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The Circulating Water Channel (CWC) is a device commonly utilized in maritime engineering for hydrodynamic experiments. The ability to generate a high-quality flow field is a critical criterion for evaluating the device, and thus, improving key parts of the CWC device can significantly increase this ability. In this paper, a numerical model based on the RANS method is established to investigate the hydrodynamic performance of the circulating water channel's finite section. First, associated analyses and optimizations for the turning vanes and contraction section are performed. Following confirmation that adding a honeycomb can greatly improve the flow field, the hole type and length diameter ratio are investigated further. After integrating the components, the flow field properties are examined at various flow velocities. The main findings demonstrate that flow field's uniformity can be enhanced using the right number of turning vanes. Applying the Witozinsky transition curve to the contraction section can produce a better pressure gradient and increase the efficiency of contraction selection. The best rectification result is achieved by a honeycomb with a square shape and a slenderness ratio of 9. By varying flow velocities, the most uniform flow field area occurs at 4 m to 16 m from the outlet of the contraction section. This model can better simulate the dynamic characteristics of the flow field in the 3D CWC and serve as the foundation for the design of multifunctional CWC equipment for wind, wave, and flow. [ABSTRACT FROM AUTHOR]

In order to establish a proper geometry of an Unmanned Underwater Vehicle (UUV) for stable motion control and energy usage reduction, this paper analyzes the hydrodynamic performance of a complex shape underwater vehicle and develops a systematic Computational Fluid Dynamics (CFD) simulation method to solve the hydrodynamic parameters of the system. Based on the simulation method and their results, the streamlines and pressure distributions of the water flow around the underwater vehicle are analyzed, and the geometric model design is improved based on the drag characteristics. Also, a comparison scheme is designed to evaluate the vehicle model before and after the geometry improvement. Simulation result shows that the design schemes brings 18% drag reductions in surge direction and 32% in heave direction. Moreover, by establishing the physical and mathematical models of the UUV on a physical simulation platform, a complete model of the underwater vehicle is constructed, laying the foundation for further simulations and experiments. [ABSTRACT FROM AUTHOR]

ObjectiveTo optimize the energy capture efficiency of a rocker-type wave energy converter, the effects of the platform pillar wave diversion scheme and shape of finned floats on energy capture are studied.MethodsFirst, a hydrodynamic model of float motion is established. Next, the hydrodynamic characteristics of different finned floats are calculated using AQWA software and analyzed under different platform pillar wave diversion schemes, different frequency domain parameters such as added mass, radiation damping, wave excitation force and response amplitude operator (RAO), and different time domain parameters such as displacement, velocity, acceleration and excitation force. Finally, the energy-capture width ratios of different finned floats under different platform pillar wave diversion schemes are investigated.ResultsThe results show that different platform pillar wave diversion schemes have little effect on the frequency domain parameters of the finned floats or the float motion displacement of time domain parameters, but a large effect on the motion period. Energy capture is optimal at a fin angle of 100° under the double-pillar wave diversion scheme.ConclusionsBased on the proposed numerical simulation model, the hydrodynamic response of multi-degree-of-freedom finned floats in wave motion can be simulated accurately, providing useful references for the optimized design of the float shape for oscillating float-type wave energy generation platforms.

Hydrodynamics analysis and control are very significant for the seabed operations, particularly for the intelligent manipulation process of streamlined intervention autonomous underwater vehicles (I-AUVs). The computation fluid dynamics simulations and verification were conducted in the consideration with water channel domain, mesh insensitivity, support straight bar connector, free surface and other boundary conditions. The variation trend of hydrodynamic coefficients in the process of manipulation is obtained, by simulations of streamlined I-AUV manipulation under dynamic manipulation state. To further realize underwater floating manipulation, a novel controller with an integral termed nonlinear sliding mode surface and disturbance observer has been developed. The disturbance observer can make quantity analysis on the interaction forces between I-AUV and the environment from hydrodynamic analysis. Simulations and experiments have verified the controller performance. • After hydrodynamics analysis, PMM experiments have been simulated through 3D computational meshing and turbulence models. • Hydrodynamic analysis on the I-AUV cruising with different manipulator postures has been expanded into infinite domain. • The interaction force between the manipulator and the environment has been estimated to improve the I-AUV operation control. • A novel controller with the integral termed sliding mode surface and the disturbance observer has been developed. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) based wave energy absorption devices are classic which have been widely used for harnessing ocean wave energy. This paper presents a numerical study on a projecting wall (PW) type OWC wave energy converter in regular waves. The computational fluid dynamics (CFD) modelling of a stationary floating PW-OWC model in a three-dimensional wave flume is achieved by the software Flow-3D. Numerical analyses are carried out based on CFD simulations and the linear potential flow solutions with modifications to account for turbine-induced damping. The present numerical solutions are validated against our previous experimental data. It is found that both the CFD and modified linear potential flow predictions are in reasonably good agreements with the experimental data in the first order results of OWC and air pressure responses. When the nonlinear responses are included in the result, the modified linear potential flow solution is found to slightly under-estimate the wave energy conversion performance at long wavelengths. Regarding the airflows above and below the chamber orifice, the CFD results suggest that they are almost unidirectional, oscillating in not only the base frequency but also subharmonic and ultraharmonic frequencies. The evolution of the OWC responses during an entire period and the phase analysis based on CFD simulations are presented. The phase results provide the crucial evidence to the reasonability of the physics-based modification of the potential flow model in modelling of OWCs. The present results and analysis are expected to be beneficial to the understanding on the physical mechanism of OWCs and the design of phase control strategies. [ABSTRACT FROM AUTHOR]

The lift-drag ratio (L/D) and volume of the shape have a significant influence on the gliding range and carrying capacity of Blend-Wing-Body Underwater Gliders (BWBUGs). In this research, a model-based shape design framework for BWBUGs with curved wings is proposed in the conceptual design stage. This framework presents a parameterisation method of BWBUGs with curved wings, and multi-software collaboration technology is adopted to automatically evaluate the shape performance (hydrodynamic analysis and volume). A data-driven optimisation technique is integrated into this design framework to enhance the performance of the shape for BWBUGs. Finally, a series of solutions are generated using the suggested framework, demonstrating its viability and effectiveness for BWBUG shape design optimisation during the conceptual design stage. [ABSTRACT FROM AUTHOR]

This paper focuses on the effect of water depth on the hydrodynamics of floating offshore wind turbines with open-hole anti-heave devices. The three-floating-body wind turbine platform is used as the primary research object in this paper. The effect of water depth on the reduction of the heave motion of a floating platform with anti-heave devices is systematically investigated through a series of experiments and numerical simulations. The results show high agreement between the test results and simulations, with larger values of heave motion in deep water. A wind turbine platform with anti-heave devices can effectively reduce the lifting and sinking motions when the wave period is large.

The current research aimed to determine the origin of ions and the type of flow system in groundwater flowing out through two types of atmospheric and hydrothermal springs by hydrochemical and hydrodynamic approaches in a volcanic aquifer. Findings revealed that the major ion types in atmospheric waters are calcic and magnesium bicarbonate, whereas hydrothermal springs predominantly indicated chloride–sodic composition, showing an evolving pattern resulting from hydrothermal and atmospheric waters mixing. Investigating the ionic ratios and the saturation index to determine the origin of ions suggests that the presence of ions in the waters can be attributed to the weathering of silicates and plagioclase-bearing minerals in the volcanic units, and in some cases, ionic exchange also plays a role. The recession curve analysis revealed a predominant conduit flow with α = 0.144 in the system feeding the representative hydrothermal spring. Two micro-regimes with α1 = 0 = 0.46 and α2 = 2.68 were detected on the hydrograph of the atmospheric representative spring, indicating the development of systems with two types of flow regimes. Estimating the Qmax/Qmin ratio for selected hydrothermal and atmospheric springs as 2.3 and 36.8, respectively, and calculating the electrical conductivity coefficient as 11% and 18% respectively, confirmed the recession curve analysis result. HIGHLIGHTS Determination of the origin and type of flow by investigating the hydrochemical and hydrodynamic characteristics of water flowing out through hydrothermal and atmospheric springs.; The major ion types in atmospheric waters are calcic and magnesium bicarbonate while in hydrothermal springs are chloride sodic indicating evolving water from mixing hydrothermal and atmospheric waters.; The ionic ratios and saturation index suggest that ions in waters resulted from the weathering of silicates and plagioclase-bearing minerals in the volcanic units.; Evaluation of maximum to minimum discharge and EC variation confirmed the type of flow system defined by the recession curve.; The recession curve analysis and type of flow regime revealed that atmospheric springs originate from precipitation while hydrothermal springs result from atmospheric and hydrothermal waters mixing.;

In order to establish a proper geometry of an Unmanned Underwater Vehicle (UUV) for stable motion control and energy usage reduction, this paper analyzes the hydrodynamic performance of a complex shape underwater vehicle and develops a systematic Computational Fluid Dynamics (CFD) simulation method to solve the hydrodynamic parameters of the system. Based on the simulation method and their results, the streamlines and pressure distributions of the water flow around the underwater vehicle are analyzed, and the geometric model design is improved based on the drag characteristics. Also, a comparison scheme is designed to evaluate the vehicle model before and after the geometry improvement. Simulation result shows that the design schemes brings 18% drag reductions in surge direction and 32% in heave direction. Moreover, by establishing the physical and mathematical models of the UUV on a physical simulation platform, a complete model of the underwater vehicle is constructed, laying the foundation for further simulations and experiments.

Hydroelectric power generation is an important clean energy source, and the stability of water turbine operation determines the quality of hydro energy utilization. For hydro turbines, the layout direction of the draft tube is often only considered from a structural perspective, ignoring the hydrodynamic characteristics. This study adopts the computational fluid dynamics method and verifies the effectiveness of numerical simulation with experimental results, analyzing the influence of asymmetric draft tube layout direction on pressure pulsation of mixed flow turbine. The results show that under different working conditions, there is basically no difference in efficiency corresponding to different inclined directions of the draft tube, and the relative difference in performance values is less than 1%. From the perspective of internal flow, the working condition has a greater impact on the flow, and the draft tube tilt has a smaller impact. Under strong swirling flow conditions in the draft tube, the variational mode decomposition of pressure fluctuation is carried out. Research has found 7 characteristic frequency bands including 140 Hz, 80–90 Hz, 40–46 Hz, 5.5–6.5 Hz, 2.5–3 Hz, 1.67 Hz, and <1 Hz. The frequency of the dominant mode corresponding to the left tilt is higher, while the amplitude of the runner frequency is slightly lower. In general, left tilt is not only more suitable for unit layout but also has a better effect on draft tube pressure fluctuation, which is worthy of application in engineering. [ABSTRACT FROM AUTHOR]

It is necessary to investigate the dynamic performance during autonomous underwater vehicle (AUV) underwater docking to aid in control and to enhance docking safety. Therefore, in this study, the docking hydrodynamic characteristics (including the docking system's streamline, velocity vector, and surface pressure during the overall docking process) of the AUV with conical hood dock are determined by using the dynamic grid technology, and the water drag force situations of the AUV docking with conical hood dock are studied from the perspectives of different velocities, accelerations, navigation modes, and structures. Additionally, the multiple nonlinear regression fits and the preliminary docking test were examined. Furthermore, brief inferences obtained are as follows: First, the maximum pressure is situated at the upstream surface of the dock conical hood and the head of the AUV, and the maximum rotation angle of the streamline is situated at the outermost ring of dock conical hood. Within a specified range, the rotation angle of the streamline affected by the conical hood progressively declines as the AUV docks deeper into the conical hood. Second, low velocity uniform docking, deceleration docking, and chase docking can reduce the drag force to a certain extent during docking. Finally, both arc shape and mesh structure can decrease the water drag force of docking to a specified degree. This research provides a theoretical basis and reference methods for the dynamic research of the docking system, and other related research can be carried out through the methods and results of this research. [ABSTRACT FROM AUTHOR]

The present research work concerns about the hydrodynamic behaviors of the open net offshore fish cages of single, double and 4-cage systems subjected to regular sinusoidal waves. The open net semisubmersible rigid cage is square in shape and analyzed numerically using ANSYS AQWA software. Frequency and time domain analyses are carried out for each case. The hydrodynamic parameters such as added mass, radiation potential damping, motion responses and mooring line tensions are considered as performance indicators to conclude as the best arrangements among three different cages. The single cage and windward side of all cages exhibit identical performance in all hydrodynamic parameters. The leeward side of each cage shows lesser parametric values than the windward side cages. Based on the performance indicators, it is concluded that the grid system containing four cage arrangements provides better performance than three other cage configurations. An experimental model of 1: 75 scale is fabricated and wave flume studies are conducted to validate the present numerical model. The cage is placed at a water depth of 55 cm and subjected to wave heights of 12 cm and 14 cm with wave periods ranging from 0.8 s to 2.2 s with an interval of 0.2 s are considered. The same wave flume boundary conditions are adopted for numerical simulations and results are in good agreement with experimental work results. [ABSTRACT FROM AUTHOR]

Assessment of the performance of arrays of wave energy converters (WECs) of the type of simple heaving floaters, operating in nearshore and coastal areas, is essential to estimate their power production capacity. The effective design and layout of such WEC farms can be supported by studying the system's operational characteristics based on the wave climatology of the deployment region. Installation along the exposed side of harbor breakwaters in sea areas with substantial incoming energy potential is a promising option. In this study, a 3D hydrodynamic model based on the boundary element method (BEM) is presented and discussed, aiming to evaluate the performance of WEC arrays consisting of multiple heaving bodies attached to the exposed side of a harbor breakwater, modelled as a vertical wall, considering its reflective effects, as well as hydrodynamic interactions between the multiple floating devices. Numerical results of the predicted power performance for various configurations, including interactions of multiple WECs with the nearshore topography and the breakwater wall, as well as the effects of power take off (PTO) parameters, are presented and discussed. Finally, a case study is presented for a selected coastal site at the port of Heraklion, located in the north-central part of Crete Island in the South Aegean Sea, characterized by relatively increased wave energy potential, using long-term climatological data, illustrating the method and its applicability as a supporting tool for optimal design of WEC arrangements. [ABSTRACT FROM AUTHOR]

This study tries to identify the effect of hydrodynamics on the absorbed wave power using a toroidal Oscillating Water Column (OWC) device. To this end, the fundamental hydrodynamic boundary value problem for an arbitrary shaped toroidal OWC device of revolution with vertical axis is solved. The described method is based on the discretization of the flow field around the device by means of ring-shaped macro-elements, in each of which axisymmetric eigenfunction expansions for the velocity potential is made. The solution sought for the corresponding diffraction and radiation velocity potentials is achieved using Galerkin's method. The applied formulation is generic and can be employed for arbitrary configurations of toroidal OWCs. Therefore, the numerical results shown and discussed in the present paper, in terms of the hydrodynamic loads and the air volume flows inside the OWC chamber, concern different types of OWCs. From the present analysis, it is demonstrated that the absorbed wave power by the examined toroidal devices is strongly affected by the geometrical parameters of the device; thus, these should be properly considered towards the maximization of the device's wave power efficiency. [ABSTRACT FROM AUTHOR]

Locomotion control of synergistical interaction between fins has been one of the key problems in the field of robotic fish research owing to its contribution to improving and enhancing swimming performance. In this paper, the coordinated locomotion control of the boxfish-like robot with pectoral and caudal fins is studied, and the effects of different control parameters on the propulsion performance are quantitatively analyzed by using hydrodynamic experiments. First, an untethered boxfish-like robot with two pectoral fins and one caudal fin was designed. Second, a central pattern generator (CPG)-based controller is used to coordinate the motions of the pectoral and caudal fins to realize the bionic locomotion of the boxfish-like robot. Finally, extensive hydrodynamic experiments are conducted to explore the effects of different CPG parameters on the propulsion performance under the synergistic interaction of pectoral and caudal fins. Results show that the amplitude and frequency significantly affect the propulsion performance, and the propulsion ability is the best when the frequency is 1 Hz. Different phase lags and offset angles between twisting and flapping of the pectoral fin can generate positive and reverse forces, which realize the forward, backward, and pitching swimming by adjusting these parameters. This paper reveals for the first time the effects of different CPG parameters on the propulsion performance in the case of the synergistic interaction between the pectoral fins and the caudal fin using hydrodynamic experimental methods, which sheds light on the optimization of the design and control parameters of the robotic fish. [ABSTRACT FROM AUTHOR]

A numerical simulation method based on CFD has been established to simulate the fully coupled motion for an attenuator-type wave energy converter (WEC). Based on this method, a detailed parametric analysis has been conducted to investigate the design of the rafts. The effects of different parameters (wave parameters, structural parameters and PTO parameters) on the hydrodynamic characteristics of the attenuator-type WEC were studied in detail. The results show that in terms of wave parameters, there is an optimal wave period, which makes the relative pitching angle amplitude of the WEC reach the maximum, and the increase of wave height is conducive to the relative pitching angle amplitude of wave energy. Under different wave conditions, the relative pitch angle of the parallelogram raft device is the maximum. In terms of structural parameters, the parallelogram attenuator-type device has the optimal values in different relative directions, different distances and different apex angle, which makes the relative motion amplitude of the device reach the maximum, and the spacing and the apex angle have influence on the motion frequency of the device, while the relative direction has almost no influence on it. In terms of PTO parameters, there is an optimal damping coefficient, which makes the power generation efficiency of the WEC reach the maximum. The research results provide a valuable reference for future research and design of the attenuator-type WEC. [ABSTRACT FROM AUTHOR]

The straight-through electric fully closed ultra-clean cut-off valve is a completely closed shut-off valve inside and outside. It relies on magnetic force as the driving force of the valve to control the opening and closing of the valve. In this paper, a new type of cut-off valve structure was designed, a mathematical model was established and its hydrodynamic characteristics were simulated by the computational fluid dynamics method. According to different inlet velocities and different valve openings, the internal cavitation phenomenon of the valve was analyzed. The external magnetic drive control device was designed in combination with the completely closed structure inside and outside the valve, and the magnetic field interference and force of the external permanent magnet and the embedded permanent magnet were studied. This study is helpful for the optimization design and practical engineering application of the ultra-clean valve. [ABSTRACT FROM AUTHOR]

The sloshing effect of fluid storage tanks of a Floating Liquefied Natural Gas (FLNG) vessel causes variations in its global motion response. These acceleration and motion alterations can affect the safe performance of the FLNG vessels. The classification societies' rules are employed to standardize the storage tanks' configuration of FLNG vessels. Herein, we report a methodology to assess the sloshing effect on the global motion response of an FLNG vessel considering four geometrical arrangements of tanks and different fluid filling fractions. This methodology includes the hydrodynamic effect in operating and storm conditions from the Gulf of Mexico using a return period of 100 years. In addition, our methodology considers the influence of the internal fluid of each tank to estimate the accelerations and motions of the vessel. This methodology can be implemented to estimate the stability of an FLNG vessel under different environmental conditions. Thereby, the naval engineers could choose the best geometrical configuration of the storage tanks for safe behavior of a vessel under different operating and extreme environmental conditions. [ABSTRACT FROM AUTHOR]

After conducting the geotechnical and hydrogeological explorations of the RTH flotation tailing dump and determining the stability of slopes for the overhanging purposes, the zone with the observed instability, an elevated and unfavorable gradient of the groundwater level, was determined and located. Due to these reasons, further design of the groundwater level regulation system was started. The hydrodynamic analysis of the task determined that in order to achieve the effects of lowering and regulating the level of underground water, it is necessary to construct fourteen wells.

Cormorants (Phalacrocoraxe), types of aquatic birds, utilize the compliance/flexibility of the flippers and exploit hydrodynamic/biomechanic processes to accomplish diverse operations. Particularly, the flipper‐propelled locomotion exhibits traits such as super‐redundancy and large deformations, necessitating depiction of both movements of the rigid skeletons as well as local deformations of the soft tissues. However, there are few well‐established kinematic/hydrodynamic framework models and constitutive equations for such rigid–flexible intrinsically coupled biosystems. Herein, combined with a skeletal skinning algorithm to handle the deformation of a flexible body attached to a rigid body, a numerical computation framework for an in‐depth fluid–structure interaction is presented, which enables the capture of viscoelastic and anisotropic characteristics of a highly compliant 3D rigid–flexible coupled model in a low‐Reynolds‐number flow. Considering the biorobotic cormorant flipper with a nonuniformly distributed stiffness as a representative, the challenging issue of controlling a biomechanically compliant flipper to synthesize realistic locomotion sequences, including rigid skeleton movements and soft tissue deformations, is addressed. Furthermore, a numerical computational hydrodynamic analysis is performed to demonstrate that the cormorant flipper can generate 5 N fluid force and 0.45 N m fluid moment during the turning operation in 0.8 s, which is consistent with the former experimental results. [ABSTRACT FROM AUTHOR]

Abstract In China at present, the 67-type railway pontoon bridge is part of the main reserve standard equipment that can be utilized for traffic emergency of cross-river bridges. Based on the potential flow theory and boundary element method (the three-dimensional source distribution method), this paper studies the dynamic response characteristics of the 67- type railway pontoon bridge under the action of waves and train loads (the live load of the Chinese Standard-18). First of all, the hydrodynamic characteristics based on application of other computation models applied in relation to floating bridges are compared with the results obtained by the present approach. Secondly, the effects of varying the related wave parameters (i.e. wave period and angle of incidence) on the dynamic response of the 67-type emergency railway pontoon bridge (as installed across the Ji Canal within the Beijing-Shanhaiguan line) are investigated. Finally, the load effects in the girder when this bridge is subjected to the action of various wave and train loads are analyzed. The calculation results indicate that the method proposed in this paper has good generality which enables it to perform a better analysis of the hydrodynamic characteristics of this kind of practical emergency pontoon. Article highlights 1. Research on dynamic response for the major railway emergency pontoon in China are carried out; 2. The three-dimensional source distribution method and FEM is adopted; 3. Simulate traffic loads with the live load of the Chinese Standard-18.

Pipelines are widely used for transporting oil resources in the context of offshore oil exploitation. The pipeline stress-strength analysis is an important stage in related design and ensuing construction techniques. In this study, assuming representative work environment parameters, pipeline lifting operations are investigated numerically. More specifically, a time-domain coupled dynamic analysis method is used to conduct a hydrodynamic analysis under different current velocities and wave heights. The results show that proper operation requires the lifting points are reasonably set in combination with the length of the pipeline and the position of the lifting device on the construction ship. The impact of waves on the pipeline is limited, however lifting operations under strong wind and waves should be avoided as far as possible. [ABSTRACT FROM AUTHOR]

The DeepCwind floating wind turbine platform has become one of the most successful structures for accommercial floating wind farms, and the stability of it is crucial for survivability. Hence, this paper studies an anti-oscillation device with the purpose of reducing the heave and surge effects of the platform. The influence of various chamfered perforations at different sizes of the anti-heave device on the floating platform was further studied by numerical and experimental methods. Furthermore, through an analysis of the surge and heave of the pedestal with anti-heave devices with different chamfered perforations under different wave heights and wave periods, the effects on the hydrodynamic performance of the pedestal were studied. Physical experiments were conducted on a pedestal with anti-heave devices with chamfered perforations under the working conditions of different wave heights and wave periods to verify the reliability of the numerical simulation. The results show that the anti-heave effect of the anti-oscillation device is obvious under the small wave period and large wave height. Under the working conditions of different wave heights and wave periods, different perforated chamfers have different effects on reducing the oscillation of the pedestal, and its effect does not change linearly with an increasing chamfer. Under most working conditions, the anti-heave effect of the 35° chamfered perforated model was found to be the most obvious. [ABSTRACT FROM AUTHOR]

Cormorants (Phalacrocoraxe), types of aquatic birds, utilize the compliance/flexibility of the flippers and exploit hydrodynamic/biomechanic processes to accomplish diverse operations. Particularly, the flipper‐propelled locomotion exhibits traits such as super‐redundancy and large deformations, necessitating depiction of both movements of the rigid skeletons as well as local deformations of the soft tissues. However, there are few well‐established kinematic/hydrodynamic framework models and constitutive equations for such rigid–flexible intrinsically coupled biosystems. Herein, combined with a skeletal skinning algorithm to handle the deformation of a flexible body attached to a rigid body, a numerical computation framework for an in‐depth fluid–structure interaction is presented, which enables the capture of viscoelastic and anisotropic characteristics of a highly compliant 3D rigid–flexible coupled model in a low‐Reynolds‐number flow. Considering the biorobotic cormorant flipper with a nonuniformly distributed stiffness as a representative, the challenging issue of controlling a biomechanically compliant flipper to synthesize realistic locomotion sequences, including rigid skeleton movements and soft tissue deformations, is addressed. Furthermore, a numerical computational hydrodynamic analysis is performed to demonstrate that the cormorant flipper can generate 5 N fluid force and 0.45 N m fluid moment during the turning operation in 0.8 s, which is consistent with the former experimental results.

Hydroelectric power generation is an important clean energy source, and the stability of water turbine operation determines the quality of hydro energy utilization. For hydro turbines, the layout direction of the draft tube is often only considered from a structural perspective, ignoring the hydrodynamic characteristics. This study adopts the computational fluid dynamics method and verifies the effectiveness of numerical simulation with experimental results, analyzing the influence of asymmetric draft tube layout direction on pressure pulsation of mixed flow turbine. The results show that under different working conditions, there is basically no difference in efficiency corresponding to different inclined directions of the draft tube, and the relative difference in performance values is less than 1%. From the perspective of internal flow, the working condition has a greater impact on the flow, and the draft tube tilt has a smaller impact. Under strong swirling flow conditions in the draft tube, the variational mode decomposition of pressure fluctuation is carried out. Research has found 7 characteristic frequency bands including 140 Hz, 80–90 Hz, 40–46 Hz, 5.5–6.5 Hz, 2.5–3 Hz, 1.67 Hz, and

Deep-sea exploring and sampling technologies have become frontier topics. Generally, the movable exploring mode near the seabed with low disturbance is an important way to improve the measurement accuracy and expand the measurement range. Inspired by fish, the fishlike propulsion method has the characteristics of low disturbance and high flexibility, which is very suitable for near-seabed detection under complex terrain conditions. However, the swimming mechanism and surrounding flow field evolution law of the robotic fish under the constraints of complex terrain are still unclear. In this paper, the confined terrain space is constructed with an undulating seabed and a narrow channel, and the hydrodynamic changing law and flow field evolution law of the autonomous swimming process of the fishlike swimmer in the confined space are analyzed. Moreover, the influence mechanism of the terrain on the motion performance of the robotic fish is revealed, and the optimal motion mode of the robotic fish under a complex terrain constraint is discussed. The results show that the propulsion force, Froude efficiency, and swimming stability of the robotic fish vary with the distance from the bottom under the undulating seabed condition lightly. When the distance from the bottom exceeds a certain value, it can be considered that the undulating seabed no longer affects the swimmer. Furthermore, when the robotic fish swims through a narrow channel with certain width, the swimming performance obviously varies with the distance from the boundary surface. During swimming in the confined terrain space, the propulsion force and swimming stability of robotic fish will decrease. In order to maintain the forward speed, the robotic fish should improve the tail-beat frequency in real time. However, considering the swimming stability, the tail-beat frequency is not the larger the better. The relevant conclusions of this paper could provide theoretical support for the development of low-disturbance bionic exploring and sampling platforms for deep-sea resources and environments.

The purpose of breakwaters is to protect the ports, beaches, or beach facilities from strong waves and storms, as they help establish calm inside the port and thus achieve safety for ships and ease of operation. This paper presents an experimenta l and numerical study of unconventional alternatives to the vertical breakwater in order to evaluate the hydrodynamic performance of the proposed models. Two proposed cases for a semi-submerged breakwater were selected in the form of a half-pipe section with an inside diameter of 20 cm and a thickness of 1 cm. Case (a) was of the concave type of semicircular breakwater, while case (b) was of the convex type. Numerical modeling FLOW 3D was used to construct numerous scenarios for numerical simulation of the proposed breakwaters. The obtained results indicates that, when comparing the wave transmission coeffi cient (Kt ) and its refl ection coeffi cient (Kr ) with the relative water depth (h/L), the transmission coeffi cient decreased with the relative height of the wave, while the refl ection coeffi cient was completely reversed. In case (a), Kt was less than in case (b) at a range of 10% to 15%, while Kr in case (a) was bigger than in case (b) at a range of 5% to 10%. When the wave hit the breakwater, it was refl ected back as its energy is dissipated in less water depth and its speed decreases as it approaches the port. The velocity of the wave decreases as it approaches the bottom, which means that the wave is aff ected by the depth of the water, i.e. the lower the water depth, the lower the wave velocity. Case (a) was more effi cient and eff ective in wave dissipation, current velocity, and bed stability than case (b), so it is recommended to use case (a) due to its effi ciency in protecting coastal areas and generating electricity. [ABSTRACT FROM AUTHOR]

Marine submerged buoys can effectively obtain various parameters of seawater, which plays an important role in the research of marine physical phenomena, marine environmental changes, and climate change. However, traditional self-contained submerged buoys usually work underwater at a depth of about 100 m, and the observation data cannot be obtained before their recovery, which cannot satisfy the needs of real-time data acquisition for marine scientific research. To solve this problem, this paper proposes a real-time communication subsurface mooring system that consists of a satellite communication buoy (SCB), conductivity–temperature–depth sensors (CTD), and an inductive coupling mooring cable. The underwater inductive coupling link collects the data from the underwater sensors and transmit it to the SCB. Then, the data will be transmitted to the station receiver via satellite communication module integrated into the SCB. In order to ensure a high success rate of data recovery, the stress analysis and hydrodynamic simulation of the SCB were carried out in this paper. The results show that the SCB maintained a relatively stable attitude in the 3–4 sea state. The attitude data obtained from the subsequent sea trial was consistent with the simulation results, and the success rate of satellite communication during this period was more than 95%. In this paper, a modular embedded hardware circuit was designed to meet the functional requirements of the subsurface mooring system. An efficient data recovery strategy was also developed, which ensured that the average power consumption of the system was low and the success rate of data recovery is not less than 90% when operating in the severe sea state for a long time. The system underwent sea trials in the South China Sea for more than 3 months from the end of 2021 to the beginning of 2022. It transmitted more than 2034 sets of seawater profile temperature, salinity, and depth data in real-time, with a success rate of over 91% of the total sample data. The CTD data returned in real-time from our system is consistent with the data of the HYCOM and World Ocean Atlas (WOA), and a cyclonic mesoscale eddy was detected in the operation area. [ABSTRACT FROM AUTHOR]

• Disengagement of the mechanical motion rectifier reduces resistance of power take-off. • Modal analysis reveals the modulation of natural frequencies and oscillation phases. • Magnetic bistable mechanism imparts suitable stiffness to WEC to improve efficiency. • WaveBob WEC achieves higher efficiency and broader bandwidth than PowerBuoy one. The self-reacting point absorber (SRPA) has demonstrated significant potential for offshore wave energy utilization. To enhance efficiency in wave energy harvesting, this study introduces a novel gyrational magnetic bistable mechanism (MBM), which is integrated into the SRPA along with an actual power take-off (PTO) based on a mechanical motion rectifier (MMR). An integrated dynamic model is developed and solved in the time domain, considering the hydrodynamics of floats, the dynamics of the MMR PTO and the nonlinear stiffness mechanism (NSM) of the MBM. Two classic SRPA types, i.e., PowerBuoy and WaveBob types, are analyzed to uncover the effect of the MMR disengagement and the NSM of the MBM on the system dynamics. The influence of the MBM and wave parameters is analyzed for both SRPA types with the underlying mechanisms discussed. Results indicate that the performance with a MMR PTO significantly surpasses that with a linear PTO due to the continuous power output and the positive effect of the MMR disengagement on relative oscillation. Besides, the modal analysis suggests that the MBM provides the appropriate stiffness for both SRPA types according to different oscillation states, thus modifying natural frequencies and phases to enhance the performance. For the PowerBuoy type SRPA, the capture width ratio (CWR) improves from 0.36 to 0.39, whereas CWR increases from 0.55 to 0.71 with the dimensionless frequency bandwidth for CWR = 0.2 nearly doubling for the WaveBob one. [ABSTRACT FROM AUTHOR]

An investigation is conducted on the propulsion performance of semi-active flapping foils in tandem configuration of the wave glider, considering different vortex effects and tandem distances. The Finite Volume Method is utilized with commercial CFD software Fluent to solve the URANS equations around the flapping foil. A mesh comprising a 2D NACA0012 foil with Reynolds number Re = 42,000 is used in all simulations. The study investigates the propulsion performance of tandem flapping foil systems, focusing on the differing impacts of wave interaction on the front and rear foils. It explores how varying tandem distances and reduced frequencies influence thrust enhancement and degradation. The research reveals that while the front foil consistently benefits from thrust enhancement, the rear foil's performance is more complex, being subject to thrust enhancement or reduction due to wake vortex interactions. Detailed analyses of instantaneous thrust coefficients, pitching trajectories, and pressure contours provide insights into the hydrodynamic performance of these systems. The findings emphasize the importance of synchronization in motion between the foils for optimal propulsion and highlight the rear foil's susceptibility to changes in the wake flow, affecting its thrust generation capability. Thus, the optimal tandem distance can fully capitalize on the thrust enhancement effect of the wake interaction to improve the performance of the tandem flapping foil propulsion system. • Semi-Active Tandem Flapping Foils : The research centers on the propulsion performance of semi-active tandem flapping foils in tandem configurations for wave gliders. • Front and Rear Foil Dynamics : Explores how thrust enhancement and degradation are influenced by tandem distances and reduced frequencies, noting distinct effects on front and rear foils. • Wake Vortex Interactions : Highlights the complex performance of the rear foil due to wake vortex interactions and the importance of motion synchronization. • Optimal Tandem Distance : Concludes that optimal tandem distance significantly improves propulsion performance by maximizing the thrust enhancement effect of wake interactions. [ABSTRACT FROM AUTHOR]

Locomotion control of synergistical interaction between fins has been one of the key problems in the field of robotic fish research owing to its contribution to improving and enhancing swimming performance. In this paper, the coordinated locomotion control of the boxfish-like robot with pectoral and caudal fins is studied, and the effects of different control parameters on the propulsion performance are quantitatively analyzed by using hydrodynamic experiments. First, an untethered boxfish-like robot with two pectoral fins and one caudal fin was designed. Second, a central pattern generator (CPG)-based controller is used to coordinate the motions of the pectoral and caudal fins to realize the bionic locomotion of the boxfish-like robot. Finally, extensive hydrodynamic experiments are conducted to explore the effects of different CPG parameters on the propulsion performance under the synergistic interaction of pectoral and caudal fins. Results show that the amplitude and frequency significantly affect the propulsion performance, and the propulsion ability is the best when the frequency is 1 Hz. Different phase lags and offset angles between twisting and flapping of the pectoral fin can generate positive and reverse forces, which realize the forward, backward, and pitching swimming by adjusting these parameters. This paper reveals for the first time the effects of different CPG parameters on the propulsion performance in the case of the synergistic interaction between the pectoral fins and the caudal fin using hydrodynamic experimental methods, which sheds light on the optimization of the design and control parameters of the robotic fish.

The sloshing effect of fluid storage tanks of a Floating Liquefied Natural Gas (FLNG) vessel causes variations in its global motion response. These acceleration and motion alterations can affect the safe performance of the FLNG vessels. The classification societies’ rules are employed to standardize the storage tanks’ configuration of FLNG vessels. Herein, we report a methodology to assess the sloshing effect on the global motion response of an FLNG vessel considering four geometrical arrangements of tanks and different fluid filling fractions. This methodology includes the hydrodynamic effect in operating and storm conditions from the Gulf of Mexico using a return period of 100 years. In addition, our methodology considers the influence of the internal fluid of each tank to estimate the accelerations and motions of the vessel. This methodology can be implemented to estimate the stability of an FLNG vessel under different environmental conditions. Thereby, the naval engineers could choose the best geometrical configuration of the storage tanks for safe behavior of a vessel under different operating and extreme environmental conditions.

Evaluating the structural safety and seakeeping performance of very large floating structures (VLFS) using the rigid module flexible connector (RMFC) method remains challenging due to the complexity of the coupled hydrodynamic–structural responses in this system. In this study, a coupled hydrodynamic–structural frequency–time domain model is developed based on the RMFC method employing the planar Euler–Bernoulli beam elements to investigate the dynamic responses of multi-module floating systems. To reveal the dynamic characteristics of the systems, the coupled hydrodynamic–structural responses are investigated using a frequency–time-domain numerical model with viscous correction, in which the mass and stiffness attributes of connectors are incorporated into the system. Given the effects of hydrodynamic interaction, consideration is given to the case of three modular boxes connected by flexible beams aligned in series in shallow water to validate the present model. Higher efficiency and accuracy can be found in the system using viscous correction in potential flow theory and introducing state–space model to replace the convolution terms in the Cummins equation for the time domain. Moreover, this model can be extended to a considerable number of floating modules, which provides possibilities to analyze N-module floating systems.

In this paper, a novel model predictive control (MPC) method based on the population normal probability division genetic algorithm and ant colony optimization (GA-ACO) method is proposed to optimally solve the problem of standard MPC with constraints that generally cannot yield global optimal solutions when using quadratic programming (QP). Combined with dynamic sliding mode control (SMC), this model is applied to the dynamic trajectory tracking control of autonomous underwater vehicles (AUVs). First, the computational fluid dynamics (CFD) simulation platform ANSYS Fluent is used to solve for the main hydrodynamic coefficients required to establish the AUV dynamic model. Then, the novel model predictive controller is used to obtain the desired velocity command of the AUV. To reduce the influence of external interference and realize accurate velocity tracking, dynamic SMC is used to obtain the control input command. In addition, stability analysis based on the Lyapunov method proves the asymptotic stability of the controller. Finally, the trajectory tracking performance of the AUV in an underwater, three-dimensional environment is verified by using the MATLAB/Simulink simulation platform. The results verify the effectiveness and robustness of the proposed control method. [ABSTRACT FROM AUTHOR]

Patinopecten yessoensis have been one of the most valuable shellfish in the North Yellow Sea. Trawling has also been one of the most widely used harvestings. However, the bycatch of benthic fish and the impact of the trawling process on the bottom ecology have posed a great threat to the sustainable development of the scallop farming industry. Eco-friendly fishing has been a high demand in recent years. It is very necessary to optimize the structure of the net gear (the disturbing device and the trawling structure) and the speed of the trawling net in the combination with the bottom of the seabed. In this study, an environment-friendly fishing optimization and transformation scheme was proposed to evaluate the performance of the disturbing device and the trawl structure for the three scallop fishing nets, such as the dredge, cutting, and hydrodynamic types. The underwater video surveillance technology was also used to clarify the causes of the problems (leakage capture, broken, and choking mud) in the underwater operation of scallop fishing nets. The gravity mesh head, torsion spring type elastic teeth/multi-strand wire rope type elastic teeth, and flexible tail trawl were transformed into the adjustable hydraulic plate mesh head structure, follow-up the disturbing device, and flexible connection of rigid tail trawl net structure. As such, there was a significant decrease in the missed catch rate, broken shell rate, sediment content, and bottom-dwelling fish bycatch. The Ansys-Fluent software was utilized to simulate the three hydraulic plates (plate, positive angle of attack arc, and negative angle of attack arc type) at four towing speeds of 1, 1.5, 2, and 2.5 m/s. The hydrodynamic characteristics demonstrated that the drag resistance of three hydraulic pressure plates increased with the increase of the outflow angle and the drag speed. There was less resistance of the positive angle of attack arc type and the negative angle of attack arc type hydraulic pressure plate, compared with the flat type. The negative lift generated by the hydraulic plate increased with the increase of the drag speed. Once the outflow angle was 22.5°, the negative lift generated by the negative angle of the attack arc hydraulic plate was smaller than those of the flat plate type and the positive angle of the attack arc. When the angles were 30° and 45°, the negative lift generated by the negative angle of attack arc type hydraulic plate was greater than those of the flat plate type and the positive angle of attack arc type. The maximum negative lift was achieved for the best solution to the shape of the negative angle of the attack arc hydraulic plate. Furthermore, the mesh head and the disturbing device were in the best working condition, when the towing speed was between 1.5 and 2 m/s. When the towing speed was 1.5 m/s and the towing water depth was 30-50 m, the optimal combination of parameters was achieved as follows: the towing critical rope length was about 34.87-58.12 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 59.35°, and the critical drag force was 3 848.16 N, the stability angle was 11.03°, and the stable drag force was 1 998.85 N. When the towing speed was 2 m/s and the towing water depth was 30-50 m, the optimal combination of parameters was: the critical towing rope length was about 35.20-58.67 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 58.45°, the critical drag force was 5 222.04 N, the stability angle was 11.03°, and the stable drag force was 2 783.93 N. When the optimal towed speed was 1.8 m/s and the towed water depth was 30-50 m, the optimal combination of parameters was: the length of the towed boundary rope was about 35.02-58.36 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 58.95°, the critical drag force was 4 148.41 N, the stability angle was 11.03°, and the stable drag force was 2 275.83 N. The scallops were expected to smoothly enter the net bag when the height of the net opening was 350 mm. As such, there was a reduced introduction of benthic fish into the net bag by the water flow, which fully met the requirements for the limiting bycatch capacity of benthic fish. A comparison experiment of sea fishing demonstrated that there was no significant difference between the modified fishing nets and the fishing nets for production (P>0.5), but a significant decrease was found in the amount of shellfish and benthic fish (P<0.5), when the towing speed was 1.5, 1.8 and 2.0 m/s. The deviation rates of the average catch of single netting were 6.30%, 0.59%, and 5.55%, respectively, the deviation rates of the average shellfish yield of single netting were 90.63%, 84.78%, and 85.29%, respectively, and the deviation rates of the average concurrent catch of single netting benthic fish were 78.57%, 81.25%, and 84.85%, respectively. There was the lowest deviation rate of the average catch per net and the average shellfish per net, as well as the significantly reduced average concurrent catch of benthic fish when the towing speed was 1.8 m/s. Consequently, the optimal combination of structure and process parameters was achieved for the environmentally friendly fishing dredge and trawling. The finding can also provide promising theoretical support for the sustainable production of scallop fishing. [ABSTRACT FROM AUTHOR]

Present study considers the performance of a truss-type octagonal platform coupled with multiple wave energy converters (WECs). A numerical model was established to investigate multi-body hydrodynamic interaction between an octagonal platform and absorber-type wave energy converters utilizing CFD tool. Firstly, we analyzed the hydrodynamic performance and absorption power of the isolated WEC under different waves. The numerical results were compared with the physical experimental data to validate the proposed numerical method. Then the effects of the connection type and connection angle between the float arm and float were investigated numerically. Based on the numerical results, the fixed-type WEC with a zero deg connection angle is selected as the final design. Finally, the numerical simulation of multi-point-absorber circularly arranged on the octagonal platform was carried out. The numerical results demonstrated that the multi-body interaction has a remarkable influence on the absorption power and, therefore, it cannot be neglected when evaluate the performance of whole system. A truss-type octagonal platform coupled with multiple-WECs is proposed for wave energy absorption. The performance of isolated WEC is studied experimentally and numerically. The influences of incoming wave headings and connection types on the behavior of the WEC is investigated using a CFD approach. The interactions between WECs and platform are numerically analyzed. [ABSTRACT FROM AUTHOR]

The high-speed water entry dynamics and hydrodynamics of the projectiles are significantly influenced by the head shape and initial velocity. In this study, we numerically investigated unsteady cavitation dynamics during the water entry process and the hydrodynamic characteristics in the initial stages of conical, hemispherical, and blunt projectiles. We covered Froude numbers between 289 and 515, which are commonly handled in various industries. A preconditioned three-phase Navier–Stokes solver based on a homogeneous flow assumption was used for the numerical simulations. A dual-time stepping algorithm was applied to the current solver for physical time accuracy. The supercavitation regime under high-speed entry conditions with three projectiles is discussed, and the influence of the head shape on the pressure distribution, velocity, drag coefficient, and projectile shape is discussed in detail. Consequently, we suggest the physics of a high-speed water entry projectile based on the flow fields, cavitation dynamics, and time-dependent hydrodynamics under the effects of the projectile head shapes and initial velocity. • High-speed water entry dynamics under influence of projectile head shape and initial velocity were numerically investigated using a preconditioned 3D NS solver. • Froude number range of 289–515 is covered. • Unsteady cavitation dynamics on supercavitation regime near the projectiles were observed and discussed. • Hydrodynamics on initial stage of water entry such as free surface splashing, pressure distributions, drag coefficients were discussed. [ABSTRACT FROM AUTHOR]

This paper addresses the dynamic characteristics of an underwater towed system under manipulations of synchronous hydrofoils, considering the coupling of instantaneous fluid forces on the complete systems and hydrofoil control, which is usually difficult to solve with existing methods. A hydrodynamic model coupled with hydrofoil control algorithms is presented, wherein the hydrodynamic forces on the hydrofoils and Underwater Towed Vehicle (UTV) are simulated using the Computational Fluid Dynamics (CFD) method, and the fluid forces on the Towing Cable (TC) are described using a flexible cable dynamic model. The dynamic behaviors of the underwater towed system under depth-undulating, depth-tracking, and depth-keeping controls of hydrofoils are analyzed using the proposed model. The results show that the towing speed has a significant effect on the control efficiency of the hydrofoils. The dynamic responses of the underwater towed system increase or decrease more significantly in submerged depth control operations than in freely towing operations. The hydrofoils can achieve satisfactory effects in submerged depth control for the UTV at an appropriate towing speed, whereas it fails at a lower towing speed and noticeable dynamic response oscillations occur simultaneously. This study is expected to provide guidance on both hydrodynamic and control issues of underwater towed systems. • A hydrodynamic model of a complete underwater towed system coupled with hydrofoil manipulations is presented. • An open-loop and closed-loop hydrofoil control system is proposed and discussed using the CFD method. • The dynamic behaviors of the underwater towed system under manipulations of hydrofoils are comprehensively analyzed. [ABSTRACT FROM AUTHOR]

The motion of pectoral fin is highly complex, and accurately describing it is essential to guide the development of bionic pectoral fin mechanisms or ray-inspired robots. Although many researchers have proposed motion equations to express its motion, most of them have limitations, such as limited displacement degrees of freedom, indistinct topology, and empirically set coefficients values, which make it difficult to develop corresponding bionic pectoral fin mechanisms. To overcome these limitations, this paper proposes a new motion equation, whose coefficients values were strictly derived from observed pectoral fin information. The motion equation accurately describes three-dimensional motion of the pectoral fin and has a distinct topology. A geometric model is drawn from the shape of the real pectoral fin, and the motion simulation of the pectoral fin is completed by combining the motion equation. The result shows that the motion equation can achieve large amplitude and large bending motion characteristics of the real pectoral fin, which indicates a favorable imitation. The validity of the motion equation is verified by computational fluid dynamics simulation method. The established motion equation and the homogeneous coordinate transformation method for establishing it are useful for the development of the bionic pectoral fin mechanisms. • A motion equation of pectoral fin with a clear topology is established by homogeneous coordinate transformation method. • Coefficients values in the equation are strictly derived from observed pectoral fin information. • Motion simulation result shows the equation accurately represents the motion of a real pectoral fin. • The maximum efficiency of the pectoral fin with the motion equation is 0.54 [ABSTRACT FROM AUTHOR]