Analysis Of The Generation Mechanism Of Freak Wave Based On Double Wave Groups Focusing

Freak wave is a single large wave contained in random wave train.It has obvious non-linear characteristics,short duration,concentrated energy and astonishing destructive power.At present,there are many hypotheses about the generation mechanism of freak wave,which can explain the generation of freak wave under specific conditions,but the generation mechanism of freak wave in irregular wave train has not been clearly and directly explained.In this paper,by analyzing the experimental data of a large number of freak waves in irregular wave train,it is concluded that the interaction between wave groups is an important mechanism for the generation of freak waves.Based on this mechanism,the experiment of freak wave formation by focusing on two-wave group is designed.And the evolution characteristics,spectrum characteristics and high-order interaction among wave group components in the process of freak wave generation are analyzed by means of wavelet transform,Fourier transform and component wave separation.On this basis,the characteristics of wave breaking are studied.Firstly,the formation process of freak wave in one-way irregular wave train is analyzed,and the generation mechanism of freak wave is observed and summarized.The results show that there may be different mechanisms for the formation of freak waves.In addition to the B-F instability mechanism,the two-wave group can also produce freak waves.Then,using the superposition of two single-focus wave groups,the process of freak waves generated by double-focus wave groups is simulated experimentally,and the interaction between wave groups is analyzed by the method of component wave separation.The experimental results show that the third-order non-linearity between two wave groups plays an important role in the formation of freak wave,which changes the dispersion relationship of waves and increases the focusing amplitude.And the smaller the peak frequency width of the wave group,the more obvious the third-order nonlinearity is,which indicates that the smaller the frequency width of the wave group,the easier the freak wave will be produced.Finally,based on the double-wave cluster focusing mechanism,the wave breaking experiments are carried out to preliminarily study the characteristics of the breaking waves under this mechanism.The results show that when the wave group evolves into wave breakage,the breakage is related to the width f between the center frequencies of the two wave groups.The greater the difference between the center frequencies,the breakage of different intensities will occur at most twice before and after the focusing position.The energy loss in the process of breaking mainly comes from the high frequency wave group.After breaking,the evolution energy of the wave group redistributes,and the spectrum peak energy of the low frequency wave group gradually transfers to the high frequency wave group,and the spectrum peak energy of the low frequency wave group decreases.The energy change between the two peaks is the most significant.The energy decreases when breaking,and the wave group recovers rapidly after breaking.The research results in this paper are helpful to further reveal the mechanism of freak wave generation and wave growth in irregular wave train.