Abstract
From the experiment of Li et al. (2015) it was observed that the generation of freak waves in random wave trains may be attributed to the focusing of double wave groups with different peak frequencies. In order to investigate this generation process, a modified wave focusing experiment is carried out, in which the focused waves are generated by two wave groups with different peak frequency differences, assumed to focus at the same point and time. By analyzing the evolutions of the free surface elevation and wavelet spectra of the experimental data, it can be verified that the focusing of double wave groups can reproduce the generation process of the freak waves in random wave trains well. Phase lags of the double wave groups focusing relative to the linear superposition of the corresponding single wave group and changes in the amplitude spectra during the focusing process are obviously observed. The method for the symmetry-based separation of harmonics illustrates that the phase shifts are mainly caused by the third-order nonlinearity due to interactions between the two wave groups, rather than the even-order nonlinearity. Third-order nonlinearity makes the amplitude of waves small in the high-frequency region, resulting in a shift of the actual focusing location from the target location. Further investigations are conducted with the numerical simulation based on the High Order Spectral (HOS) method. The wavenumber-frequency spectra explain the evolution of the amplitude spectra and changes in the dispersive properties both in time and space, demonstrating that the third-order nonlinearity changes the dispersion relationship of the wave components more intuitively. The above phenomena become obvious for the cases with smaller peak frequency differences wave spectra. All these observations will lead to a better understanding of the mechanism of freak wave generation and lay an important foundation for the low-cost and large-scale development and utilization of ocean wave energy resources.
Highlights
Given the future of conventional energy sources, there is a great need to establish sustainable energy systems for substitution (Cruz, 2008)
This implies that the focusing of the double wave groups can reproduce the generation process of the freak waves in random wave trains well
By comparing the free surface elevations and the amplitude spectra of the double wave groups focusing with those of the linear superposition of the corresponding single wave group, phase lags and spectral changes in the high frequency part of the double wave groups can be obviously observed during the focusing process
Summary
Given the future of conventional energy sources, there is a great need to establish sustainable energy systems for substitution (Cruz, 2008). Ocean wave energy, as a large, widespread, and environmental renewable resource, is in the spotlight with large potential and more and more wave energy devices come into being (Tunde and Hua, 2018). The higher the Double Wave Groups Focusing wave height is, the greater the ocean wave energy that can be converted. As human activities (such as the exploration and development of marine resources) move toward the deep ocean and become frequent, the chance to encounter freak waves increases. How to avoid the damage caused by extreme waves is one issue that must be faced in the process of promoting the development of ocean wave energy. Study on the freak wave generation is of great practical significance, and it should attract more and more attention
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