The Research On Simulation Of Multidirectional Irregular Wave Groups

Wave groupiness is an important characteristics of the ocean waves.It has obvious effects on the damage of the seawall,harbor resonance and the response of floating structures, etc. Hence, tests on the effects of wave groups on the structures have to be involved for the design of the ocean structures, which requires simulating wave groups. So far, the studies on the simulation of random wave groups are mostly confined to the unidirectional waves.However, the real ocean waves are multidirectional waves.So, the study on the simulation of multidirectional random wave groups has very important theoretical importance and application value for engineering.At the beginning of this paper, the main approaches to research wave groups and the parameters commonly used to describe the wave groupiness degree are introduced. Correlation of the main characterisitic parameters to describe wave group height and group length is analyzed based on the field measured wave data. At the same time, two new factors to describe wave groupiness based on the wave energy history deduced by wavelet transform are defined.Through numerically simulated wave and the field wave data, their effectiveness is confirmed by the comparation with the commonly used wave groupiness parameters. Therefore, the ability of the wavelet transform to analyze wave groupiness in time-frequency domain is demonstrated.Further, the effects of the wave record length on the stability of the wave groupiness factors are discussed in detail.The analyzed results indicate that the wave record length has great effects on the wave groupiness parameters,and it is suggested that, for the simulation and analysis of waves considering wave groupiness,the number of waves during the simulation time length should be at least400to500.An empirical wave envelope spectrum suitable for wind waves is obtained by fitting to the reasonablely normalized wave envelope spectra of the measured sea waves at the mouth of the Zhu Jiang River and in the Bohai Sea. In the proposed wave envelope spectrum,both the wave group height and group length are considered.In addition, more reasonable parameter selection principle is given according to the analysis results of the wave group chracterisitics of the field wave data. It can be determined provided that the wave parameters and wave groupiness factors are given. So, it is feasible for practical application.Then, based on the improved empirical wave envelope spectrum, a method to simulate unidirectional irregular wave groups is developed.Simulation examples demonstrate that, irregular wave groups which includes not only its group height but also its length can be conveniently simulated by the empirical wave envelope spectrum,provided that the wave parameters and the parameters for the description of the wave group height and group length are given. Then, the calculation method of the wave maker signals for generating irregular wave groups at the specified position in the wave flume is proposed.Random waves containing expected wave groupiness and wave spectrum are satisfactorily simulated in laboratory wave flume. In addition, the variations of wave groupiness for the simulated waves with different wave groupiness during propagation in the wave flume are investigated.The analyzed results demonstrate that wave groupiness tends to be an limted range along with wave propogation due to the modulation of harmonic waves.It means that, for physical model tests considering wave groupiness, the waves with desired wave groupiness should be generated at the specified position in the wave flumeFurther, based on the study of the simulation of unidirectional wave groups,the method to simulate multidirectional wave groups is developed by introducing directional spreading function. The calculation method for the incident boundary condition of the numerical wave basin used to generate multidirectional waves containing expected wave groupiness is proposed. Using the numerical model, the multidirectional wave propogation is numerically simulated and analyzed. Multidirectional wave groups can be generated at the specified position in the numerical wave basin. Then, the calculation method of the wave maker driving signals for generating multidirectional wave groups in physical wave basin is proposed.The physical simulation of multidirectional wave groups is performed in physical wave basin. The multidirectional waves with expected wave groupiness and directional spreading can be satisfactorily generated at the specified position in the physical wave basin. The variations of wave groupiness for the experimentally simulated waves during propogation in the physical wave basin are also analyzed.Last, the effects of wave groupiness on the characteristic wave heights and wave height distribution are analyzed through numerically simulated multidirectional wave groups and the field wave data. The results show that the characteristic wave heights increase with the wave group height, moreover, the maximum wave height is effected mostly by the wave group height. But, they have little corelation with the wave group length.The relation between the parameters α,β of the Welbull distribution and wave group height is obtained.