Phase-Averaged Flow Properties Beneath Microscale Breaking Waves |
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| Authors: | Kamran Siddiqui Mark R Loewen |
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| Institution: | (1) Department of Mechanical and Industrial Engineering, Concordia University, H3G 1M8 Montreal, QC, Canada; |
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| Abstract: | The phase-averaged characteristics of the turbulent velocity fields beneath steep short wind waves are investigated. A scheme
was developed to compute the phase of individual wind waves using spatial surface displacement data. This information was
used to analyze the two-dimensional velocity data acquired using particle image velocimetry (PIV) in a wind-wave tank. The
experiments were conducted at a fetch of 5.5m and at wind speeds that ranged from 4 to 10ms−1. Under these conditions previous studies have shown that a significant percentage of the waves are microscale breaking waves.
An analysis of the phase-averaged results suggests under these conditions (short fetches and moderate wind speeds) a wind-driven
water surface can be divided into three regions based on the intensity of the turbulence. These are the crests of microscale
breaking waves, the crests of non-breaking waves and the troughs of all waves. The turbulence is most intense beneath the
crests of microscale breaking waves. In the crest region of microscale breaking waves coherent structures were observed that
were stronger and occurred more frequently than beneath the crests of non-breaking waves. Beneath the crests of non-breaking
waves the turbulence is a factor of two to three times weaker and beneath the wave troughs it is a factor of six weaker. These
findings provide additional support for the hypothesis that approximately two-thirds of the gas and heat fluxes occur across
the turbulent wakes produced by microscale breaking waves. |
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