Simultaneous particle image velocimetry and infrared imagery of microscalebreaking waves

We report the results from a laboratory investigation in which microscale b reaking waves were detected using an infrared (IR) imager and two-dimension al (2-D) velocity fields were simultaneously measured using particle image velocimetry (PIV). In addition, the local heat transfer velocity was measur ed using the controlled flux technique. To the best of our knowledge these are the first measurements of the instantaneous 2-D velocity fields generat ed beneath microscale breaking waves. Careful measurements of the water sur face profile enabled us to make accurate estimates of the near-surface velo cities using PIV. Previous experiments have shown that behind the leading e dge of a microscale breaker the cool skin layer is disrupted creating a the rmal signature in the IR image [Jessup , J. Geophys. Res. 102, 23145 (1997) ]. The simultaneously sampled IR images and PIV data enabled us to show tha t these disruptions or wakes are typically produced by a series of vortices that form behind the leading edge of the breaker. When the vortices are fi rst formed they are very strong and coherent but as time passes, and they m ove from the crest region to the back face of the wave, they become weaker and less coherent. The near-surface vorticity was correlated with both the fractional area coverage of microscale breaking waves and the local heat tr ansfer velocity. The strong correlations provide convincing evidence that t he wakes produced by microscale breaking waves are regions of high near-sur face vorticity that are in turn responsible for enhancing air-water heat tr ansfer rates. (C) 2001 American Institute of Physics.