Effects of Swell on Turbulence Structure and Mass Transfer Across the Wind-Driven Air-Water Interface

Abstract

The effects of swell on both turbulence structure and mass transfer across the wind-driven air-water interface were investigated through laboratory experiments in a wind-wave tank. The swell was mechanically generated and propagated in the same wind direction. The CO2 transfer coefficient on the water side was measured through reaeration experiments of CO2 and fluid velocities in both air and water sides were measured using a laser Doppler velocimeter (LDV) and a particle image velocimeter (PIV). The results show that the mass transfer across the air-water interface is damped by the swell. The swell effect is significant in the wind-speed region less than 9 m/s. The damping effect is due to the fact that the shear stress at the water side is reduced by swell, and consequently the frequency of the appearance of surface renewal eddies that control mass transfer across the air-water interface is reduced. However, in the wind-speed region higher than 9 m/s the damping effect disappears owing to wind waves with strong energy and wave breaking.