Publicações

Cherneva, Z. e Guedes Soares, C. (2001), “Local Non-Stationary Properties of the Wind Wave Groups”, Proceedings of the RINA Conference on Design and Operation for Abnormal Conditions II, 6-7 Novembro, London – United Kingdom, pp. 137-145

The studies of wind wave groups may be categorised into two approaches: one considers the phenomenon as a level crossing problem associated with the envelope of a random process; the other treats a sequence of large wave heights as a Markov chain problem. The basic idea for wind wave spectrum enhances application of the both basic descriptions for evaluating group wave characteristics to a great extend. Although the spectrum does contain information about the spectral components in a time series, for a large class of practical applications, this information is inadequate. Wind wave groups are a particular example. Fourier transformed spectral information for them in frequency components and their phases. This is due to assumption that the wind wave is a stationary process. It implies that wind wave spectral characteristics are constant during a given time period. This is a mathematical idealisation that is never realised. Analysis of experimental sea data shows that the local properties of wave group, in particular their frequency and phase, have non-stationary properties. The better understanding of this process may contribute to the understanding of the mechanisms that lead a wave group to be transformed into a freak wave. The study presents an analysis of ocean data in which the phases of waves in a time series are studied and it is shown that the phase changes are associated with the occurrence of wave groups and that by studying the time variation of the phase of waves one is able to detect different kind of wave groups. The paper discuss some of the features of the local wave phase and suggests some guidelines to use that information to identify severe wave groups that can be critical for ship design. This work has been developed in the scope of the MAXWAVE project, which is partially financed by the European Commission.

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