Abstract
In order to solve the weak nonlinear problem in the simulation of strong nonlinear freak waves, an improved phase modulation method is proposed based on the Longuet-Higgins model and the comparative experiments of wave spectrum in this paper. Experiments show that this method can simulate the freak waves at fixed time and fixed space coordinates. In addition, by comparing the target wave spectrum and the freak wave measured in Tokai of Japan from the perspective of B-F instability and spectral peakedness, it is proved that the waveform of the simulated freak waves can not only maintain the spectral structure of the target ocean wave spectrum, but also accord with the statistical characteristics of the wave sequences. Then, based on the Kirchhoff approximation method and the modified Two-Scale Method, the electromagnetic scattering model of the simulated freak waves is established, and the normalized radar cross section (NRCS) of the freak waves and their background sea surfaces is analyzed. The calculation results show that the NRCS of the freak waves is usually smaller than their large-scale background sea surfaces. It can be concluded that when the neighborhood NRCS difference is less than or equal to −12 dB, we can determine where the freak waves are.
Highlights
Freak waves are high and steep waves in the ocean. e duration of freak waves is very short, but the contingency and great destructiveness are extremely threatening to shipping and marine engineering structures. erefore, the study of freak waves has attracted more and more attention [1]. e occurrence mechanism and engineering prediction of freak waves have become a hot research topic in the field of physical oceanography and ship hydrodynamics [2]
Combined with the comparative data of the freak wave measured in Tokai of Japan (144∘− 145∘E, 35∘− 36∘N04: 0023JUNE(UTC)), the effectiveness of the numerical simulation model and scattering calculation model proposed in the work is analyzed
Compared with the smooth transition coefficient of the target spectral scattering model, the freak wave simulation method based on LonguetHiggins model is more in line with the variation law of sea surface, which proves the effectiveness of the proposed method
Summary
Freak waves are high and steep waves in the ocean. e duration of freak waves is very short, but the contingency and great destructiveness are extremely threatening to shipping and marine engineering structures. erefore, the study of freak waves has attracted more and more attention [1]. e occurrence mechanism and engineering prediction of freak waves have become a hot research topic in the field of physical oceanography and ship hydrodynamics [2]. E method based on the Longuet-Higgins model is effective in simulating the freak waves in the laboratory, which is simple and practical [16]. In the early practical application, based on the numerical simulation of one- and two-dimensional space freak waves, we have calculated and analyzed the electromagnetic scattering coefficient of freak waves to study the formation mechanism, remote sensing recognition, and other related characteristics [21]. Based on the Two-Scale Method (TSM) and the Harger distribution surface, Franceschetti proposed two kinds of sea surface SAR simulator models [22]; this research method has great limitations because of its failure to fully consider the strong non-Gaussian statistical characteristics and velocity bunching effect of the height distribution of freak waves [23, 24]. Based on the normalization method of Z-score, the influence of the deviation coefficient a1∼a4 on the height of the freak waves is measured in this work
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