ABSTRACT

The present paper presents the result of a freak wave and its impact by wavelet transform. Wavelet transform is used as a tool for analyzing signals in the time as well as in the frequency domain. The scheme was applied to the laboratory generated freak waves. The wavelet transform of the time history of the freak wave and impact force revealed that wide range of frequency components were contained in them. The coherence analysis was done on the wave and its impact force time histories. The coherence analysis reveals that some high frequency components are highly correlated with impact forces. The present study demonstrates that the wavelet transform can be an alternate tool in the analysis of strongly nonlinear freak wave and its impact.

INTRODUCTION

Freak wave is characterized by its nonlinear, steep nature of its shape. It is believed to cause severe damage to the offshore structures and ships. Haver et al. (Haver and Andersen, 2000) mentioned that it should be modeled as a transient phenomenon. Stansberg(2000) modeled a freak wave based on the wave evolution and nonlinear focusing of an energetic random wave group propagating along the wave tank, at a large distance from the wave flap. The Draupner freak wave was studied by one of the present authors (Kim and Kim, 2003a; Kim and Kim, 2003b). The experiment was done in the wave flume. They measured particle horizontal velocity field and wave impact forces, placing the cylinder against the freak wave to obtain the maximum horizontal force. In addition, high frequency components contained in the impact force and freak wave have been examined by continuous wavelet transform (CWT). It was concluded in the study that the large high frequency components make the freak wave and its impact force strongly nonlinear.

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