An experiment on reconstruction and analyses of in-situ measured freak waves

Abstract

Freak waves are destructive to marine and offshore structures, but their generation mechanisms are still unclear. To explore the possible generation mechanism of freak waves, some in-situ measured waves are reconstructed in a wave flume at model scale. To generate high-quality target waves, a modified time reversal (TR) method including a correction step is proposed. Five scaled in-situ measured waves, containing three freak waves and two non-freak waves, are reconstructed successfully.As reconstructed waves provide good agreement with the scaled in-situ measured waves, their formation mechanisms are analyzed by applying various dispersion relations. The results show the key contributions of the dispersive focusing in the freak wave formation. Then, to explore the nonlinear interactions of the quadratic order of these reconstructed waves, the wavelet-based bicoherence and its total bicoherence are presented, and large surface elevations are found to be related to high total bicoherences, indicating inescapable effects of wave-wave interactions in the wave formation. Furthermore, a phase coincidence level F is proposed to evaluate the phase coincidence level of these reconstructed waves. The results show that a low F (high phase focus level) means a high wave height, while a high wave height does not always exhibit a low F.