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.

In order to investigate the hydrodynamic characteristics of submarines moving in ice regions, a mathematical model and corresponding numerical method, considering the coupling interaction of submarine, water and ice, were established based on RANS equations for level ice and brash ice conditions. The ice sheet was regarded as a no-slip wall boundary with a tangential velocity equal to the incoming flow speed, while the discrete element method incorporated with a linear elastic model was employed to simulate the motion and collision of brash ice. The SUBOFF submarine model was selected as the research object. The influence of sea ice on the resistance was numerically investigated first after mesh convergence analysis. Then further calculations were conducted on resistance components for different submerged depths and speeds under various ice conditions. And wave patterns of the SUBOFF were analyzed for free surface and brash ice condition. Numerical results show that the sea ice predominantly affects the pressure resistance, especially the wave-making component, while there are little differences in frictional resistances for different submerged depths and ice conditions. As the submarine moves under level ice, a significant decrease in pressure resistance and total resistance is observed owing to the lack of wave disturbance. In contrast, when navigating in water covered by brash ice, waves are generated. However, the random and uncertain collisions of floating ice lead to strong fluctuations in the pressure resistance and total resistance curves. And the corresponding average resistance values are slightly smaller than those in open sea due to the wave attenuation induced by brash ice. Furthermore, the submerged depth and forward speed are found to have a significant impact on the hydrodynamic characteristics of the submarine sailing in ice regions. With the increase of submerged depth, both pressure and total resistances will gradually decrease and tend to be consistent. Generally, the increase of forward speed will lead to larger resistance. However, when the submarine navigates beneath the open sea or brash ice, the pressure resistance increases at first and then decreases slightly, which makes the total resistance to grow at a slower rate.

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.

Computational bottlenecks and geographical restrictions can be effectively overcomed by cloud-enabled CFD software, while collaborative resource sharing is promoted through integrated cloud ecosystems. This paper investigated the cloud-based application of CFD software for predicting added resistance and motion response of surface ships, utilizing a Browser/Server (B/S) architecture. The study presented an overall architecture for the cloudification of large-scale CAE software, along with component-based flow construction, web-based lightweight CFD data techniques. Key challenges in the cloudification of ship CFD software, including efficient allocation of computing resources, complex human-computer interactions, and effective cloud-based CFD data visualization, etc., had been addressed. In this research, ship CFD software was transitioned from local installations to cloud-based solutions, enabling user interaction through browser interfaces and computations are performed on server nodes. This work provides valuable insights and a reference framework for the cloud-based deployment of other CAE softwares.

As a magnetic spring to provide restoring force for the rigid oscillator, the magnetic levitation support system is of great significance in the flow-induced vibration ocean current energy capture. To investigate the influence of different magnetic spring stiffness on the flow-induced vibration of cylindrical oscillators, the coupling model of the flow-induced vibration of a rigid cylindrical oscillator and its magnetic suspension support was constructed by using the RANS and the Maxwell-Ampere law without free current. The amplitude ratio, vibration frequency and wake vortex shedding pattern of the rigid cylindrical oscillator were analyzed under the action of magnetic spring stiffness of different magnetic suspension support forces. The results show that (1) the amplitude ratio of the oscillator increases first and then decreases with the increase of the flow velocity, and the maximum amplitude ratio decreases gradually with the increase of the spacing; (2) the oscillator reaches the maximum amplitude ratio of 0.844 at a flow velocity of 0.8 m/s and a spacing of 3.8D; (3) the vibration frequency of the oscillator increases with the increase of the spacing, and the growth trend tends to be gentle; (4) the oscillator reaches the maximum vibration frequency of 1.62 Hz at a flow velocity of 0.9 m/s and a spacing of 3.4D; (5) the wake vortex mode becomes more complex with the increase of spacing and velocity, and four wake vortex modes have appeared in the whole velocity range.

Based on the slicing theory, a time domain model of flow-induced vibration of 3D rotating slender structure is established by combining computational Fluid dynamics (CFD) and finite element method, and the flow vibration characteristics under the action of water flow and rotation are studied. Under the action of water flow, the trajectory of non-rotating elongated structure is mainly "8" shaped. Under the combined effects of flow and rotation, the motion direction of rotating slender body is opposite to its rotation direction, resulting in backward whirling. When the flow velocity is 0.46 m/s, vibration is jointly influenced by the flow and rotation. As the rotational frequency increases, the trajectory of the rotating elongated body transitioned gradually from a "8" shape to a circle. When the flow velocity is 1.02 m/s, the frequency is close to the theoretical intrinsic frequency, and the main cause of vibration is Vortex-Induced Vibration (VIV). The vortex motion is completely suppressed. There is a frequency-locking interval near the intrinsic frequency of the cylinder. The relative amplitude of the transverse vibration of the rotating cylinder increases with the flow velocity in the locking interval, while the frequency ratio remains unchanged.

Ship structures operate continuously in the marine environment, where they are prone to fatigue crack growth (FCG) under complex alternating loading, therefore it is of great significance to accurately predict the FCG and ensure the safety of structures. In this paper, the load spectrum constructed by the spectral method was combined with an improved unique curve crack growth model, and a method was proposed to more accurately predict the FCG in the near-threshold regime for ship structures under spectral loading. A balcony opening corner in a cruise ship was taken as an example; the method for determining the shape exponents in the improved model was given, the FCG of this structure under spectral loading was predicted, and the effects of the initial crack length and crack growth model on the FCG were discussed. The results show that the prediction method can more accurately predict the FCG in the near-threshold regime, and the prediction result is more conservative than that predicted by the unique curve model recommended in the regulations of CCS. The method presented in this paper can also provide a reference for the fatigue life assessment of other marine structures.

In order to study the effect of different stress ratios on the fatigue crack growth of HTS-A steel in low temperature environment, low-temperature fatigue crack growth tests of HTS-A steel CT specimens at stress ratios of 0.1 and 0.3 were carried out. The test results show that with decreasing temperature, the crack growth rate decreases and the fatigue life increases. At the same time, with the increase of the stress ratio, the fatigue crack growth rate also increases accordingly. However, with the decrease of temperature, the effect of stress ratio on fatigue crack growth rate becomes smaller and smaller. On the basis of experiments, an improved McEvily model considering the effects of temperature and stress ratio was proposed in this paper. The model can predict the fatigue crack growth rate of HTS-A steel under different low temperatures and different stress ratios. The predicting results were compared with the experimental data. This prediction model lays a foundation for the fatigue life assessment of marine equipment in low temperature environment.

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.

The multimode vibration characteristics associated with slender marine risers/pipes are widely seen in the case of complex marine environments. Firstly, by using a two-way fluid-structure interaction technique, numerical simulations were performed on the vortex-induced vibration of a flexible marine riser model subjected to shear and uniform flows. Secondly, based on the analysis of position-frequency-energy spectra, the multimode characterized vibration along the pipe span was investigated. Then, an empirical mode decomposition technique called FB-EMD, based on the Fourier transform and band-pass filter, was used to adaptively decompose the vibration data into some intrinsic mode functions. By analyzing the time-frequency characteristic information, the transient evolution characteristics of the vibration modes associated with the flexible riser model were then discovered. It is found that the phenomena of multimode vibration along the slender flexible pipes are quite common. With the increase of flow velocity, higher order vibration modes are continuously triggered, and with the energy transfer between modes, the apparent modes at different periods compete fiercely, often resulting in transient multimode coexistence. The time-frequency-energy Hilbert spectral analysis shows that the random and natural vibration modes caused by the combined effects of local vortex shedding and energy transfer from adjacent pipe sections are excited in a wide frequency band, and the energy of each dominant vibration mode is usually time-varying.

When submarine cables are suspended in the air and dragged by ship anchors, they will be deformed or even fractured, which seriously threatens the power transmission and information communication. Therefore, it is of great application value to investigate the backfill protection scheme for the suspended section of submarine cables. Based on ABAQUS, we established a three-dimensional finite element model simulating the process of a ship anchor dragging a three-core composite submarine cable through soil. Using nonlinear dynamics, we obtained the stress changes in the cables armorlayer copper conductor, and fiber optic armoring when it was dragged through different soil materials by a ship anchor. We conducted comparative analysis of the protection effect of four kinds of soil properties on the marine cable. Then by applying different speed loads, we investigated the relationship between the longitudinal compression rate of the cable and the burial depth under different ratios, to propose a layered backfill protection scheme. The results show that the difference of the protection effect on the cable of different seabed soil material properties is significant, clay can effectively reduce the tensile stress on the cable, and gravel can effectively reduce the stress change amplitude and strain time of the plastic yielding stage of the cable, and it is proposed to backfill the submarine cable with layers of clay and gravel to protect the submarine cables; when the ratio of clay to gravel is 1∶2, the backfill protection can withstand the towing speed of 90 cm/s. The backfill protection can also be used to protect the submarine cable against the tugging of a ship with a speed of 90 cm/s. The backfill protection is also used for the protection of the submarine cable. When the ratio of clay to gravel is 1∶2, the backfill protection depth of 0.7 m can withstand the damage caused by the anchor drag with a speed of 90 cm/s, which provides theoretical references for the backfill protection engineering and operation and maintenance of submarine cable.

In the process of oil and gas development, semi-submersible platforms operate relocation between wells by retracting and releasing mooring chains. In this paper, a fast well relocating optimization method of semi-submersible platform based on greedy algorithm was proposed in view of the traditional method that cannot obtain relocation strategy quickly. Three sets of relocation operations were selected in this study, an optimization analysis of relocation strategy was carried out for windlasses normal operation and windlasses failure operation. The results show that under normal operation, the optimized strategy has a significant improvement in terms of optimization effect and calculation cost compared with the original strategy and the best strategy. Under windlasses failure operation, it shows that the feasibility of relocation is closely related to the distance and direction of the relocation operation. A comparison of relocation strategy was made between the normal operation and the windlasses failure operation, it was found that the retracted length of the mooring chains and the number of operating steps increased under windlasses failure operation, and the stability keeps changing with different working conditions.

In order to investigate the effect of fillets at the leading and trailing edges of flow holes on the noise characteristics of submerged body, a submerged body with a single flow hole was taken as the research object. Based on the large eddy simulation turbulence model and FW-H (Ffowcs Williams-Hawkings) acoustic model, numerical simulations were carried out using STAR CCM+ software. After verifying that the simulation accuracy meets the requirements, the influence of different fillet positions and radii of the orifice on flow noise was then deeply explored at a flow rate of 10 m/s. The results show that the treatment of fillets at the leading and trailing edges of the flow hole can reduce the flow noise of the flow hole, especially the second-order line-spectrum noise. Regarding fillet position, the noise reduction amplitude by the fillets at the trailing edges is greater than that at the leading edge, and the noise reduction amplitude at the outer rounded corners is greater than that at the inner edge. In the studied operating conditions, when the outer edges of the leading and trailing edges of the orifice are simultaneously rounded with a radius of 10 mm, the minimum second-order line-spectrum noise is 94.03 dB, which is 9.48 dB lower than that in the reference operating condition, and the corresponding frequency is reduced by 4.72 Hz. The far-field radiation noise of the orifice is mainly affected by the pressure pulsation on the back wall and bottom of the orifice, and the influence of the orifice shear layer oscillation is relatively small. The research conclusions provide guidance for the optimization of submerged body flow hole structure and the low-noise design.

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.