Structural analysis of floating pipes of the fish cage in currents

A numerical model is developed to investigate the structural performance and stress distribution of floating pipes of fish cage subjected to the flow. The modeling approach is based on the joint use of the finite element method using the shell elements to simulate the floating pipes and the hydrodynamic force model improved from the Morison’s equation and lumped-mass method. The hydrodynamic response of the fish cage and forces on the floating pipes can be obtained by the Morison’s equation and lumped-mass method. The stress and deformation of the floating pipes can be evaluated using the finite element method. Using an appropriate iterative scheme, the stress distribution and maximum stress of the floating pipes can be obtained using the proposed model. To validate the numerical model, the numerical results were compared with the data obtained from corresponding physical model tests. The comparisons show the numerical results agree well with the experimental data. On that basis, the simulations of floating pipes in currents are performed to investigate the maximum stress and the critical locations. Simulations of the fish cage in different flow velocity are performed. The effect of the velocity on the deformations and stress of the floating pipes is analyzed. The simulations results show that the stress and deformations drastically increases with the increase of flow velocity. Comparing results of floating pipes with different mooring line arrangements indicates that increasing mooring lines can efficiently lower the stress of the floating pipes. The simulation of the SPM cage system with multiple net cages in current is preformed and the results show the middle cage is most dangerous for the tripartite-cage system

Numerical Simulation of Freak Waves in Random Sea State

AbstractThe generation of freak waves in a 2-dimensional random sea state characterized by the JONSWAP spectrum are simulated employing a nonlinear fourth-order Schrödinger equation. The evolution of the freak waves in deep water are analyzed. We investigate the effect of initial wave parameters on kurtosis and occurrence of freak waves. The results show that Benjamin-Feir index (BFI) is an important parameter to identify the presence of instability. The kurtosis presents a similar spatial evolution trend with the occurrence probability of freak waves. Freak waves in a random sea state are more likely to occur for narrow spectrum and small values of significant wave height

On hydrodynamic characteristics of transient harbor resonance excited by double solitary waves

The harbor resonance triggered by double solitary waves (DSWs) with different wave parameters (including various wave heights and relative separation distances) is simulated based on the fully nonlinear Boussinesq model, FUNWAVE-TVD. A long and narrow harbor with different topographies is adopted. In the current study, effects of incident wave height, relative separation distance and bottom profile on hydrodynamic characteristics related to the transient oscillations are mainly investigated. The hydrodynamic characteristics considered include the evolution of the maximum free-surface elevation, the maximum runup, the wave energy distribution and the total wave energy inside the harbor. Results show that Green's law can accurately estimate the evolution of the maximum free-surface elevation in most part of the harbor area. The impacts of the topography on the maximum runup exhibit a strong dependence on the incident wave height. The smaller mean water depth inside the harbor, the larger relative separation distance, and the higher incident wave height tend to result in greater uniformity of the wave energy distribution. The normalized total wave energy is always shown to decrease gradually with the incident wave height, and to increase remarkably at first and then decrease slightly with the increase of the mean water depth.<br/

Topographic influences on transient harbor oscillations excited by N-waves

The main objective of this paper is to comprehensively study influences of the variation of the bottom profile inside the harbor on the transient harbor oscillations excited by normally-incident N-waves. The specific physical phenomena investigated consist of wave profile evolution, maximum runup, relative wave energy distribution and total wave energy inside the harbor. A series of numerical experiments are implemented using a fully nonlinear Boussinesq model, FUNWAVE-TVD. Results show that when the harbor is subjected to the leading-elevation N-waves (LEN waves), the evolution of the maximum free surface elevation during the wave shoaling process inside the harbor coincides well with Green's law overall. When the incident wave amplitude is small, the maximum runup inside the harbor is almost only determined by the incident wave amplitude. As the incident wave amplitude increases, effects of the bottom profile on the maximum runup closely depend on both the incident wave type and amplitude. As the mean water depth inside the harbor decreases, the relative wave energy distribution tends to become more uniform, regardless of the incident wave amplitude and type. Finally, the variation trend of the total wave energy with the bottom profile is found to depend on the incident wave amplitude

Investigation on the effects of Bragg reflection on harbor oscillations

Periodic undulating topographies (such as sandwaves and sandbars) are very common in coastal and estuarine areas. Normally incident water surface waves propagating from open sea to coastal areas may interact strongly with such topographies. The wave reflection by the periodic undulating topography can be significantly amplified when the surface wavelength is approximately twice the wavelength of the bottom undulations, which is often called as Bragg resonant reflection. Although the investigations on the hydrodynamic characteristics related to Bragg reflection of a region of undulating topography have been widely implemented, the effects of Bragg reflection on harbors have not yet been studied. Bragg resonant reflection can effectively reduce the incident waves. Meanwhile, however, it can also significantly hinder the wave radiation from the harbor entrance to the open sea. Whether Bragg reflection can be utilized as a potential measure to alleviate harbor oscillations is unknown. In the present study, Bragg reflection and their interactions with the harbor are simulated using a fully nonlinear Boussinesq model, FUNWAVE 2.0. For the purpose, an elongated harbor with constant depth is considered, and a series of sinusoidal bars with various amplitudes and numbers are deployed outside the harbor. The incident waves considered in this paper include regular long waves and bichromatic short wave groups. It is revealed for the first time that for both kinds of incident waves, Bragg resonant reflection can significantly alleviate harbor resonance. The influences of the number and the amplitude of sinusoidal bars on the mitigation effect of harbor resonance and on the optimal wavelength of sinusoidal bars that can achieve the best mitigation effect are comprehensively investigated, and it is found that the former two factors have remarkable influences on the latter two parameters. The present research provides a new option for the mitigation of harbor oscillations via changing the bottom profile, which is feasible as long as the navigating depth is guaranteed.</p

Numerical investigation of harbor oscillations induced by focused transient wave groups

Focused wave groups are traveling waves characterized by extremely-large transient wave amplitudes and very short durations. These waves usually cause serious damage to marine/offshore structures and coastal infrastructures, and can even result in human casualties (Nikolkina and Didenkulova, 2011). The studies on natural disasters related to the focused wave groups near the coastal zone have been mostly confined to wave evolution over beaches, wave runup, overtopping, and their impact forces acting on the coastal infrastructures (e.g., the seawall and the circular cylinder); the influence of focused transient wave groups on harbors has not yet been studied. In this study, the generation and propagation of focused transient wave groups and their interactions with the harbor are simulated using a fully nonlinear Boussinesq model, FUNWAVE 2.0. To this end, four elongated harbors with constant depth and a series of focused wave groups with various focused wave amplitudes, spectral width parameters, and incident directions are considered. Based on the Morlet wavelet transform and discrete Fourier transform techniques, the capability of focused transient wave groups to trigger the harbor resonance phenomenon is revealed for the first time. Subsequently, the influences of spectral width parameter, incident wave direction, and resonant mode on different resonant wave parameters (including maximum runup and resonant intensity of various resonant modes inside a harbor) are comprehensively investigated, and it is found that these three factors have significant effects on resonant wave parameters.</p