RESEARCH ARTICLE Turbulent structure of air flow over wind-induced gravity waves

Abstract This is the second paper in a group of three that reports the systematic measurements of wind-generated water waves in a wind tunnel experiment. Here, the structure of the boundary layer on the air side of the water– air interface was analysed and compared with the boundary layer over a smooth plane rigid wall. The contribution of the wave-induced Reynolds stress was detected through filtering the spectrum of velocity fluctuations. Wave-induced Reynolds stresses became negligible for z [ 5 Hrms. The intermittency factor in the boundary layer over water waves was similar to that in a boundary layer over a rigid plane wall, with several differences near the interface. Here, the presence/absence of water damps out the turbu-lence. The quadrant analyses revealed that ejection and sweep events were dominant and more concentrated. At small fetches, the large-amplitude negative streamwise perturbations were preferentially lifted. Turbulence energy production peaked at z/d = 0.2 and had a distribution similar to that observed for a self-preserving boundary layer with a strong adverse gradient pressure. The quadrant analysis contribution to the energy production revealed that ejections still dominated the balance and that the produc-tion was spatially modulated in the wind direction with a couple of cells and with a minimum in the area of the free surface wave height reduction. Abbreviations... Time average operator f... Oscillating term operator c... Phasic average operator U Volume fraction or concentration for water d Boundary layer thickness c Intermittency factor q Mass density h Angle between wind and wave propagation direction j Turbulent kinetic energy m Kinematic fluid viscosit