Applied Ocean Research最新文献

The gravity collapse method is a commonly used approach to generate internal solitary waves (ISWs) in experimental or numerical tanks. Presently, the correlation between the wave-making parameters and the amplitude of the ISW produced via this approach is not well understood. The research presents a suggested amplitude expression that is derived from direct numerical simulations, dimensional analysis, and restricted linear regression. To our best knowledge, the expression is first presented in this paper and can be utilized as a useful tool to produce ISW with a specific amplitude. While the formula is often effective in predicting the ISW amplitude, it exhibits a slight inaccuracy in situations where the initial level difference is small and the collapse region is long. This is because the initial potential energy of the collapse region is not fully transformed into ISW energy. Furthermore, this research illustrates the mechanics of the collapse process and concludes that the near-bed horizontal velocities and transport properties induced by large-amplitude ISWs are inherent characteristics rather than being caused by the unstable process of wave generation.

This paper presents a comprehensive verification and validation study applied to the hydrodynamic analysis of a semi-submersible Floating Offshore Wind Turbine (FOWT) platform. The numerical simulation is conducted using a coupled Computational Fluid Dynamics-Finite Element Method (CFD–FEM) approach. The CFD program incorporates an overset grid interpolation method, facilitating a more systematic grid refinement in the estimation of discretization uncertainties. A mooring analysis program, based on the FEM scheme, is integrated with the CFD program to simulate the dynamic responses of mooring system. The necessity of the nonlinear mooring model is demonstrated by comparing solutions with a comparative test using a linear mooring model. The verification study, based on a least-squares-based extrapolation method, quantifies the spatial and temporal discretization errors of the numerical model. The numerical solutions based on the baseline model are further validated against the model test measurements. A pitch free decay model test and a regular wave model test conducted within the OC5 project are utilized for the verification and validation study. Results from both tests demonstrate the reliability and accuracy of the coupled method.

The problem considered in this computational study is for the bearing capacity determination of three-dimensional (3D) rigid tripod-pile groups subjected to combined vertical (V), horizontal (H), and bending moment (M) loads. Using the adaptive 3D finite element limit analysis (FELA) coupled with lower bound, upper bound and mixed Gauss element formulations, numerical results are compared with those of the previous study under a single-component loading (V, or H, or M). The failure envelope in the 3D V–H–M loading space is then constructed, and several failure mechanisms presented to complement the discussions on the complex 3D responses. This is followed by a parametric study for the various factors such as the pile length, pile spacing, pile-soil adhesion factors as well as the loading direction angle. Finally, a closed-form design expression of failure envelopes considering the abovementioned influential parameters and the combined load are derived. Noting that the current industry-based design procedures for tripod-pile groups are rarely found, the outcome of the present study can be used with great confidence in design practice.

A finite element model based on high-level Green-Naghdi (GN) theory is developed and used to investigate the nonlinear wave force on a vertical cylinder. In this model, auxiliary variables are introduced to remove the third spatial derivative terms in the governing equations of GN theory to employ the Galerkin finite element method with linear elements. The impermeability boundary condition is exactly satisfied on the curved cylinder surface by a direct numerical method. The stream-function theory is used to generate nonlinear incident waves, and a relaxation zone is placed near the outer boundary to absorb the reflected waves. Numerical results are compared with experimental data, demonstrating the accuracy of the present method. Numerical simulations were carried out to examine the influences of the ratio of wave height to water depth, the ratio of water depth to wavelength, and the ratio of cylinder radius to wavelength on the wave force on the vertical cylinder.

Two-dimensional nonlinear free surface flows are nowadays routinely simulated on computers. In the context of the potential flow theory, fine details of the free surface motion can be captured. However, few studies focus on the flow beneath the free surface. The novelty of the present work lies in identifying lines where the pressure gradient is colinear with one eigenvector of the pressure Hessian matrix. The spatial and temporal evolution of these lines offers new insights into the complexity of highly nonlinear flows. This pressure characteristic is used to revisit four more or less recent research studies on breaking waves.

Ocean temperature data anomaly detection is instrumental in monitoring environmental changes and implementing measures to alleviate adverse consequences. This holds immense importance for marine environmental observation and scientific inquiry. However, existing anomaly detection methods encounter significant challenges in extracting features from data, which severely affects the performance of anomaly detection. Existing models have limitations in capturing the local contextual distribution features and high stochastic distribution trends of ocean temperature data. Therefore, this paper introduces the ConvTrans-CL model, which integrates the Transformer encoder with causal convolution and employs a contrastive learning approach. Causal convolution extracts the distributional features of short-time subsequences within a sliding window and integrates them into the self-attention mechanism, enabling the model to focus on the localized features of the data. Contrastive learning efficiently captures the long-range dependencies of time-series data by distinguishing between pairs of subsequences with adjacent time intervals and pairs of non-adjacent subsequences. This enables the model to capture the trend of high stochasticity in the distribution of the temperature data. Finally, we select temperature data from two sea areas that are susceptible to multiple environmental factors for experiments and compare the feature extraction and anomaly detection capabilities of ConvTrans-CL with other methods, confirming the superior performance of our method.

After the underwater vehicle detaches from the launch tube and starts its engine, the interaction between the tail cavity and the high-speed jet significantly impacts its motion stability and engine efficiency. Therefore, it is crucial to study the evolution mechanism of cavities in vertical motion. However, in traditional water tunnel experiments, the evolution of the cavity differs from the actual situation of vertical launch due to the influence of buoyancy. This study investigates the dynamics and motion trajectories of tail cavities and high-speed gas jets in the vertical movement of vehicles under different Froude numbers and nozzle stagnation pressure ratios through experimentation. The experimental results show two modes of tail cavity evolution: intact cavity (IC) and foam-conical cavity (FC-CC). Increasing the Froude numbers and stagnation pressure ratios facilitates the transition of the cavity from the IC mode to the FC-CC mode, effectively suppressing the formation and intensity of re-entrant jets, thereby reducing its impact on the bottom of the vehicle and enhancing motion stability. By deriving the formula for the length of the jet core area, it was found that the jet length is linearly related to the nozzle stagnation pressure, further revealing the mechanism of cavity evolution. These findings offer a new perspective for a deeper understanding and prediction of the dynamic behavior of underwater vehicles.

Laboratory model tests were conducted to investigate the stability of the L-shaped caisson quay wall installed on the sandy soil with clay interlayer. A total of three L-shaped caisson models were cast to explore the effects of the loading position and the position and thickness of the clay interlayer on their stability under plate loading. The ultimate bearing capacity of the L-shaped caisson quay wall on clay interlayer foundation was reduced by a maximum of 41 % compared to sand foundation. The ultimate bearing capacity was significantly reduced when the distance of the clay interlayer to the rubble mound was less than 1/5 of the quay wall height. The applied surcharge made the lateral earth pressure coefficient higher than the active earth pressure coefficients and triangularly distributed along the L-shaped caisson wall. The modified Beton's method considered the effect of the toe length of the L-shaped caisson could more accurately predict the lateral earth pressure. The failure of the L-shaped caisson quay wall on the clay interlayer foundation has three forms Type-I, Type-II, and Type-Ⅲ which are influenced by the loading position, L-shaped caisson dimensions, and positions of the clay interlayer.