stiffened ribs on the vibration response of stiffened plate was also studied in this paper. Compared with an unreinforced plate, the vibration response of a stiffened plate at low frequency was effectively reduced. The results can serve as a theoretical reference for the analysis of the vibration characteristics of rectangular stiffened plates excited by turbulent boundary layer under flexible boundary constraints.

In view of the fact that it is difficult to obtain analytical solutions for vibration problems of combined shells and it is hard to solve strongly coupled acoustic and vibration control equations, a Ritz-Legendre spectral element method was proposed to discuss the vibration characteristics of underwater conical-cylindrical-spherical shells. Based on Reissner shell theory, virtual spring technology and the displacement angle relationship of adjacent subshells, the theoretical structural model of the combined shells was established. The Legendre spectral element method was introduced to avoid the problem of discontinuity of normal derivative and discretize the Kirchhoff-Helmholtz boundary integral equation, then the theoretical model of underwater external sound field was constructed. Based on Fourier transform and coupled surface Euler equation, the coupled vibration control equation of underwater combined shells was obtained. Compared with FEM simulation results, the convergence, reliability and correctness of this method were verified. This method can provide theoretical reference for engineering application in the design stage.

Underwater target azimuth estimation is a critical technology in array signal processing, with wide applications in military operations, marine resource development, and environmental monitoring. A comprehensive review of the current development status of underwater target azimuth estimation methods is provided in this paper. Firstly, an introduction to the acoustic mathematical model based on an uniformly distributed sound pressure line array was given. Next, azimuth estimation methods are classified into four categories: classical beamforming, statistical, subspace, and AI-based Direction of Arrival (DOA) estimation methods. Key factors affecting azimuth estimation accuracy, such as array calibration errors, array geometry, signal processing techniques, and underwater acoustic channel characteristics, were also analyzed. Finally, the paper discussed the limitations of current azimuth estimation technologies and proposed future research directions, including multimodal data fusion, integration of deep learning with physical models, and the development of new array structures etc, to enhance the accuracy and robustness of underwater azimuth estimation.

Tip clearance flow is a complex phenomenon that occurs between the rotor blade tip and the inner surface of the duct of a pump-jet propulsor. The tip clearance size significantly influences both the tip clearance flow and the performance of the pump-jet propulsor. Previous studies on tip clearance flow primarily focused on cases with tip clearance sizes less than 4 mm on model scale. Tip clearance flow of pump-jet propulsors with tip clearance sizes of 1 mm and 16 mm were simulated based on large eddy simulation in this paper. The study focuses on the characteristics of tip clearance flow in the large tip clearance pump-jet propulsor and the effects on cavitation inception, hydrodynamic performance, and duct pressure fluctuation. The results indicate that, compared to smaller tip clearance, the starting position of tip-separation vortex of pump-jet propulsor with large tip clearance is closer to the leading edge of rotor, while the intersection position of tip-separation vortex and tip-leakage vortex is closer to the trailing edge of rotor. Furthermore, the propulsion efficiency of the pump-jet propulsor behind SUBOFF is reduced by approximately 10%. The vorticity and circulation of tip-leakage vortex are larger, and cavitation inception of tip-leakage vortex occurs earlier. The amplitude of fluctuating pressure on duct inner surface is significantly decreased by about 80%. Therefore, the design of the pump-jet propulsor should be made based on comprehensive balance of the above-mentioned performance characteristics to find the optimal tip clearance size.

Marine operations, such as takeoff and landing of carrier-based aircraft, ship fuel supply, and ship lifting operations, have strict requirements for the movement of ships and need to be carried out within the quiescent period window to ensure operational safety. However, the incidence probability of a quiescent period window under high sea conditions is relatively low. Therefore, it is crucial to assess the probability of the target sea area’s quiescent period in advance for the planning and deployment of offshore operations. This article establishes a ship motion probability model based on the statistical characteristics of ship motion to study the short-term statistical characteristics of quiescent periods. Simulation data is used to verify the statistical characteristics of motion, the joint statistical features of motion amplitude-period, proving the accuracy of the model. Subsequently, the probability of continuous small movements below the operational threshold during the quiescent period have been analyzed. The results indicate that the established ship motion probability model and the ship continuous small amplitude motion probability model can accurately reflect the motion characteristics and probability of quiescent periods of ships under corresponding sea conditions. This study can provide the probability of a quiescent period meeting the operational requirements before the ship enters a certain sea area for operation, and also provide the probability of a quiescent period occurring in a certain period of time before the operation, providing auxiliary decision-making for the navigation operation plan of offshore ships.

To investigate the turning maneuverability of polar ships in floating ice area, in this study a combined CFD-DEM approach was adopted to numerically simulate the turning motion process of a medium-sized polar ship in floating ice area. In the simulation the ship’s turning motion at different rudder angles and ice concentrations have been considered, and the parameters of the ship’s turning motion have been predicted. The results show that the existence of floating ice will significantly hinder the ship’s turning motion, and the range of the tactical diameter in floating ice area is 1 to 1.5 times than that in open water conditions. The forces and moments acting on the hull exhibit strong randomness, and the instantaneous fluctuations of the ship’s speed and yaw rate are more pronounced. At the same rudder angle, the ice longitudinal force increases with the increase of ice concentration, while the variation of the fluid longitudinal force is not significant. The average total lateral force and total yawing moment are in the same direction as the turning maneuver, while the average ice yawing moment tends to be in the opposite direction.