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The hydrodynamic characteristics of autonomous underwater vehicles (AUVs) play a significant role in the design and analysis of their maneuverability. This paper evaluates the effects of the Reynolds (Re) number on the hydrodynamic characteristics of AUV for various angles of attack (AOA). To estimate the hydrodynamic parameters, a numerical modelling based on computational fluid dynamics (CFD) is employed. Reynolds numbers between 2106 and 150106 were examined at -10º to 10º AOAs. Experimental tests for the same AUV in Re = 2106 in the water tunnel were carried out for CFD validation. A comparison of the results showed an acceptable agreement between the numerical method and the experimental results. The results show that hydrodynamic parameters can be a function of Re and converge on a constant in a limited value when the Re number increases. Results of independent parameters, can be used for full-scale without the establishment of dynamic similarity.

As an important part of the river ecosystem, vegetation has a significant influence on hydrodynamic characteristics, water quality, river morphology, and ecological habitat. Combining vegetation survey with the verified numerical model, this study aims to analyze the impact of floodplain vegetation patches on hydrodynamic characteristics in the old course of Fuhe River under various combinations of incoming flow discharges, and flood diversion discharges, and changes in the land use type. The equivalent Manning coefficient was adopted to quantify the additional resistance induced by plants in the vegetation module of the numerical model. According to simulating results, vegetation patches would cause the water level to rise and velocity to decrease, which mainly affects the upstream of the old course of Fuhe River. And with the increase in incoming discharge, water level difference and velocity difference have an upward trend. It is also found that the resistance of Zizania latifolia to river flow is strongest followed by sugarcane, crops, and weeds because of the differences in vegetation characteristics. Furthermore, compared with existing vegetation conditions, converting farmland to Zizania latifolia and expanding farmland induce a moderate rise in water level upstream while the decreasing velocity happens in the area where land use type is changed. And there are areas where velocity increases located opposite to the velocity decreasing area because of the adjustment of cross-section velocity distribution caused by plants.

A theoretical and numerical study of complex sliding flows of yield-stress fluids is presented. Yield-stress fluids are known to slide over solid surfaces if the tangential stress exceeds the sliding yield stress . The sliding may occur due to various microscopic phenomena such as the formation of an infinitesimal lubrication layer of the solvent and/or elastic deformation of the suspended soft particles in the vicinity of the solid surfaces. This leads to a ‘stick–slip’ law which complicates the modelling and analysis of the hydrodynamic characteristics of the yield-stress fluid flow. In the present study, we formulate the problem of sliding flow beyond one-dimensional rheometric flows. Then, a numerical scheme based on the augmented Lagrangian method is presented to attack these kind of problems. Theoretical tools are developed for analysing the flow/no-flow limit. The whole framework is benchmarked in planar Poiseuille flow and validated against analytical solutions. Then two more complex physical problems are investigated: slippery particle sedimentation and pressure-driven sliding flow in porous media. The yield limit is addressed in detail for both flow cases. In the particle sedimentation problem, method of characteristics – slipline method – in the presence of slip is revisited from the perfectly plastic mechanics and used as a helpful tool in addressing the yield limit. Finally, flows through model and randomized porous media are studied. The randomized configuration is chosen to capture more sophisticated aspects of the yield-stress fluid flows in porous media at the yield limit – channelization.

Locomotion is essential for the survival of fish because it influences the success rate of avoiding danger and predation. In particular, differences in the hydrodynamic properties of the caudal fin have a significant impact on swimming, since the caudal fin is the primary propulsion organ. The hydrodynamic characteristics of shark caudal fins have been studied. However, comparisons have been limited to a few species, and more caudal fin morphologies need to be investigated to determine the relationship between caudal fin morphology and hydrodynamic characteristics in sharks with diverse morphologies. Therefore, we performed computational fluid dynamics analysis on the caudal fin morphologies of 30 species in 9 orders of sharks to investigate the relationship between caudal fin morphology and hydrodynamic characteristics. We found that caudal fin morphologies with large AR L (ratio of vertical to the horizontal length of caudal fin) had higher thrust and swimming costs and caudal fin morphologies with small AR S (ratio of the product of the length and height of the caudal fin to the surface area) had higher propulsive efficiency. The results of this study will help in selecting caudal fin morphology for fish-like underwater robots and in studying the relationship between shark ecology and caudal fin morphology.

Artificial reefs are beneficial to restore fishery resources and increase fishery production. Meanwhile, they play a significant role in improving ocean ecology and accelerating the evolution of fishery industries. Since they are generally affected by currents, waves, and other hydrological factors, the flow field around artificial reefs and their stabilities have become a research hotspot in recent years. Research on artificial reefs is a systematic process consisting of four aspects: Firstly, the significance, the definition, the mechanism, and the present research progress were introduced for artificial reefs in detail. Secondly, the development trend of the sit-bottom artificial reef and that of the floating artificial reef were summarized, respectively. Thirdly, it was found that the combination of traditional artificial reefs and emerging ocean engineering has a great development potential in practical engineering. Finally, the existing problems related to the hydrodynamic characteristics of the artificial reefs in China were summarized, and the prospects of artificial reefs were proposed. The purpose of this study is to provide a scientific reference for the ecological and sustainable development of the large-scale construction of artificial reefs in the ocean.

The composite propeller has attracted much interest due to its excellent mechanical properties such as high specific stiffness and high specific strength, hence there is an increasing interest in utilizing the composite materials to improve the hydrodynamic and structural performance of marine propellers. The objective of this paper is to study the cavitation performance of composite propellers based on the unsteady simulation method considering the cavitation-composite structure interaction. The typical cavitation patterns around the composite propeller are studied, which include blade sheet cavitation and tip vortex cavitation. The unsteady flow characteristics of tip vortex cavitation and structural dynamic response of composite propeller are studied, and the mechanism of composite propeller for the cavitation suppression and efficiency improvement is revealed. The results show that compared with rigid propellers, composite propellers have smaller cavity volume and higher propulsion efficiency under the same conditions. The unsteady cavitating flow characteristics under non-uniform wake are periodic, and the phase lag of hydrodynamic coefficients of composite propeller can be observed compared with that of the rigid propeller. The bending-torsional coupling deformation of the composite propeller makes the pressure pulsation of the flow field gentler, which reduces the influence of the cavitation load on the composite propeller.

As ocean exploration deepens, new demands on propulsion methods are proposed due to the complex underwater environments. Marine animals exhibit excellent locomotion properties, which provide a promising direction for the development of high-performance underwater robots. The unique hydrofoil-paddle propulsion mode of the sea lions foreflippers is a key contributor to their efficient locomotion. Inspired by this, the present work simulates the hydrofoil motion of California sea lion foreflippers with a transient computational fluid dynamic model using dynamic mesh technology to investigate its hydrodynamic characteristics and performance. The result shows that thrust generation during the recovery and power phases is dominated by lift-based propulsion, while during the paddle phase it is dominated by drag-based propulsion. Maximum thrust in a stroke cycle occurs in the power phase, while maximum efficiency at the same motion speed of the hydrofoil is achieved in the paddle phase. The effects of the motion parameters, including Strouhal number (St) and the dimensionless flapping amplitude (h), on the hydrodynamic performance are also investigated. Analysis results show that the optimal thrust and efficiency are achieved in the St range of [0.220, 0.293] and h range of [1.0, 1.864], with the highest efficiency of 25.27% occurring at St = 0.293  and h = 1.846. The present work provides valuable theoretical guidance for designing the bionic robotic foreflippers.

This paper selects SST k-w turbulence model and VOF wave to construct a numerical calculation model of moving body planning on a flat free surface based on STAR-CCM+ numerical simulation software. The construction model is checked through foreign classic literature, and the numerical simulation results are in good agreement with the experimental results. The hydrodynamic numerical errors are less than 5%, which is within the engineering error range. The model can be used for the numerical simulation of the planning cylinder. In this paper, it is used to simulate the planing process of cylinder with different speeds and different submerged depths, and the flow field characteristics and hydrodynamic characteristics in the planing process are obtained. The results show that waves appear at the tail and the tail liquid splashes to form a water splash during the planing process of the cylinder on a flat surface. The higher the speed of the planning cylinder is, and the deeper the submersion depth, the more pronounced waves at the tail. When the cylinder has a Fr number C v ≥8, the hydrodynamic force of the cylinder is almost unchanged, and it is not affected by the speed. But when C v =3, the hydrodynamic characteristic coefficient is higher. The drag coefficient is 20% higher than that in the high-speed ( C v ≥8) planing process. The lift coefficient is 3 times of high-speed planing lift coefficient. It is related to the surface pressure and frictional force distribution of cylinder during the low-speed planing. There is a linear relationship between the drag coefficient and the submerged depth during the cylinder planing at different submerged depths. 基于STAR-CCM+数值仿真软件，选用SST k - w 湍流模型，采用VOF波构建运动体在静水面滑水数值计算模型。通过国外经典文献对构建模型进行校核，数值模拟结果与实验结果吻合性较好，流体动力数值误差均小于5%，在工程误差范围内，模型可用于回转体滑水航行工况的数值模拟计算。对回转体在不同速度、不同淹没深度滑水工况进行数值模拟，研究其流场特性和流体动力特性。结果表明，回转体在静水面滑水过程中，尾部波浪兴起，液体飞溅形成水花，滑水速度越高、淹没深度越深，尾部兴波越明显；回转体在 C v ≥8时，流体动力系数几乎不变，其不受速度大小的影响，而在 C v =3时，流体动力系数较高，阻力系数高于高速( C v ≥8)滑水阻力系数20%，升力系数是高速滑水升力系数的3倍，其与回转体在低速滑水过程中表面压力和摩擦力作用分布有关；回转体在不同淹没深度滑水过程中，阻力系数与淹没深度呈线性关系。

Due to the limited availability of land resources, offshore wind turbines have become a crucial technology for the development of deep-water renewable energy. The multi-floating body platform, characterized by its shallow draft and main body located near the sea surface, is prone to significant motion in marine environments. The proper chamfering of the heave plate can effectively enhance its resistance during wave action, thereby improving the stability of the floating platform. The optimal chamfer angle is 35°. Considering the complexity of the floating body’s motion response, this study focuses on the damping characteristics of the heave plate with 35° chamfered perforations. Using the NREL 5 MW three-column semi-submersible floating wind turbine platform as the research model, the hydrodynamic characteristics of the floating body with a perforated heave plate are systematically studied through theoretical analysis, numerical simulation, and physical tests. The amplitude of vertical force under various working conditions is measured. Through theoretical analysis, the additional mass coefficient and additional damping coefficient for different working conditions and models are determined. The study confirms that the heave plate with 35° chamfered perforations significantly reduces heave in the multi-floating body.