Wave Groupiness Analysis Of The Process Of Freak Wave Generation

Freak wave is defined as a huge wave whose height is more than twice the significant wave height, and it has sharp peak, highly concentrated energy, and strong destructive force. It is an important cause of the wrecks of great ships and marine structures. Scholars have made some achievements by carrying out simulation study, but most are based on the focusing mechanism of waves with different initial phases, which does not accord with the actual conditions of the random waves on the sea. In addition, there exist relations between wave group and freak wave generation in random wave trains, but few studies on quantitative investigation of this are available. Therefore, base on Benjamin-Feir instability, this study respectively simulate the formations of freak waves in the unidirectional and multi-directional irregular wave trains. Then the evolutions and characteristics of wave groups are analyzed by wavelet transform, and the formation mechanism of freak wave is studied and discussed.In this dissertation, firstly freak wave and wave group are introduced briefly, and two new groupiness parameters:group height parameter GFW and group height parameter GLw, are defined based on wavelet transform. The experimental studies of unstable random waves in the unidirectional and multi-directional conditions are respectively carried out based on Benjamin-Feir instability. Then the evolutions of freak waves in the unidirectional and multi-directional irregular waves are analyzed and compared. And the effects of BFI (BFI=(?)2/(2△f/fp)), direction distribution and relative water depth on wave groupiness are also studied. The results of this dissertation are summarized as follows:1.The modulation and demodulation of wave groups play an important role of the generation and evolution of freak wave in unidirectional irregular waves, and the formation mechanism of freak wave is mainly self focusing and nonlinear caused by modulational instability.2. GFw reflects the instability of wave train very well, GFw is larger when wave train is more unstable and GFW increases more significantly with the propagation and evolution of waves.3. The generation mechanism of freak wave in multi-directional irregular waves is different from unidirectional, the instability weakens and the spatial aggregation of different directional waves plays a more important role.4. The direction distribution, BFI and relative water depth have a great impact on wave groupiness. With the wide distribution of direction, the instability of wave trains is restrained, so the variation of GFW is very slow and the impact of BFI is not obvious. With the directional distribution narrowing down, the variation of GFW along the wave flume becomes steeper and faster, meanwhile the impact of BFI becomes increasingly significant. In addition, the shallow relative water depth also can restrain the instability of waves, the variation of GFW becomes smaller and slower.5. The variation tendencies of GFW/GLW and kurtosis are basically the same, and both of them have a very good correlation.