Experimental And Numerical Study Of Freak Waves

Freak waves are both extremely large waves and highly transient in space and time.Such a wave may lead to damage of ships and offshore platforms and to deaths.The study on the freak waves is not developed extensively own to the rare data in situ or remote observations of the actual wave fields and unexpected occurrence of freak waves.The investigation on freak wave still lies in its early age so far,the physical mechanics,probability of occurrence and the effects factors are still unclear.Thus,it is necessary to investigate this problem thoroughly.This thesis will conduct the experimental and numerical study on the freak wave.The experimental study is conducted on the generation of freak,waves in a random sea. The major emphasis of the investigation is on the influence of wave breaking on the occurrence of freak waves and the statistics of random wave train.Effects of non-dimensinal water depth,wave steepness,frequency spectra enhance factor,peak period on wave statistics for finite-depth random waves and occurrence of freak waves are also presented.It is found that the occurrence probability of freak wave events in non-breaking waves is much larger than that in breaking waves and such occurrence in deep water is larger than that in shallow water.The results show that skewness of surface elevations is independence of wave breaking and kurtosis is suppressed by wave breaking.The results of experiments indicate that the maximum of abnormal index lies in moderate wave train and increased with increasing kurtosis.A conclusion is drawn that the freak waves can be generated from different physical processes that can be of linear or non-linear wave systems.A numerical wave model is developed for modeling nonlinear water waves based on the High-Order Spectral(HOS) method proposed by Dommermuth and Yue(1987) and West et al(1987).The numerical method is an efficient technique in water wave computation,which is based on Taylor expansion of the Dirichlet problem and FFT algorithm.Specific care has especially been paid to error considerations and numerical time integrator,using the fifth-order Adams-Bashforth- Moulton predictor-corrector adjustment time integrator instead of the fourth-order Runge-Kutta method.This allows the simulations to be faster convergence and owning higher computational efficiency.The validation and efficiency of the numerical scheme is illustrated by a number of wave and wave train configuration including evolution of fifth-order Stokes waves and the instability of Stokes wave with finite slope.The results are consistent with those obtained in other studies. An efficient model is proposed for generating freak waves based on the enhanced HOS method.Considering the wave dispersion and wave directionality,physical scaled freak waves are generated and the evolution of freak wave events in open seas is then realized. A second-order wave model is derived for the wave generation and propagation,based on the spectral expansion.A boundary-value problem,for the generation of water wave by a wavemaker,is simplified by decomposing the total velocity into two parts and two efficient solutions are achieved for the wavemaker problem and the free surface problem.The system, correct up to second order in wave steep,is then solved by applying spectral expansions based on Fast Fourier Transform.In order to validate our numerical scheme,laboratory experiments are carded out and a reasonable agreement is observed between experimental data and numerical results.The initialization of nonlinear free-surface simulations and wave propagation and transformation in a numerical wave flume are presented.The characters of the wave propagation are also investigated based on wavelet analysis.Due to the mismatch between the linear input wavemaker motion and the kinematics of fully nonlinear waves,direct numerical simulations of progressive waves,generated by a sinusoidally moving wavemaker,are prone to suffering from high-frequency instability unless the flow is given sufficient time to adjust. A time ramp is superimposed on the wavemaker motion at the start that allows nonlinear free-surface simulations to be initialized with linear input wavemaker.The duration of the ramp are adjusted to test its efficiency for generated waves of different steepness and different length.Numerical results show that the time ramp is efficient to stabilize the initial propagation of generated wave in a wave flume.Four wave focusing models for freak wave generation(1.extreme wave model +random wave model;2.extreme wave model +regular wave model;3.phase interval modulation wave focusing model;4.Number modulation wave focusing model with the same phase) are proposed.By using different energy distribution techniques in the four models,freak wave events are obtained with different H_max/H_s in finite space and time.