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Publicações
Veltcheva, A. e Guedes Soares, C. (2016), “Nonlinearity of abnormal waves by the Hilbert-Huang Transform method”, Ocean Engineering, Vol. 115, pp. 30-38
The nonlinearity of full-scale abnormal wave data is studied by the Hilbert-Huang Transform method. The variation of local frequency within one single wave is denoted as intra-wave frequency modulation. The vertical asymmetry of abnormal waves is studied in relation to the level of intra-wave frequency modulation of different Intrinsic Mode Functions. The Draupner New Year wave data time series is used as an example of vertically asymmetric abnormal waves, while record 37 from hurricane Camille dataset is chosen as an example of vertically symmetric abnormal waves. A measure of intra-wave frequency modulation is proposed based on the instantaneous frequency variations. The study of variation of local frequency in the vicinity of abnormal waves demonstrates that larger intra-wave frequency modulation is associated with a higher asymmetry of the wave profile. The analysis of intra-wave frequency modulation of abnormal waves of larger data sets (six wave records from hurricane Camille and six wave records from the North Sea storm, 1997) confirms that the intra-wave frequency modulation increases with increasing the vertical asymmetry of abnormal wave profile. It is also found that the vertical asymmetry of the abnormal New Year wave is reflected in the spreading of energy in a wider frequency range in contrast to the Hilbert spectrum of the symmetrical abnormal wave of record 37 of Camille.
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