Experimental Study On The Characteristics Of Freak Waves Under Adverse Currents Influence

As one of the most important environmental dynamic factors in the ocean,wave has farreaching influence on the safety of offshore structures,ships and ports.Freak wave is a kind of disastrous ocean wave which has attracted more and more attention.It often has the characteristics of sudden high wave height,big wave steepness and asymmetry of wave surface,which poses a great threat to human activities at sea.The study of mechanism and occurrence probability to freak waves has obvious theoretical and engineering significance.Focused on the wave-current interaction and modulation instability,this paper will design physical experiment to analyze the characteristics of freak waves generated in unidirectional propagation random waves under adverse currents influence.This paper first briefly describes the research status of freak waves and data analysis methods.Secondly,experimental designs are introduced for the generation mechanism of freak waves.The selection of random wave parameters takes into account different JONSWAP spectral parameters,wave steepness,and BFI values(BFI is a parameter related to wave steepness and relative spectral width)and countercurrent flow rate.The analysis of the experimental data mainly included the morphological characteristics of freak waves,the probability of appearance and the influencing factors of freak waves,and the influence of water flow on them.The results show that:(1)According to the classification of zero up-crossing method and zero down-crossing method,high peaks and deep valleys,the proportions of the four waveform classifications were counted.More than 85% of the freak waves have the characteristics of high peaks.The number of freak waves obtained by the zero up-crossing method or the zero downcrossing method is not much different;however,the existence of the countercurrents will lead to a lower probability of occurrence of deep troughs.Wave height and period dimensionless parameters and the averaged subsequent dimensionless curves show that the use of the zero upcrossing method can meet the definition of the freak wave,so using the zero up-crossing method can explain the scientific nature of the freak wave.The wave heights of the adjacent waves before and after are very different and the regularity is not strong;However,the existence of the countercurrent does not change the waveform of the dimensionless parameter.(2)The evolution of the freak wave in the random wave is an energy self-focusing process.Linearity,or modulation instability,has a significant effect on the generation of freak waves.The existence of flow makes the non-linear focusing more sudden and violent;(3)With the increase of wave steepness or BFI value,the number of freak waves has an increasing trend;countercurrent will increase the probability of appearance of freak waves.The larger the number of freak waves,the greater the increase in the number of freak waves;with the increase in steep wave values,BFI,or the presence of countercurrents,most of the freak waves generated are located farther away from the wave plate;(4)Along with the evolution of waves,the kurtosis value of the wave train shows an increasing trend,while the skewness value is relatively stable at the first 12-wavelengths,and the fluctuation after 12-wavelength increases significantly.The existence of countercurrent will increase the kurtosis and skewness values.And the greater the counter-flow velocity,the greater the degree of the increase;(5)The spectral response is the change of energy.By comparing the energy change in the wave propagation process,it is found that the peak value of the spectrum will decrease with the propagation of the wave train,and the spectrum width will be relatively narrow,this is due to the energy dissipation caused by friction;the existence of the reverse flow makes the peak of the spectrum diagram at the measurement point along the flow path significantly increase with respect to the no-flow conditions.and the greater the flow rate,the more the peak value increases,which reflects the input energy of the flow;(6)The increase in wave steepness or BFI value,and the presence of countercurrents will increase the probability of occurrence of freak waves.The Rayleigh distribution or Rayleigh-I distribution can describe the random waves that are not sufficiently growing in the nonlinearity.However,as the distance from the wave plate increases,the distribution curve gradually becomes higher than the theoretical curve.The existing several theoretical distributions cannot describe the probability of occurrence of a freak wave under conditions of strong non-linearity and countercurrent conditions.