Applied Ocean Research最新文献

Three common types of greenwater events – plunging dam breaker (PDB), hammer fist (HF) and plunging wave (PW) – are experimentally modeled in a laboratory wave flume on a rectangular structure, with a focus on investigating their fluid kinematics. To ensure high repeatability for type PW and type HF, a specific wave focusing method was employed, while PDB-type events were generated using a regular wave train. Utilizing a combination of PIV (particle image velocimetry) and BIV (bubble image velocimetry) techniques, ensemble-averaged flow fields were obtained from 20 repeated tests for each event type. The flow patterns at high speed, along with corresponding velocity fields, facilitated a comprehensive examination of flow behaviors, particularly for HF-type events which have received limited study. The maximum dominant speed for type PW was measured at up to 2.76$C$ during the run-up phase, where $C$ denotes the celerity of the incoming wave. For type HF and type PDB, the maximum dominant speeds occurred during the greenwater phase, with magnitudes of 1.37$C$ and 0.79$C$, respectively. The velocity deviation during the greenwater phase is <0.62C for all type events. The greenwater front velocity was measured at 1.36$C$ for type PW and approximately 0.8$C$ for both type HF and type PDB. Moreover, an attempt to evaluating the potential greenwater loads of high spatial resolution is demonstrated by the measured velocity fields for all event types. In this study, the simplest dam break solution is found to effectively capture the horizontal greenwater velocity distribution for all event types. Additionally, other mathematical expressions for the horizontal greenwater velocity have been derived based on flow self-similarity.

The closed sump is a vital inlet structure for low-head tidal pumping stations in coastal regions. The flow instability caused by roof-attached vortices (RAVs) and floor-attached vortices (FAVs) within the sump significantly affects the reliability of the unit operation. Mitigating and eradicating these detrimental vortices is deemed imperative in the realm of engineering applications. Reducing the sources of vortices, improving operating modes, and adding anti-vortex devices (AVDs) are the general ways to suppress the occurrence of vortices. However, few reports exist on the closed pump sumps' joint vortex elimination methods for RAVs and FAVs. Based on a deep understanding of the dynamic evolution behavior of the RAVs and FAVs, a hybrid RANS-LES numerical prediction method is adopted to comprehensively compare the suppression effects of different AVDs on the vortex structure. An effective "elliptical line anti-vortex cone combined with underwater cover plate" joint anti-vortex device (JAVD) is proposed and verified through model experiments. The research results provide analytical ideas for improving the flow field of the pump sump and optimizing hydraulic design.

Wave motion simulators have various applications in the development of marine industrial products. The main factor limiting its performance to meet the needs for extreme sea states simulation is the lack of large stroke, heavy duty, and high response actuators. Therefore, a novel actuator mechanism is proposed in this paper to realize the dynamic output of large stroke, heavy duty and high response. In this paper, a (2P(nR)+PPR)P actuator mechanism composed of 2P(nR)P and PPRP mechanisms is proposed, with the input-output relationship analyzed. Then, this actuator mechanism is applied to a 6-PUS platform. The Newton-Euler method is employed to model and simulate the dynamics of the platform to verify the input-output relationships. Finally, a 6-PUS platform based on (2P(nR)+PPR)P mechanism was designed, built and tested under extreme operating conditions. The results show that the 6-PUS platform with this actuator mechanism can achieve a large stroke of ±45° within 7 s cycle time and a high response motion of ±30° within 3 s under a heavy duty of 10t, which demonstrates that it has the performance of large stroke, heavy duty and high response. This actuator mechanism and its platform are of significant value in wave motion simulators for extreme sea states.

The real-time hybrid model (RTHM) test is adept at addressing the scale contradiction, the lack of fidelity in wind modelling in hydrodynamic testing facilities and spatial constraints inherent in conventional monopile-type offshore wind turbine (OWT) model testing methods, thus emerging as an effective avenue for conducting physical model tests of Monopile-type OWTs. This method entails the reproduction of aerodynamic loads or platform motions using loading device or vibration tables. Time delays in the physical attributes of the loading device and signal transmission processes within the system can result in error accumulation, with the potential to impact overall system stability. Moreover, time delay compensation algorithms for hybrid model test systems with force control loading can easily generate excessive noise, leading to system divergence. As a result, time delay has emerged as a technical challenge in the RTHM test. To address this issue, this paper has developed second-order and third-order polynomial extrapolation algorithms, alongside an adaptive compensation algorithm. The adaptive compensation algorithm employs the least squares method to identify parameters of the loading system, enabling it to address variations in the time delay of the experimental system caused by the nonlinearity of the loading system and changes in the physical properties of the model. The feasibility and effects of time delay compensation for various algorithms are validated through numerical simulation. Results indicate that the adaptive compensation algorithm surpasses second and third-order polynomial extrapolation compensation algorithms in terms of accuracy and compensation effectiveness. To validate the applicability of the adaptive compensation algorithm, a RTHM test was conducted. Across rotor thrust force (RotThrust) and tower top displacement, there was an average reduction of approximately 5 % and 9 % in the maximum and minimum synchronization errors, respectively. This highlights the efficacy of the delay compensation algorithm in practical applications, notably diminishing time delay errors within the experimental system. The adaptive compensation algorithm continuously adjusts and updates parameters, enhancing the adaptability of the compensation process to time-varying systems.

The wave-driven vehicle is a surface vehicle powered by capturing wave energy, which is required to face harsh sea conditions when performing tasks in parts of the ocean. However, wave-driven vehicles are usually small in size, and their seakeeping and speed are generally poor in high sea conditions. Wave driven vehicles are usually equipped with rigid connected hydrofoils to capture wave energy, which can provide power for wave driven vehicles and enhance seakeeping. Aiming at the long-term survival and operation requirements of wave-driven vehicle under high sea conditions, this paper studies the effect of high sea conditions launching wing on the self- propelled performance of wave-driven vehicle. After installing rigid connected hydrofoils on wave-driven vehicles, the structural parameters of the hydrofoils are changed, and the kinematic and dynamic responses of wave-driven vehicles at 0–90 ° wave encounter Angle are numerically simulated based on CFD method. The effects of underwater wing depth, hydrofoil spacing and hydrofoil span length on the self- propelled performance of wave-driven vehicles are analyzed. Based on this, the structural parameters of rigidly connected hydrofoils are optimized, which improves the seakeeping and rapidity of wave-driven vehicles in high sea conditions.

This paper presents an effective approach for quantitatively assessing the structural performance of the corroded reinforced concrete (RC) columns with a circular cross-section in aggressive marine environments. Firstly, the material damage and structural strength deterioration models are constructed by analyzing the degradation mechanisms of the materials due to rebar corrosion. The general method for calculating the load-bearing capacity of the circular RC columns at various corrosion levels with different loading conditions is proposed, based on the assumptions of the equivalent steel ring and the rectangular stress block of the concrete. In order to improve computational efficiency, a simplified method is then developed by introducing a linear relation to replace the complex expression of the load-bearing capacity coefficients of rebar in the general method. Finally, a numerical example is employed to investigate the effect of design parameters on the load-bearing capacity and performance deterioration rate of the corroded RC columns, and the effectiveness of the proposed simplified method is examined. The obtained results show that the corrosion level, loading condition and design parameters have a significant impact on the residual load-bearing capacity of the circular RC columns, and the simplified method with the introduced linear expression can be used for the preliminary assessment of the residual resistance of the corroded circular RC columns.

Natural gas hydrates stored in the subsurface seabed of continental margins are one of the most important carbon reservoirs on Earth. Research on natural gas hydrates is of great significance to global warming and ecological protection. Methane plumes caused by crustal dynamics are usually considered as a sign of existence of natural gas hydrates. Detection of methane plumes thus becomes the first step of cold seep research. This paper conducts comprehensive research on detection of methane plumes based on deep learning methods and optical images. First, we proposed a method of creating high quality and balanced datasets for methane plumes detection tasks using open-source videos. We then proposed a FMAW-YOLOv5s method for methane plumes detection. The FMAW-YOLOv5s method improves the traditional YOLOv5s in design of backbone network, neck network and loss function. The FMAW-YOLOv5s method can realize accurate and fast detection of methane plumes with a precision of 96.9% and FPS of 141.7. The lightweight feature of FMAW-YOLOv5s also enables the deployment in edge computing devices such as AUVs and ROVs. This research can not only promote the study of cold seep activities, but also provide meaningful insights for detection of other underwater events such as gas pipelines leakage.

Near-trapping is an essential resonant phenomenon associated with multiple-column structures in water waves, which exhibits high wave profiles in the area enclosed by multiple columns. For engineering safety, a straightforward scenario is proposed in this study to suppress the near-trapping phenomenon by allowing the multiple columns to move longitudinally with respect to the symmetric axes. To evaluate the effectiveness of the scenario, a stable and efficient second-order numerical model in the time domain is developed and adopted, which is also robust for the simulation of multiple structures with complex geometry and undergoing individual motions. Since both the first-order and second-order boundary value problems are solved, the second-order nonlinear properties are highlighted and the second harmonic induced near-trapping is the main focus of this study. For the cases in this study, the numerical results obtained by the validated numerical model confirm that this scenario can reduce the maximum second harmonic of the wave elevation by 63% and the maximum second-order wave elevation by 59% at the second near-trapping frequency. The first-order wave elevation is also reduced, and it is even smaller than the incident wave in a large portion of the enclosed region. As a mass–spring system is considered in the simulation of body responses, by testing different body masses and stiffnesses, it is revealed that the wave profile is insensitive to those parameters and the reduction in the wave profile occurs for all those parameters tested. It is interesting to find out that the near-trapping frequency can shift in the suppression scenario, and a remarkable reduction (32%) in the second-order wave elevation is still observed at the shifted near-trapping frequency.

This study examines the effectiveness of buffer blocks in dissipating the flow due to extreme events along the coasts using the numerical modelling approach. Initially, the study explores the effect of the number of rows of buffer blocks on the reduction of momentum flux. After establishing that the three rows configuration are the most efficient, subsequent analysis were carried out which include the understanding in the variations of block heights, including adjustments in height ratios between rows and by arranging blocks in increasing or decreasing height within the rows. Through a systematic examination of these configurations, the study aims to determine the most effective setup for maximizing energy dissipation of flow characteristics during extreme events such as tsunamis and storm surges.

The wave glider is an unmanned surface vehicle propelled by wave energy, consisting of three main components: a surface float, a submarine glider, and a tether. The submarine glider serves as the primary propulsion mechanism, converting the wave-induced motions of the float into forward thrust, which is crucial for the wave glider’s energy absorption efficiency. However, predicting the motion performance of the submarine glider presents a significant challenge due to its complex and unique structure. In this study, we establish a kinematic and dynamic model of the submarine glider’s hydrofoils, considering the elastic effects such as spring stiffness, spring preload, and spring attachment positions. To support this model, wind tunnel tests were conducted to determine the lift and drag coefficients of the submarine glider under various motion states. Utilizing the elastic hydrofoil model and the experimentally obtained lift and drag coefficients, we developed a comprehensive kinematic and dynamic simulation model of the submarine glider under heave excitation forces. To validate the accuracy of this model, performance tests for the submarine glider were designed under different vertical excitation forces , with results compared to simulation outcomes. The findings indicate that the deviation between simulated and experimental outcomes is less than 5%, demonstrating the model’s precision. This accurate simulation capability allows for detailed analysis of the effects of various design parameters on the glider’s performance and lays a solid foundation for high-accuracy motion simulation of the entire wave glider.