DNS and Measurements of Scalar Transfer Across an Air-water Interface During Inception and Growth of Langmuir Circulation 

Amine Hafsi, University of South Florida, Civil & Environmental Engineering, Tampa, FL, United States, Andres E Tejada-Martinez, University of South Florida, Civil and Environmental Engineering, Tampa, FL, United States, Fabrice Veron, University of Delaware, Newark, DE, United States and Marc P Buckley, Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Geesthacht, Germany
Abstract:
Direct numerical simulations (DNS) of an initially quiescent coupled air-water interface driven by an air flow with free stream speed of 5 m/s have been conducted and scalar transfer from the air side to the water side and subsequent vertical transport in the water column have been analysed. Two simulations are compared: one with a freely deforming interface (giving rise to gravity-capillary waves and aqueous Langmuir turbulence (LT) characterized by small-scale (centimeter-scale) Langmuir cells (LC)) and the other with the interface intentionally held flat (i.e. without LC). It is concluded that LT serves to enhance vertical transport of the scalar in the water side and in the process increases scalar transfer efficiency from the air side to the water side relative to the shear-dominated turbulence in the flat interface case. Furthermore, transition to LT was observed to be accompanied by a spike in scalar flux characterized by an order of magnitude increase. These episodic flux increases, if linked to gusts and overall unsteadiness in the wind field, are expected to be an important contributor in determining the long-term average of the air-sea gas fluxes.
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