In order to investigate the characteristics of high-speed trimaran’s resistance and flow field under shallow water conditions, a numerical method was established based on URANS equation and overset grid technology. Numerical simulations were carried out and the influences of water depths on resistance, navigational attitude and flow field of high-speed trimaran were analyzed. The results indicate that the resistance of high-speed trimaran under shallow water is mainly affected by wave-making and depicts different variation characteristics at different speeds. At Fr=0.439, the hull resistance in shallow water increases by 4.64%～23.04% compared to resistance in deep water and reaches the peak at critical water depth. At Fr=0.658, the hull resistance increases by 2.07% at critical water depth. With the depth continuing to decrease, the hull resistance decreases by 3.61%～4.86% compared to resistance in deep water. The change of water depth has little effect on the frictional resistance, while has a great effect on the residual resistance. With the decrease of water depth, the heave of the high-speed trimaran changes from sinking to lifting, the trim increases and the axial nominal wake fraction decreases. Around the critical water depth, there is a significant wave crest in the area between the bow of the main hull and the front of the side hull at Fr=0.439.

The interaction between waves and marine structures involves complex geometric boundaries, wave impacting, overtopping. To accurately simulate this complex phenomenon, DualSPHysics is employed to build a numerical wave flume based on Smoothed Particle Hydrodynamics (SPH), which is a grid-free particle method. In order to validate the accuracy of the model, regular waves are generated by a piston-type wavemaker. By analyzing the wave height with different particle spacing, the model was demonstrated that it can generate stable regular waves. Furthermore, the impact of regular waves on a vertical wall is simulated with the model. Under the action of wave impact and inertial force of the water, the pressure at different positions on the vertical wall shows the following characteristics: it quickly reaches the peak in a short time, then decreases, increases again, and finally decreases. The time series of pressure have a saddle shape. By comparing the wave shape and velocity field at the same time, it is found that the angle between the wave and the vertical wall, as well as the water head rise, are the main factors leading to this saddle-shaped pressure curve.

In order to study the ship seakeeping and hydroelastic responses in realistic random waves, a CFD-FEM two-way fluid-structure coupling method is employed to simulate the ship motions and wave load response in head irregular waves. First, a numerical tank and a ship finite element model are established in CFD and FEM solvers, respectively. A numerical simulation scheme based on high-performance computing platform is addressed for large-scale numerical computations. Then, the long-crested irregular waves are generated in the CFD solvers. The reliability and accuracy of the numerical waves is validated by comparison with theoretical results. Furthermore, the obtained results of the motion and load response of the ship hull in irregular waves, including the time series, frequency spectrum and probability of exceedance of the signals, such as heave and pitch motions, vertical acceleration and sectional bending moments, etc., are comprehensively studied and discussed. These results are also validated by comparison with those of the segmented model tests. The analysis reveals that the results of ship motions and VBM at midship simulated by the presented method are in good agreement. The errors of the significant amplitudes of heave, pitch motions and VBM amidships between experimental and numerical results for a typical case are 4.8%, 0.8% and 1.6%, respectively.

Cavitation is a special physical phenomenon that exists in fluids, and the cavitation model is a key technology in cavitation flow calculation, which describes the cavitation phase transition process. The rate of cavitation phase transition is directly related to the nucleus density. In this paper, the R-P equation and nucleus transport equation are used to improve the bubble dynamics-based cavitation model, so that the new model includes the influence of nucleus density evolution with the cavitation process. The improved cavitation model can effectively simulate the collapse process of two-dimensional vapor bubbles. In the final stage of vapor bubble collapse, the evolution of gas phase volume occupancy can reach 0 in a limited time while maintaining the stability of the calculation process. The simulation results are in better agreement with the theoretical results.

Unsteady numerical simulations were systematically conducted to investigate the transient flow characteristics around a staggered heat exchanger tube bundle. The vorticity distribution derived from numerical simulations was analyzed using Dynamic Mode Decomposition (DMD) for modal extraction and flow field reconstruction. The effects of the number of snapshots and singular value truncation order on the accuracy of reconstructed flow fields were studied. The results indicate that DMD enables accurate extraction of both vortical mode structures and their associated characteristic parameters, including frequencies and growth rates, from transient flow fields surrounding a staggered tube bundle. Flow field reconstructions obtained from dominant coherent modes exhibit strong agreement with high-fidelity numerical benchmarks. This confirms the effectiveness of the DMD methodology in resolving vortex-driven flow interactions in staggered tube bundle configurations. A minimum snapshot criterion is proposed based on the dimensionless period number, allowing direct determination of an appropriate number of snapshots with minimal computational error, which in turn substantially reduces computational costs. In terms of the energy retention ratio, a criterion for determining the optimal singular value truncation order is established, alleviating errors caused by overly aggressive truncation thresholds. This study provides theoretical insights into parameters selection for DMD analysis of complex flow fields around heat exchanger tube bundles.

This paper establishes a three-dimensional turbulence model using the CFD method in OpenFOAM. It compares and analyzes the effects of spacing ratios (L/D = 1.5~6.0) at Reynolds number Re = 3900 on the wake flow characteristics, force coefficients, Strouhal number, mean pressure coefficient, and mean velocity distribution together with the flow mechanims, for both the single-row three-cylinder and the three-row three-cylinder configurations. The research results show that the wake flow patterns are classified into four types: reattachment flow (reverse flow reattachment), reattachment flow (reverse flow without reattachment), narrow wake impact, and synchronized detachment. The critical spacing ratio for the three-row three-cylinder configuration is L/D = 3.5, which is smaller than the critical spacing ratio of L/D = 4.0 for the single-row three-cylinder configuration. The drag coefficient, Strouhal number, and lift coefficient root-mean-square values of the three-row three-cylinder are generally higher than those of the single-row three-cylinder structure, and a desynchronized Strouhal number phenomenon is observed in the three-row three-cylinder configuration. The trend of mean pressure coefficient distribution of the single-row three-cylinder is similar to that of the central column of the three-row three-cylinder. For the outer columns of the three-row three-cylinder at small spacing ratios, the location of positive pressure is closer to the central column side. The gap flow mean velocity in the three-row three-cylinder configuration is relatively higher. This study provides a reference for the hydrodynamic calculation of pile groups in engineering applications.

To extend the effective wave attenuation period range of free surface breakwaters, a study on the wave attenuation performance of a combined structure consisting of multiple surface-piercing boxes and submerged bars was conducted using Bragg resonance. A numerical wave flume was constructed utilizing the Desingularized Boundary Integral Equation Method (DBIEM) based on time-domain potential flow theory, and the interaction between regular waves and the combined structure of multiple equally spaced free surface boxes with submerged bars was calculated numerically. The occurring condition of Bragg resonance, reflection strength and wave attenuation performance of the combined structure were analyzed through reflection and transmission coefficients. The effectiveness of the numerical calculation method in this paper was verified by comparing the numerical results with existing literature data. The variation trend of transmission and reflection coefficients with 2S/λ (S is the center distance of adjacent structures, λ is the incident wavelength) is given. The results indicate that Bragg resonance occurs when regular waves propagate through multiple structures, and the wave attenuation performance of these multiple structures is significantly better than that of single structures. For multiple structures, as the number, width, and immersion depth of the boxes increase, the reflection intensity of Bragg resonance also increases. This not only reduces the transmission coefficient but also enhances the ability to dissipate long-period waves. Furthermore, increasing the gap between adjacent combined structures mainly aims to expand the effective range of the wave attenuation period. The submerged bars improve the wave-dissipating efficacy against long-period waves. This study provides valuable insights for the design of composite breakwaters.

The collection of seabed ore particles is a critical phase in the development of deep-sea mineral resources. The wall-jet nodule-collection device has significant engineering value due to its high collection efficiency and minimal disturbance to the surface sediment of the seabed. In this study, high-speed imaging and computer-based image processing techniques were employed to investigate the influence of the jet Reynolds number (Re) on the dynamic behavior of ore particles. Based on Particle Image Velocimetry (PIV), the impact of Re, height-to-particle-diameter ratio (h/d), and jet thickness-to-particle-diameter ratio (b/d) on the distribution characteristics of the mining flow field were analyzed. The experimental results reveal that ore particles demonstrate distinct movement characteristics in different regions of the collection flow field. These regions are categorized into a translation zone, a startup zone, and a continuation zone. Under the same Re, in the range of 1.5 ≤ h/d ≤ 2.25, decreasing h/d had a limited impact on the flow characteristics near the lowest point of the convex curved wall (y/d = 0), and facilitated ore particle entry into high-velocity gradient flow regions near the wall, thereby effectively promoting particle elevation. Under the same Re, for the range of 0.1 ≤ b/d ≤ 0.4, reducing b/d increased the velocity gradient of the high-speed flow region near the wall, and enhanced the vertical lifting force on particles, improving collection efficiency. This study offers valuable insights for the design and engineering application of new, efficient, and low-disturbance collection devices.

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.