Yousefi, K., Veron, F., & Buckley, M.P. (2020): Momentum flux measurements in the airflow over wind-generated surface waves. Journal of Fluid Mechanics 895, doi:10.1017/jfm.2020.276
The air–sea momentum exchanges in the presence of surface waves play an integral role in coupling the atmosphere and the ocean. In the current study, we present a detailed laboratory investigation of the momentum fluxes over wind-generated waves. Experiments were performed in the large wind-wave facility at the Air–Sea Interaction Laboratory of the University of Delaware. Airflow velocity measurements were acquired above wind waves using a combination of particle image velocimetry and laser-induced fluorescence techniques. The momentum budget is examined using a wave-following orthogonal curvilinear coordinate system. In the wave boundary layer, the phase-averaged turbulent stress is intense (weak) and positive downwind (upwind) of the crests. The wave-induced stress is also positive on the windward and leeward sides of wave crests but with asymmetric intensities. These regions of positive wave stress are intertwined with regions of negative wave stress just above wave crests and downwind of wave troughs. Likewise, at the interface, the viscous stress exhibits along-wave phase-locked variations with maxima upwind of the wave crests. As a general trend, the mean profiles of the wave-induced stress decrease to a negative minimum from a near-zero value far from the surface and then increase rapidly to a positive value near the interface where the turbulent stress is reduced. Far away from the surface, however, the turbulent stress is nearly equal to the total stress. Very close to the surface, in the viscous sublayer, the wave and turbulent stresses vanish, and therefore the stress is supported by the viscosity.
Yousefi, K., Veron, F., & Buckley, M.P. (2020): Measurements of Airside Shear- and Wave-Induced Viscous Stresses over Strongly Forced Wind Waves. In: Vlahos, P., & Monahan, E. (eds): Recent Advances in the Study of Oceanic Whitecaps. Springer, Cham, doi:10.1007/978-3-030-36371-0_6
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.