A Free-Floating PIV System: Measurements of Small-Scale Turbulence under the Wind Wave Surface

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Abstract
An in situ free-floating underwater miniature particle image velocimetry (UWMPIV) system is developed and applied to measure the structure of turbulence in the aqueous side of the wind wave surface boundary layer. The UWMPIV system provides a direct way to measure the aqueous side turbulence dissipation rate and vortex structures immediately below the air–water interface, which are important parameters that determine the gas exchange rate across the air–water interface subjected to a low-to-moderate wind shear. The impact of platform motion on the measurement of small-scale turbulence is discussed and found to be insignificant. A series of field experiments under a near “zero-fetch” wind wave condition and one open water experiment under a low wind condition were conducted on Lake Michigan to demonstrate the capabilities of the free-floating particle image velocimetry (PIV) system. The dissipation rate estimated with a “direct method” and with a “spectra fitting” method are compared. Vertical profiles of the turbulence dissipation rate suggest a power-law dependency with depth below the water surface. Surface shear velocities estimated through the aqueous side Reynolds stress distribution agreed well with wind stresses estimated by the classic drag law for zero-fetch wind wave conditions, where the primary source of turbulence was wind shear. For the open water experiment under a very low wind condition, a high dissipation rate was observed near the water surface, suggesting a high turbulence production rate by surface waves, and the profile of dissipation rate showed a slower decay rate with depth in the presence of waves.

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