Air Flow Structure Over Short-gravity Breaking Water Waves |
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Authors: | Nicolas Reul Hubert Branger Jean-Paul Giovanangeli |
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Institution: | 1.Laboratoire d’Océanographie Spatiale,Institut Fran?ais de Recherche et d’Exploitation de la Mer (IFREMER),Plouzané,France;2.Laboratoire IOA,Institut de Recherche Sur les Phénomènes Hors Equilibre (IRPHE),Marseille,France |
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Abstract: | Despite its importance for momentum and mass transfer across the air–sea interface, the dynamics of airflow over breaking
waves is largely unknown. To fill this gap, velocity and vorticity distributions above short-gravity breaking waves have been
measured in a wind-wave tank. A Digital Particle Image velocimetry technique (DPIV) was developed to accomplish these measurements
above single breaking waves, propagating in mechanically-generated wave groups and forced by the wind. By varying the wind
speed and initial characteristics of the groups, the airflow structure was captured over waves at different stages of the
breaking process, and breaking with various intensities. The instantaneous airflow that separates from a sharp breaking crest
is very similar to that occurring over a backward facing step. The separation bubble is however strongly unsteady: the steeper
the wave crest and the larger the Reynolds number based on the crest-height, the higher the separated layer and the farther
downwind the reattachment point. Instantaneous flow topology displays specific features of three-dimensional separation patterns.
The tangential stress above the wave profile does not exhibit spikes at reattachment but grows progressively downwind from
zero at reattachment to a value at the next crest approximately that found at the upwind breaking crest. Static pressure measurements
revealed that large pressure falls are generated by vortices in the separated layer, as found in separated flows over solids.
This study may provide useful data for theoretical and numerical modelling of the flow and associated phenomena. |
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Keywords: | Breaking wind waves Separation of the air flow Sea drag |
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