The use of PIV to analyse post breaking flow fields is presented A method to resolve the problems of directional ambiguity and very small or zero velocities is discussed An instantaneous record of a post breaking vortex has been obtained From this record a vorticity map has been calculated along with the map of the streamlines in the flow The calculation of energy in the flow field is discussed

INTRODUCTION

Knowledge of the post breaking process is limited, however it is clear that large amounts of energy are dissipated when a wave breaks Instrumental in this process of dissipation are vortices, visible in the water after the passage of a breaking wave An understanding of the movement and spread of these vortices will help us to determine more accurately how the wave energy IS dissipated Similarly such an understanding will help us to predict the spread and mixing of pollutants released into the sea and assist with the problems of containment It is also possible that energy, in the form of a vortex may be swept up into an oncoming wave following a breaking wave and so may contribute to a changed set of conditions for that wave perhaps forcing it to break prematurely and thus altering the forces that it may excert [I]

As Basco [l] pointed out only a few papers have been written on this subject More often than not papers jumped straight to the turbulence following breaking without recognising the existence of an organised intermediatory state - that of the vortex Sawaragi and Iwata [14] noticed In their experiments that a vortex motion descended towards the bed The early work carried out by Miller [9] in 1976 showed that a vortex was formed by the breaking wave which could be seen if the entrained bubbles were observed Further vortices were also formed, up to five in total, by the splash-up from the plunging jet entering the water Miller traced the motion of the vortices and noted that they traveled slower than the wave velocity and drifted downwards whilst expanding He also observed that there was a graduation in vortex magnitude from spilling through to steeply plunging breakers

Sawaragi and Iwata [14] in 1974, estimated that the energy in the vortex motion contained between 15% and 30% of the energy dissipated in breaking In 1982 Nadaoka and Kondoh [11] also observed, from the entrainment of air bubbles, that there were definite ‘large scale eddies’ associated with breaking waves In 1985 Hattori and Aono [5] noted that long lived vortex-like motion is formed under both spilling and plunging breakers and that the strength of this motion appears to be related to the breaker type Also the size of the vortex motion was related to the breaking height They confirmed these observations using a flow visualisation technique

Papers by Okayasu, Shibayama and Mimura [12], and Mizuguchi [10] are less qualitative and studied the plunging vortex in the surf zone using two-component LDA and hot film velocimetry

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