The effect of surfactants on microscale wave breaking and the aqueous boundary layer.

摘要

This study reports on a laboratory investigation of the effect of surfactants on the properties of the aqueous boundary layer, the properties of microscale-breaking waves, the characteristics of coherent structures and the air-water gas transfer rate. Digital particle image velocimetry and surface wave profile measurements were gathered for clean and surfactant-influenced water surfaces. At all wind speeds (3.8 to 9.8 m·s-1) the flow in the aqueous boundary layer was found to be in the transition regime for both water surface conditions.; The fraction of the total vertical momentum transferred to the aqueous boundary layer decreased from 0.8 to 0.2 for clean water surfaces and from 0.9 to 0.3 for surfactant-influenced water surfaces as the wind speed increased from 3.8 to 9.8 m·s-1. A strong negative correlation was observed between the fraction of the total momentum transferred to the aqueous boundary layer and the mean square wave slope irrespective of water surface condition.; The ratio of the rate of dissipation of turbulent kinetic energy in surfactant-influenced water, to that in clean water decreased from 1.0 to 0.70 as the wind speed increased from 3.8 to 9.6 m·s-1. A surfactant reduced the thickness of the enhanced layer of near-surface turbulence on average by 35%. The surfactant reduced the percentage of wave breaking by approximately one third and reduced the wave amplitude and the maximum wave slope on the forward face of the waves by an average factor of 25% and 28%, respectively for breaking waves and 22% and 19%, respectively for non-breaking waves. A strong correlation was observed between the fraction of the total momentum transferred to the wave field and the percentage of breaking waves irrespective of the water surface cleanliness.; The frequency of occurrence of coherent structures increased by a factor of ∼10 for both clean and surfactant-influenced water surfaces as the wind speed increased from 3.8 to 9.6 m·s-1. The surfactant reduced the frequency of occurrence of coherent structures by approximately 20% and the measured bulk gas transfer velocities by approximately 55%.