Measurements of Airside Shear- and Wave-Induced Viscous Stresses over Strongly Forced Wind Waves

Abstract

Detailed knowledge of the airflow over the surface of the ocean is paramount to evaluate and predict air-sea fluxes. The flux of momentum is of particular interest because it involves phenomena over a large spectrum of length and temporal scales from aerodynamic drag in large storm systems, down to the wind-wave generation problem at sub-centimeter scales. At the smaller scales, while there is a body of theoretical and experimental work which suggests that the wind-wave generation process is linked to the instability of the coupled air-water surface flow, progress has been hindered by the difficulties associated with making reliable measurements or simulations near the air-water interface at scales at which viscosity plays a role. In this paper, we present recent measurements of the two-dimensional velocity field in the turbulent airflow above wind waves. Improvements in measuring techniques have allowed us to detect the viscous sublayer in the airflow near the interface and make direct measurements of the airside viscous tangential stress (analogous to those made by (Banner ML, Peirson WL, J Fluid Mech 364:115–145, 1998) on the water side). Furthermore, we were able to separate mean, turbulent, and wave-coherent motions, and this decomposition yielded wave-coherent flow measurements as well as wave-phase averages of several flow field variables. We present the relationship of the varying surface viscous stress with the dominant wave phase. Also, to the authors’ knowledge, we present the first measurements of airside wave-induced viscous stresses. We conclude that at low wind speed, surface viscous effects are substantial and likely need to be accounted for in the early stages of the wind-wave generation process.