The stern tube seal of a ship often generates abnormal vibration and noise due to the friction excitation between the static ring and the dynamic ring. Through shipboard testing, it was found that this abnormal vibration appears in the form of the torsional vibration natural frequency of the static ring and its harmonics in the vibration spectrum. In order to reveal the cause of this phenomenon, this paper establishes a nonlinear torsional vibration analysis model for the stern tube seal considering the contact-friction effect between the static ring and the dynamic ring. The influence of parameters such as shaft system rotational speed, friction coefficient, Ω spring preload, Ω spring torsional stiffness, and structural damping ratio on the vibration of the stern tube seal was studied using multiscale method and numerical simulation. The results show that when the equivalent damping ratio induced by the friction excitation is smaller than the structural damping ratio, the static ring of the stern tube seal will exhibit friction-induced self-excited vibration, that is, the occurrence of the torsional vibration natural frequency of the static ring and its harmonics in the vibration spectrum. Increasing the Ω spring preload, and increasing the difference in static and dynamic friction coefficients between the static ring and the dynamic ring, or reducing the rotational speed of the shaft system and the torsional stiffness of the Ω spring will make the static ring more susceptible to friction-induced self-excited vibration. Increasing the structural damping ratio can effectively suppress the friction-induced self-excited vibration of the static ring. Finally, this paper provides a critical formula for the occurrence of friction-induced self-excited vibration in the static ring. This research has important guiding significance for the design and maintenance of stern tube seals.

The machining deformation of a large pressure cylindrical shell might affect its subsequent machining accuracy and application requirements. A large titanium alloy pressure cylindrical shell after deformation was studied in this paper. Based on the thermal elastic-plastic and creep finite element theory, the numerical simulation of thermal correction of the deformation correction process of the cylindrical shell was carried out through the calibration tooling and thermal correction method. The results show that the error of elliptic end calibration of the deformed large titanium alloy pressure cylindrical shell is controlled within 1%, which could meet the engineering requirements. The overall calibration effect achieved with the calibration tooling of ring-shaped is better, and the error is reduced by 0.2% compared to the calibration tooling of cross-shaped. The holding time, heating rate and cooling rate of heat treatment have little effect on the calibration effect of the deformed cylindrical shell, while the spring stiffness has a great influence. Finally, a set of good heat treatment straightening process for large pressure cylindrical shell was obtained.

To investigate the damage characteristics of supercavitating projectiles against underwater structures, we conducted a simulation study of a 12.7 mm supercavitating projectile impacting typical targets at different speeds. The reliability of the simulation model was verified based on experimental results, and a detailed analysis of the projectile-target penetration characteristics under full water conditions was performed. The results show that compared to the unreinforced curved target, the ballistic limit velocity of the weakest unreinforced part of the reinforced curved target increased by 3.1%, and the surface depression caused by water load squeezing decreased by 27.3%. As the projectile velocity increases, the energy absorption ratio of the target plate before penetration increases to varying degrees, but the unreinforced curved target is most affected by the water load before penetration. Under the same projectile velocity, the reinforced curved target effectively restrained the overall deformation of the target plate, but its crack growth energy consumption was greater than that of the unreinforced curved target, leading to a deeper crack propagation along the target thickness. The higher the projectile velocity is, the narrower the crack extension around the hole is, indicating reduced crack growth energy consumption, with bending energy dissipation becoming dominant. The results can offer guidance for the design of underwater structures.

Stereo vision wave measurement technology has become one of the most promising methods for monitoring wave fields at present. However, traditional feature extraction and matching methods rely on feature point extraction, therefore cannot meet the requirements of rate and accuracy in practical applications, thereby affecting the effect of 3D reconstruction. This paper introduces the LoFTR (Local Feature Transformer) algorithm. Taking the wave images captured by the binocular vision wave measurement system installed on a certain fixed offshore wind turbine as the image set, an efficient wave feature extraction and matching technology based on artificial intelligence algorithms and 3D wave field dense reconstruction technology is constructed. The algorithm has been verified under various lighting environments. The results show that the LoFTR algorithm can extract an average of 4300 pairs of matching points per frame for wave images under normal lighting, while taking 0.9 s per frame. The matching effect is significantly better than that of traditional methods. Moreover, for images with unclear textures in the night environment, the algorithm can still stably match more than 2000 pairs of matching points, achieving dense reconstruction of the 3D wave field and providing core technical support for the intelligent monitoring of ocean waves.

Compared with an expansion muffler with a rigid back cavity, the expansion muffler with a flexible back cavity has better low-frequency noise reduction performance. However, most of the current calculation methods for expansion mufflers with the flexible back cavity are based on transfer matrix methods under the plane wave assumption. The calculation error of such methods increases with acoustic-structure coupling, and it is difficult to obtain the modal function of the flexible structure under elastic boundary conditions with traditional calculation methods. Therefore, this paper proposed a calculation method based on the energy principle that does not rely on the plane wave assumption. The model of the muffler is decomposed into three sub-acoustic cavities, and these cavities are coupled to each other through coupling surfaces. Then, the sound pressure function of the sound field and the displacement function of the flexible structure are expanded into three-dimensional and two-dimensional Chebyshev series respectively, and the Rayleigh-Ritz method was used to solve the unknown coefficients in the Chebyshev series. The sound pressure and transmission loss of the muffler were obtained, and the correctness of the theoretical model was verified by comparing it with the FEM results. Finally, the coupling characteristics were analyzed, and the effects of boundary constraints and muffler parameters on transmission loss were studied. The results show that the expansion muffler with a flexible back cavity has a lower natural frequency and stronger low-frequency coupling effect than that with a rigid back cavity. The impact of boundary constraints on transmission loss is mainly reflected above 1000 Hz. When boundary constraints are released, the peak value of transmission loss moves to low frequencies and increases. This shift is conducive to improving sound attenuation performance. As the back cavity’s length or radius increases, the transmission loss curve moves to the low frequency, and the influence of back cavity’s radius on the acoustic performance of the muffler is more obvious. As the thickness of the flexible wall or Young's modulus decreases, the transmission loss curve will move further to the low frequency.

This paper investigates the vibration reduction characteristics of spiral acoustic black holes (SABH) on ventilation ducts and the damping mechanism. Compared with conventional vibration damping methodologies, SABH possesses distinct advantages, including its lightweight nature, ease of installation, and the capacity for low-frequency broadband vibration absorption. The finite element simulation results demonstrate the efficacy of SABH in suppressing the vibration of ventilation ducts. Compared with the original duct without SABH, the resonance peak of the first three orders of acceleration levels at the monitoring point are reduced by more than 10 dB after installing SABH. Additionally, the total level of vibration velocity at the monitoring surface decreases by 3.73 dB. The experimental results of the vibration analysis indicate that after the implementation of SABH, the resonance peaks of the acceleration level at the measurement point in the low-frequency band are reduced by more than 10 dB. Furthermore, the high-frequency effects are found to be more pronounced. The study demonstrates that SABH exhibits superior low-frequency broadband vibration damping performance for ventilation ducts, and displays effective convergence and dissipation of wave energy within the pipe structure.

Large Eddy Simulation (LES) method was employed to simulate the pressure fluctuations on the wall of NACA 0015 hydrofoil. The spatio-temporal correlation characteristics were studied using wavenumber-frequency spectrum (WFS), and six empirical models were used to predict the WFS on the hydrofoil. The results show that at zero angle of attack, as the position of sensors moves towards the trailing edge, the dimensionless convection velocity gradually approaches the theoretical predicted value of 0.7. At the same incoming velocity, the spectral level of the WFS at the ridge increases with the increase of the angle of attack. The predictions of the six empirical models show differences from the simulation results, with the Efimtsov model having the minimum values and the Ffowcs-Williams model having the maximum values. The simulation results fall between the predicted values of the Efimtsov model and those of the Corcos model.

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.

When an underwater vehicle navigates near a wall, the flow field around the underwater vehicle will be changed, which greatly affects its stability and maneuverability. Based on the CFD method, the hydrodynamic performance, including straight-ahead performance, oblique navigation performance, was analyzed under the conditions of near upper wall, bottom wall and vertical narrow channel. Finally, the following conclusions are drawn: when the underwater vehicle sails at a constant speed near the upper or bottom wall, it generates increased resistance, body suction and tail suction. When the vehicle approaching the upper wall at different distances, the directions of vertical force and pitching moment will be changed, and the ratio of direction transition interval is between 5-6 and 3-4, respectively. When the underwater vehicle sails at a constant speed near the vertical narrow channel, the influence of the wall effect on its vertical force and pitching moment has a relatively obvious competition relationship, and the competitive percentage accounts for about 50%. All hydrodynamic coefficients can be fitted by quadratic polynomials based on the reciprocal of the spacing ratio. The hydrodynamic coefficient of wall effect can be obtained from a graph by monitoring the combinations of different upper and lower spacing ratios when simulating the maneuverability of underwater vehicles.

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.