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Applied Ocean Research

Applied Ocean Research

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Vortex transportation and wave deformation after the interaction of a solitary wave with an inclined bottom mounted plate
Chih-Hua Chang; Keh-Han Wang; Chang Lin; Jassim M. Jaf;
Abstracts:This study is aimed to investigate numerically and experimentally the interaction between a solitary wave and a thin submerged plate with various inclinations. The obtained results on wave deformations and formed vortex patterns are analyzed. To extend the very limited study on a solitary wave encountering an inclined plate, the developed streamfunction-vorticity formulations with free surface (SVFS) model is applied to compute the flow field, wave profiles, and induced forces. In addition, the experimental measurements of wave elevations and flow visualizations using particle image velocimetry (PIV), particle-tracking, and planar laser-induced fluorescence (PLIF) were conducted. This paper presents qualitative and quantitative comparisons between numerical solutions and experimental data, including the recorded images. The agreements are generally good. The influence of tilting angle, ranging from −60° to 60°, on the flow patterns, formed vortices, and free-surface elevations are examined and discussed. For a given incident wave and the setting of the same vertical level of the plate tip, it is found that the wave attenuations under a plate with a downstream tilting angle (positive angle) is larger than that when a plate is placed with an upstream tilting angle (negative angle). Also, the occurrence of maximum intensity of the main vortex is observed to be within the tilting angles of −30° to −10° depending on the setting of the plate height. In terms of hydrodynamic forces, when keeping the same tilting angle, the results suggest the maximum horizontal force per unit width on a downstream tilting plate is larger than that on an upstream tilting plate.
Motion of a floating body in a harbour by domain decomposition method
Y.Y. Shi; Z.F. Li; G.X. Wu;
Abstracts:A three-dimensional domain decomposition method is used to solve the problem of wave interaction with a ship floating inside a harbour with arbitrary shape. The linearized velocity potential theory is adopted. The total fluid domain is divided into two sub-ones: one for the harbour and the other for the external open sea. Boundary integral equations together with the free surface Green function are used in the both domains. Matching conditions are imposed on the interface of the two sub-domains to ensure the velocity and pressure continuity. The advantage of the domain decomposition method over the single domain method is that it removes the coastal surface from the boundary integral equation. This subsequently removes the need for elements on the coastal wall when the equation is discretized. The accuracy of the method is demonstrated through convergence study and through the comparison with the published data. Extensive results through the hydrodynamic coefficients, wave exciting forces and ship motions are provided. Highly oscillatory behaviour is observed and its mechanism is discussed. Finally, the effects of incident wave direction, ship location as well as the harbour topography are investigated in detail.
Effects of operating parameters and injection method on the performance of an artificial upwelling by using airlift pump
Yongfa Qiang; Wei Fan; Canbo Xiao; Yiwen Pan; Ying Chen;
Abstracts:Artificial upwelling by using airlift pump is considered a sustainable way in actualizing ocean fertilization, which could potentially alleviate the pressures on the fish stocks and human-driven climate change. However, few experimental data about the effects of operating parameters and injection method on the performance of the airlift artificial upwelling were found in the literature. In this paper, an airlift pump for artificial upwelling was investigated through three field experiments, in which airlift pumps of pipe length ranges from 20 to 28.3 m and pipe diameter ranges from 0.4 to 2 m, were designed and tested. Pumped water flow rate and injected air flow rate were measured to study the effects of different factors on the performance of airlift pump. The experiment results show that airlift pump efficiency is closely related to pipe diameter, submerged depth, submergence ratio and nozzle designs. There is no best nozzle for all the range of air flow rate, and the recommended nozzles are double-ring-shaped or star-shaped nozzles when the air flow rate is less or more than 220 L/min, respectively. Moreover, the lift effectiveness increases when the hole size of nozzle is enlarged, which indicates a relatively large bubble size will enhance the lift effectiveness.
QUUV: A quadrotor-like unmanned underwater vehicle with thrusts configured as X shape
Jingwei Bian; Ji Xiang;
Abstracts:A quadrotor-like unmanned underwater vehicle (QUUV) is presented in this paper. The four fixed thrusters are configured as X shape through which the QUUV has vertical and horizontal movements decoupled. Its mathematical model and motion analysis are introduced accordingly. The differences with conventional underwater vehicles are also discussed to show its merits. Sliding mode control is adopted to design its controller. Various motions with the sliding mode controller are simulated, followed by an experiment test in a small pool.
Experimental study of forces on a multi-column floating platform in internal solitary waves
Min Chen; Ke Chen; Yun-Xiang You; Hai-Ting Yu;
Abstracts:Experiments were conducted to study the forces exerted by internal solitary waves (ISWs) on a floating platform. Depression ISWs were generated by a piston-type wave maker in a density stratified two-layer fluid in a 30-m-long wave flume. Experimental ISWs were compared with ISW theoretical models in terms of frequency-amplitude relationships. The forces and moments on the multi-column floating platform model were measured. Based on the experimental results, the empirical coefficients Cm and Cd are obtained as a function of KC, Reynolds number and the layer depth h1/h. The force prediction based on Morison’s equation and pressure integral were also performed, and the calculated results are well consistent with the measurements. Besides, the forces and moments increase linearly with the wave amplitude, and the maximums of the horizontal forces and moments increase with the layer depth h1/h decreasing.
Hydrodynamic modeling of a novel dual-chamber OWC wave energy converter
Dezhi Ning; Yu Zhou; Chongwei Zhang;
Abstracts:A novel cylindrical oscillating water column (OWC) wave energy converter (WEC) with double chambers is proposed to harvest the wave energy effectively in deep water. An analytical model is developed to investigate its hydrodynamic characteristics based on the linear potential flow theory and eigenfunction expansion technique. The computational domain is divided into six sub-domains. The unknowns are solved by matching the continuous conditions of the fluid velocity and velocity potential between neighboring sub-domains. A pneumatic model is adopted to describe the relationship between the air pressure in the chamber and turbine characteristics. Effects of the chamber volume and parameters of the turbine on the energy conversion efficiency are investigated. It is found that the chamber volume affects the OWC hydrodynamics seriously in the case of large turbine rotating speed. Three typical free-surface oscillation modes in the chamber are found, two of which contribute much to the energy conversion. The comparison between results of the single- and dual-chamber OWC-WECs shows that the effective frequency bandwidth of the dual-chamber OWC-WEC is broader than that of the single-chamber OWC-WEC.
Probability analysis of submarine landslides based on the Response Surface Method: A case study from the South China Sea
Bin Zhu; Huafu Pei; Qing Yang;
Abstracts:As deep-sea engineering develops, it becomes essential to analyze the stability of submarine slopes when considering the stability of submarine foundations or evaluating the safety of offshore structures. However, the traditional method for analyzing slope stability does not give adequate consideration to the uncertainty of soil properties, and so the reliability method has been proposed and used to settle the variation of soil parameters. Regarding the northern slope of the South China Sea, the present paper summarizes its geomorphic features, seismic characteristics, and the soil strength at certain boreholes. A typical slope section is chosen with which to conduct probability analysis using the polynomial-based Response Surface Method (RSM) and the Advanced First Order Second Moment method(AFOSM). A novel form of the RSM based on Gaussian Process Regression (GPR) is also proposed and applied in this case to approximate the limit state function, and its efficiency is confirmed. The simulation results of Latin-hypercube analysis are set as the benchmark for the other methods. The influences of slope gradient and seismic action on the stability of submarine slopes are also investigated.
Prediction of the mean heading of a turret moored FPSO in bi-modal and bi-directional sea states
Razieh Zanganeh; Krish Thiagarajan;
Abstracts:Ship-shaped platforms like Floating Production Storage and Offloading (FPSO) vessels are deployed in many offshore locations where the sea states are dominated by wind driven local seas combined with long period swells that are incident from different directions. Many of these vessels are held in station by a mooring system that is connected to the platform by a rotating turret. The heading stability of the vessel and its ability to self-align with respect to oncoming environmental forces is studied in this paper. Experiments at 1:120 scale conducted in a wave basin facility for two storm conditions offshore Brazil and West of Africa show that the heading angle is self-limiting to a range of ±20° with respect to head sea direction. Time domain simulations conducted using industry-standard software by modeling the wind-sea and swell with suitable spectrum models approaching from different directions show almost double the heading range seen in experiments. If similar numerical analyses were part of a design process, it is conceivable that this could result in incorrect predictions of the weathervaning of the platform, and hence the latter’s global motions in response to design storm conditions.
On the heuristic based electronic control of marine cycloidal propeller
Subhashis Nandy; Vishwanath Nagarajan; Om Prakash Sha;
Abstracts:The design and feasibility of electronically controlled Marine Cycloidal Propeller (MCP) by implementing heuristic-based approach is demonstrated. Adding closed-loop control enhances performance. This is shown by simulation. The chances of improving efficiency are more in electronically controlled MCP, as pitch angle of each blade can be independently controlled. Multiple configurations of MCP can be anchored within the same model without changing hardware. Simulations are used to finalize the technical specifications of the system. Experimental results demonstrate the acceptable trajectory generation (using online feedback) and synchronizations of all instruments.
Dynamic model identification of unmanned surface vehicles using deep learning network
Joohyun Woo; Jongyoung Park; Chanwoo Yu; Nakwan Kim;
Abstracts:In this paper, a deep learning–based dynamic model identification method is proposed. The proposed method is designed to capture higher-order dynamic behaviors that result from the coupling of hydrodynamics and actuator dynamics. By adopting recent advancements in deep learning, our model addresses problems such as the regression problem in machine learning. Among various deep learning algorithms, long short-term memory (LSTM)–based recurrent neural network was used to deal with the hidden latent state of the USV dynamic model. The model validation was performed using free running test data of a USV. Analysis result shows that proposed model reduces surge speed prediction error by 76.9%, yaw rate prediction error by 60.7% and sway velocity prediction error by 27.9% over the conventional linear dynamic model.
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