ISSN  0890-5487 CN 32-1441/P

2012 Vol.(1)

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Infra-Gravity Wave Generation by the Shoaling Wave Groups over Beaches
LIN Yu-Hsien, HWUNG Hwung-Hweng
2012, (1): 1-18.
[Abstract](530)
Abstract:
A physical parameter, μb, which was used to meet the forcing of primary short waves to be off-resonant before wave breaking, has been considered as an applicable parameter in the infra-gravity wave generation. Since a series of modulating wave groups for different wave conditions are performed to proceed with the resonant mechanism of infra-gravity waves prior to wave breaking, the amplitude growth of incident bound long wave is assumed to be simply controlled by the normalized bed slope, βb. The results appear a large dependence of the growth rate, α, of incident bound long wave, separated by the three-array method, on the normalized bed slope, βb. High spatial resolution of wave records enables identification of the cross-correlation between squared short-wave envelopes and infra-gravity waves. The cross-shore structure of infra-gravity waves over beaches presents the mechanics of incident bound- and outgoing free long waves with the formation of free standing long waves in the nearshore region. The wave run-up and amplification of infra-gravity waves in the swash zone appear that the additional long waves generated by the breaking process would modify the cross-shore structure of free standing long waves. Finally, this paper would further discuss the contribution of long wave breaking and bottom friction to the energy dissipation of infra-gravity waves based on different slope conditions.
Artificial Neural Network Ability in Evaluation of Random Wave-Induced Inline Force on A Vertical Cylinder
Lotfollahi-Yaghin, M. A., Pourtaghi, A., Sanaaty, B., Lotfollahi-Yaghin, A.
2012, (1): 19-36.
[Abstract](564)
Abstract:
An approach based on artificial neural network (ANN) is used to develop predictive relations between hydrodynamic inline force on a vertical cylinder and some effective parameters. The data used to calibrate and validate the ANN models are obtained from an experiment. Multilayer feed-forward neural networks that are trained with the back-propagation algorithm are constructed by use of three design parameters (i.e. wave surface height, horizontal and vertical velocities) as network inputs and the ultimate inline force as the only output. A sensitivity analysis is conducted on the ANN models to investigate the generalization ability (robustness) of the developed models, and predictions from the ANN models are compared to those obtained from Morison equation which is usually used to determine inline force as a computational method. With the existing data, it is found that least square method (LSM) gives less error in determining drag and inertia coefficients of Morison equation. With regard to the predicted results agreeing with calculations achieved from Morison equation that used LSM method, neural network has high efficiency considering its convenience, simplicity and promptitude. The outcome of this study can contribute to reducing the errors in predicting hydrodynamic inline force by use of ANN and to improve the reliability of that in comparison with the more practical state of Morison equation. Therefore, this method can be applied to relevant engineering projects with satisfactory results
Numerical Investigation of the Effect of Current on Wave Focusing
LI Jin-xuan, LIU Dian-yong, LIU Shu-xue
2012, (1): 37-48.
[Abstract](656)
Abstract:
In the present study, a numerical wave tank is developed to simulate the nonlinear wave-current interactions based on High Order Spectral (HOS) method. The influences of current on wave focusing are investigated by use of numerical model. The current is assumed to be constant in space. Focused waves with different amplitudes and frequency spectra are simulated with and without current. The focused wave characteristics, such as surface elevation, the maximum crest and frequency spectrum, with different current are compared. The results show that the opposing current increases the maximum crest and the energy transform during wave focusing process, and vice versa for the following current.
A Maximum-Entropy Compound Distribution Model for Extreme Wave Heights of Typhoon-Affected Sea Areas
WANG Li-ping, SUN Xiao-guang, LU Ke-bo, XU De-lun
2012, (1): 49-58.
[Abstract](631)
Abstract:
A new compound distribution model for extreme wave heights of typhoon-affected sea areas is proposed on the basis of the maximum-entropy principle. The new model is formed by nesting a discrete distribution in a continuous one, having eight parameters which can be determined in terms of observed data of typhoon occurrence-frequency and extreme wave heights by numerically solving two sets of equations derived in this paper. The model is examined by using it to predict the N-year return-period wave height at two hydrology stations in the Yellow Sea, and the predicted results are compared with those predicted by use of some other compound distribution models. Examinations and comparisons show that the model has some advantages for predicting the N-year return-period wave height in typhoon-affected sea areas.
Numerical Analysis of Hydrodynamic Behaviors of Two Net Cages with Grid Mooring System Under Wave Action
CHEN Chang-ping, ZHAO Yun-peng, LI Yu-cheng, DONG Guo-hai, ZHENG Yan-na
2012, (1): 59-76.
[Abstract](523)
Abstract:
Based on rigid kinematics theory and lumped mass method, a mathematical model of the two net cages of grid mooring system under waves is developed. In order to verify the numerical model, a series of physical model tests have been carried out. According to the comparisons between the simulated and the experimental results, it can be found that the simulated and the experimental results agree well in each wave condition. Then, the forces on the mooring lines and the floating collar movement are calculated under different wave conditions. Numerical results show that under the same condition, the forces on the bridle ropes are the largest, followed by forces on the main ropes and the grid ropes. The horizontal and the vertical float collar motion amplitudes increase with the increase of wave height, while the relationship of the horizontal motion amplitude and the wave period is indistinct. The vertical motion amplitude of the two cages is almost the same, while on the respect of horizontal motion amplitude, cage B (behind cage A, as shown in Fig. 4) moves much farther than cage A under the same wave condition. The inclination angle of the floating system both in clockwise along y axis and the counter one enlarges a little with the increase of wave height.
Investigation on the Hydrodynamic Performance of An Ultra Deep Turret-Moored FLNG System
ZHAO Wen-hua, YANG Jian-min, HU Zhi-qiang, XIAO Long-fei, PENG Tao
2012, (1): 77-93.
[Abstract](656)
Abstract:
Hydrodynamic performance of an ultra deep turret-moored Floating Liquefied Natural Gas (FLNG) system is investigated. Hydrodynamic modeling of a turret-moored FLNG system, in consideration of the coupling effects of the vessel and its mooring lines, has been addressed in details. Based on the boundary element method, a 3-D panel model of the FLNG vessel and the related free water surface model are established, and the first-order and second-order mean-drift wave loads and other hydrodynamic coefficients are calculated. A systematic model test program consisting of the white noise wave test, offset test and irregular wave test combined with current and wind, etc. is performed to verify the numerical model. Owing to the depth limit of the water basin, the model test is carried out for the hydrodynamics of the FLNG coupled with only the truncated mooring system. The numerical simulation model features well the hydrodynamic performance of the FLNG system obtained from the model tests. The hydrodynamic characteristics presented in both the numerical simulations and the physical model tests would serve as the guidance for the ongoing project of FLNG system
Effect of Upward Internal Flow on Dynamics of Riser Model Subject to Shear Current
CHEN Zheng-shou, KIM Wu-joan, XIONG Cong-bo
2012, (1): 95-108.
[Abstract](582)
Abstract:
Numerical study about vortex-induced vibration (VIV) related to a flexible riser model in consideration of internal flow progressing inside has been performed. The main objective of this work is to investigate the coupled fluid-structure interaction (FSI) taking place between tensioned riser model, external shear current and upward-progressing internal flow (from ocean bottom to surface). A CAE technology behind the current research which combines structural software with the CFD technology has been proposed. According to the result from dynamic analysis, it has been found that the existence of upward-progressing internal flow does play an important role in determining the vibration mode (/dominant frequency), vibration intensity and the magnitude of instantaneous vibration amplitude, when the velocity ratio of internal flow against external current is relatively high. As a rule, the larger the velocity of internal flow is, the more it contributes to the dynamic vibration response of the flexible riser model. In addition, multi-modal vibration phenomenon has been widely observed, for asymmetric curvature along the riser span emerges in the case of external shear current being imposed.
Tidal Level Response to Sea-Level Rise in the Yangtze Estuary
GONG Zheng, ZHANG Chang-kuan, WAN Li-ming, ZUO Jun-cheng
2012, (1): 109-122.
[Abstract](601)
Abstract:
The rise of tidal level in tidal reaches induced by sea-level rise has a large impact on flood control and water supply for the regions around the estuary. This paper focuses on the variations of tidal level response along the tidal reaches in the Yangtze Estuary, as well as the impacts of upstream discharge on tidal level response, due to the sea-level rise of the East China Sea. Based on the Topex/Poseidon altimeter data obtained during the period 1993~2005, a stochastic dynamic analysis was performed and a forecast model was run to predict the sea-level rise of the East China Sea. Two- dimensional hydrodynamic numerical models downscaling from the East China Sea to estuarine areas were implemented to analyze the rise of tidal level along the tidal reaches. In response to the sea-level rise, the tidal wave characteristics change slightly in nearshore areas outside the estuaries, involving the tidal range and the duration of flood and ebb tide. The results show that the rise of tidal level in the tidal reaches due to the sea-level rise has upstream decreasing trends. The step between the stations of Zhangjiagang and Shiyiwei divides the tidal reaches into two parts, in which the tidal level response declines slightly. The rise of tidal level is 1~2.5 mm/a in the upper part, and 4~6 mm/a in the lower part. The stations of Jiangyin and Yanglin, as an example of the upper part and the lower part respectively, are extracted to analyze the impacts of upstream discharge on tidal level response to the sea-level rise. The relation between the rise of tidal level and the upstream discharge can be fitted well with a quadratic function in the upper part. However, the relation is too complicated to be fitted in the lower part because of the tide dominance. For comparison purposes, hourly tidal level observations at the stations of Xuliujing and Yanglin during the period 1993~2009 are adopted. In order to uniform the influence of upstream discharge on tidal level for a certain day each year, the hourly tidal level observations are corrected by the correlation between the increment of tidal level and the increment of daily mean upstream discharge. The rise of annual mean tidal level is evaluated. The resulting rise of tidal level at the stations of Xuliujing and Yanglin is 3.0 mm/a and 6.6 mm/a respectively, close to the rise of 5 mm/a according to the proposed relation between the rise of tidal level and the upstream discharge
A Long-Term Numerical Model of Morphodynamic Evolution and Its Application to the Modaomen Estuary
MO Wen-yuan, WEI Xing, QIU Li-guo
2012, (1): 123-138.
[Abstract](528)
Abstract:
Because of the influence of human activities, the evolution of the Modaomen Estuary is no longer a purely natural process. We used a long-term morphodynamic model (PRD-LTMM-10) to study the evolution of the estuary from 1977 to 1988. The model incorporated modules for riprap-siltation promotion and waterway dredging. The model can simulate the morphodynamic evolutionary processes occurring in the Modaomen Estuary during the period of interest. We were able to isolate the long-term influences of various human engineering activities and the roles of natural factors in estuarine evolution. The governance projects had the largest effect on the natural development of the estuary, resulting in larger siltation on the west side. Installation of riprap and reclamation of submerged land resulted in scouring of the main Hengzhou Channel causing deep trough out-shift. Severe siltation narrowed the upper end of the Longshiku Deep Trough.
A Flexible-Segment-Model-Based Dynamics Calculation Method for Free Hanging Marine Risers in Re-Entry
XU Xue-song, WANG Sheng-wei
2012, (1): 139-152.
[Abstract](583)
Abstract:
In re-entry, the drilling riser hanging to the holding vessel takes on a free hanging state, waiting to be moved from the initial random position to the wellhead. For the re-entry, dynamics calculation is often done to predict the riser motion or evaluate the structural safety. A dynamics calculation method based on Flexible Segment Model (FSM) is proposed for free hanging marine risers. In FSM, a riser is discretized into a series of flexible segments. For each flexible segment, its deflection feature and external forces are analyzed independently. For the whole riser, the nonlinear governing equations are listed according to the moment equilibrium at nodes. For the solution of the nonlinear equations, a linearization iteration scheme is provided in the paper. Owing to its flexibility, each segment can match a long part of the riser body, which enables that good results can be obtained even with a small number of segments. Moreover, the linearization iteration scheme can avoid widely used Newton-Rapson iteration scheme in which the calculation stability is influenced by the initial points. The FSM-based dynamics calculation is timesaving and stable, so suitable for the shape prediction or real-time control of free hanging marine risers.
Numerical Simulation of Irregular Wave Overtopping Against A Smooth Sea Dike
GUO Xiao-yu, WANG Ben-long, LIU Hua
2012, (1): 153-166.
[Abstract](508)
Abstract:
Based on the filtered Navier-Stokes equations and Smagorinsky turbulence model, a numerical wave flume is developed to investigate the overtopping process of irregular waves over smooth sea dikes. Simulations of fully nonlinear standing wave and regular wave's run-up on a sea dike are carried out to validate the implementation of the numerical wave flume with wave generation and absorbing modules. To model stationary ergodic stochastic processes, several cases with different random seeds are computed for each specified irregular wave spectrum. It turns out that the statistical mean overtopping discharge shows good agreement with empirical formulas, other numerical results and experimental data.
Wave Height Transformation and Set-up Between a A Submerged Permeable Breakwater and A Seawall
TSAI Ching-piao, YU Chien-hung, CHEN Hong-bin, CHEN Hsin-yu
2012, (1): 167-176.
[Abstract](535)
Abstract:
In this study, we investigated wave transformation and wave set-up between a submerged permeable breakwater and a seawall. Modified time-dependent mild-slope equations, which involve parameters of the porous medium, were used to calculate the wave height transformation and the mean water level change around a submerged breakwater. The numerical solution is verified with experimental data. The simulated results show that modulations of the wave profile and wave set-up are clearly observed between the submerged breakwater and the seawall. In contrast to cases without a seawall, the node or pseudo-node of wave height evolution can be found between the submerged breakwater and the seawall. Higher wave set-up occurs if the nodal or pseudo-nodal point appears near the submerged breakwater. We also examined the influence of the porosity and friction factor of the submerged permeable breakwater on wave transformation and set-up.
Research on the Hydrodynamic Characteristics of Heave Plate Structure with Different Form Edges of A Spar Platform
SHEN Wen-jun, TANG You-gang, LIU Li-qin
2012, (1): 177-184.
[Abstract](563)
Abstract:
The hydrodynamic characteristics of heave plates with different form edges of Truss Spar Platform are studied in this paper. Numerical simulations are carried out for the plate forced oscillation by the dynamic mesh method and user defined functions of FLUENT. The added mass coefficient and the damping coefficient of heave plate with tapering condition and the chamfer condition are calculated. The results show that, in a certain range, the hydrodynamic performance of heave plate after being tapered is better.

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