Study On Generation、 Evolution And Internal Structures Of Freak Waves

Freak wave is a type of large and short-lived water wave. Such wave has high energy density, which can bring serious damages to vessels, maritime structures and other facilities in the ocean. So far, its physical mechanics and probability of occurrence are still unclear. The further study on freak waves cannot be carried out smoothly due to the insufficient in-situ data, devastating force and unexpected occurrence. This work aims to conduct a systematic investigation on the generation, evolution, internal structure and the relationship between internal and external features. The thesis work can be described as the following6major parts.Both numerical and experimental simulations of freak waves are realized. The numerical model is built by solving for the variables from a finite-difference approximation of the Reynolds time-averaged N-S equations, the k-ε model is for the turbulence closure, and the volume of fluid (VOF) method is used for free surface tracking. The simulated results of the fission phenomenon of a solitary wave over a change of water depth through a bottom slope, a conventional random wave train, the wave profile and velocity of a freak wave show good agreement with the experimental results, which indicates the model’s capability to resolve the wave profiles and velocities of freak waves. The numerical and experimental simulations are effective tools for the study of the generation, evolution and internal structure of freak waves.The present numerical model is implemented to reproduce3in-situ random wave trains containing a freak wave (or a deep trough), and to simulate5non-designed freak waves. Analysis of the simulated results attests that the generation and evolution of freak waves can be summarized as:large wave sequence�'deep trough�'quasi-freak wave�'freak wave�'quasi-freak wave�'deep trough�'large wave sequence. During that process, the maximum wave is not always in the form of free surface propagation, there are transformations between the wave with high crest and deep trough, between the deep trough and freak wave. The difference between propagation speeds of energy and free surface results in sudden change of the maximum wave. It can be seen that freak waves do not come alone, always accompanied by other abnormal events such as successive large waves and deep troughs.On the basis of the calculation results of the speeds of284experimental freak waves and64supplementary numerical freak waves, a semi-theoretical and semi-empirical formula is proposed to predict the freak wave speed by using regression analysis method. Through analyzing the difference between the nonlinearity effects on the speeds of freak waves and3rd-order Stokes waves, it is found that the nonlinearity effect on the freak wave speed is much greater than on the3rd-order Stokes wave speed, and that the increase of the modified wave steepness enhances the difference between the nonlinearity effects on the speeds of freak waves and3rd-order Stokes waves.Quantitative analysis of the time-frequency energy spectrum characteristics (transient energy density and the ranges of focused energy distribution in both time and frequency domains) of the abnormal waves occurring during the generation and evolution of freak waves (wave with high crest, deep trough, quasi-freak wave and freak wave) is performed by wavelet analysis method. Comparison of the time-frequency energy structure characteristics of abnormal waves and the maximum wave in a conventional random wave train (a random wave train contains no abnormal wave) attests that the energy densities of abnormal waves are much higher than the maximum wave in a conventional random wave train. It is suggested that the abnormal wave with energy parameter αE≥6is considered as the "general freak waves". The energy parameter αE is presented to describe the transient energy densities of waves, which is correlated linearly with the external characteristic parameter α1.The maximal velocity and acceleration of general freak waves appear close to the free surface. The velocity and acceleration fields of the general freak waves exhibit strong front/back asymmetry, which is different from regular wave. Although the general freak wave has a height to significant wave height ratio more than2, the kinematic and dynamic wave-breaking criteria are not met. Comparison of the horizontal velocity profiles below wave crest of general freak wave and5th-order Stokes wave (identical crest height and period) attests that the horizontal velocity of general freak wave is larger than5th-order Stokes wave close to the free surface and smaller below the still water level, and that the vertical variation of the horizontal velocity of general freak wave is more obvious than5th-order Stokes wave. Present numerical results show that the internal structure parameters of general freak waves have relation with the external characteristic parameters ηc/Lp, α1and α4. Further studies are needed to thoroughly analyze the relationship between the internal and external characteristic parameters.As the characteristic altitudes of the uneven topographies (slope topography and curved topography) increase, the external characteristic parameters show no obvious trend except a valley value for α4. For internal structure, the increase of the characteristic altitudes of the uneven topographies results in the decrease of the range of the focused energy distribution in time domain, the increase of the high-frequency energy, no obvious trend for αE and the range of the focused energy distribution in frequency domain.