Laboratory observation of coherent structures beneath microscale and large-scale breaking waves under wind action

Abstract

This study presents the results of an experimental investigation of coherent structures generated beneath microscale and large-scale spilling breaking waves. By analyzing PIV velocity data of wind-generated waves that are acquired at three different wind speeds, the similarity and difference in spatio-temporal evolving characteristics of the coherent structures according to the occurrence of the two types of wave breaking are investigated. There have been some researches on the coherent structures that are generated by microscale breaker. However, little has been known about the coherent structures formed underneath large-scale breaking waves. In this study, it is shown that a single strong coherent structure, whose direction of rotation is the same as the wave orbital motion, is formed immediately underneath a large-scale breaking crest. This is different from the processes occurring underneath microscale breakers that produce a series of coherent structures whose direction of rotation is not fixed. However, the overall evolving characteristics of the spatio-temporal evolution of the coherent structures are qualitatively the same between the two wave breaking events. As one of either microscale or large-scale breaker advances across the PIV field of view, the generated coherent structures interact with pre-existing vortices and subsequently disappear or reform in the downwind side of the breaker.