Study On Focused Waves And Its Actions On A Vertical Wall

In recent years,extreme waves occur more and more frequently with global warming and sea level rising.Extreme waves usually have short duration,large wave heights and strong slamming loads,which are a serious threat to ocean building and the safety of seafarers’lives.Therefore,investigation on its generating mechanisms and evolution is becoming a frontier research direction of ocean engineering.However,no matter for the study of extreme waves or wave loading induced by it,the first thing that needs to be solved is how to generate extreme waves.Nowadays a method widely used by researchers is dispersion focusing method,due to its easy operation,clear principle,high efficiency to generate extreme waves,and reserving random features in wave groups.Based on the classification in Chaplin（Int.J.Offshore Polar Eng.,1996,6（02）:131-137）,the dispersion focusing method can be further divided into three kinds:phase velocity method,group velocity method,and time-reversal method.However,Many studies were focused on the phase velocity method.The latter two methods and other possible methods are not gotten much attention.In this thesis,based upon the phase velocity method,a time-domain Focused Wave model by gathering Steady-State components in transient waves（FWSS）is proposed by utilizing the transient fundamental solution and linear superposition principle.This model reveals each wave component evolution during focused wave propagation,and solves the asymmetry problem of focused wave profiles induced by the use of unreasonable focused time.Furthermore,a time-domain focused wave model with Ramp Functions（FWSS-RF）is obtained by considering ramp functions in the traditional phase velocity method.This model confirms theoretically that the focused wave profile will not have much changes when using a reasonable ramp function to smooth those small amplitude but high frequency wavemaker signals at the beginning of wave generation.Based on the evolution law of wave envelope crest for the transient fundamental solution,a new focused wave generation method,namely maximum wave velocity method,is proposed.And then a time-domain focused wave model（FWWF）is derived based on the maximum wave velocity method.Also this kind of focused waves is generated successfully in both a physical flume and a numerical wave flume.This solves the problem on gathering each maximum wave within transient waves for a given frequency range of focused waves.Compared with the traditional phase velocity method,the maximum wave velocity method has the advantages of less focused time,higher focused wave amplitude,and less wave reflections.A numerical wave flume equipping with the functions of wave generation and wave absorption,is established,based upon a commonly two-phase solver in the open-source code Basilisk.The accuracy of simulating strongly nonlinear focused waves with wavefronts using the numerical wave flume is verified by comparison with the theoretical solutions and experimental data.Based on the spatial-temporal information of wave elevations,wavenumber-frequency joint spectra for the evolution of focused wave with wavefronts are obtained with different wave nonlinearity.The phenomenon of energy increase within the frequency band f_1,2+（the frequency band between the input free-waves and the second-order sum components）is found.To quantization the amount of wave energy in f_1,2+,a new dimensionless variable（35）G is introduced,and a relationship between（35）G and BFI（Benjamin–Feir Index）is obtained.It is found that the value of（35）G increases with increasing of BFI values for nonbreaking cases,nevertheless the value of（35）G does not having much change with increasing of BFI values for breaking cases,and its values stabilize around 12.5%.The whole breaking evolution process is simulated accurately.The breaking onset criterion B_x proposed by Barthelemy et al.（J.Fluid Mech.,2018,841:463-488）,is verified,which can be used to distinguish nonbreaking cases and breaking cases in focused waves with wavefronts.Based on image methods,a time-domain theoretical model on focused waves with wavefronts acting on a vertical wall is proposed.This model can be used to obtain the evolution of transient wave elevations on focused waves with wavefronts acting on a vertical wall,and can give an accurate time-domain theoretical solution to verify the validity of numerical simulation results.By the two-phase flow simulation,it is found that wave runup in 64.3%cases can reach up to three times theoretical focused amplitude,and it thus needs to consider the influence of extreme wave runup in practical engineering.Moreover,by separating wave runup into two parts,namely linear wave runup（two times maximum wave elevation at that position without the vertical wall presence）and nonlinear wave runup,it is found that the maximum of the proportion of nonlinear wave runup can reach up to 27.42%for nonbreaking cases,and35.28%for breaking cases.The variations of dynamic water pressures and fluid velocity profiles at the boundary of the vertical wall are also investigated.