Preliminary investigation on statistical properties of extreme waves under unsteady wind

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.