非定常风作用下极端波浪统计特性初探

Preliminary investigation on statistical properties of extreme waves under unsteady wind

为探究风对极端波浪产生的影响,基于波浪的Benjamin-Feir不稳定性,在二维海洋工程水池中开展实验,对非定常风作用下具有随机相位的波场进行了长时间的模拟.本文通过对波浪有效波高、峰度及偏度的统计,分析了风对波浪演化过程中统计特征的影响,并进一步研究了波浪能量谱对风的响应.实验结果表明,具有较大Benjamin-Feir指数的波浪,在非定常风作用下常伴有破碎现象,导致波浪有效波高显著下降,峰度则呈现增大趋势.通过对波浪能量谱的分析得到,波浪破碎导致能量谱峰值的下降,波浪从风中所获得的能量主要集中在波浪的高频部分.本研究旨在通过对极端波浪的统计学研究,为海上浮式结构物的设计、建造及极端波浪的预报提供依据,降低极端波浪所造成的危害.

Freak waves have the properties of strong nonlinearity and suddenness,which are often accompanied by extreme weather conditions.As an important environmental load,wind has a significant transient characteristic and plays an important role in the generation of freak waves.In order to understand the statistical properties of waves under the action of wind,especially the influence of wind on freak waves,laboratory experiments were carried out.In this paper,based on the Benjamin-Feir instability,long time series of wave field under unsteady wind are simulated by Joint North Sea Wave Project(JONSWAP)spectra with random phases in a two-dimensional wave tank.The freak waves generated in the experiment are similar in shape to those measured at sea,and are more than twice the significant wave height of the sea state.Through the statistical analysis of wave significant wave height,kurtosis and skewness,the influence of unsteady wind on the statistical properties of wave evolution is analyzed,and the response of wave energy spectrum is further investigated.The results of experiment show that the waves with larger Benjamin-Feir index are often broken under the unsteady wind,resulting in the loss of wave energy and the significant decrease of wave significant wave height.The skewness of waves depends on the steepness and decreases slightly during wave evolution.It is worth noting that the wave kurtosis increases under the unsteady wind.As an important parameter to measure Benjamin-Feir index,kurtosis will play an important role in the generation of freak waves.The increase of kurtosis will increase the probability of freak waves.Through the analysis of wave energy spectrum,it is found that there is energy dissipation in the process of wave evolution.The wave breaking under the unsteady wind leads to the decrease of the peak value of energy spectrum,and the energy obtained by the wave from the wind is mainly concentrated in the high frequency part of the wave.