TANGENTIAL STRESS BENEATH WIND-DRIVEN AIR-WATER INTERFACES

The detailed structure of the aqueous surface sublayer flow immediatel y adjacent to the wind-driven air-water interface is investigated in a laboratory wind-wave flume using particle image velocimetry (PIV) tec hniques. The goal is to investigate quantitatively the character of th e flow in this crucial, very thin region which is often disrupted by m icroscale breaking events. In this study, we also examine critically t he conclusions of Okuda, Kawai & Toba (1977), who argued that for very short, strongly forced wind-wave conditions, shear stress is the domi nant mechanism for transmitting the atmospheric wind stress into the w ater motion - waves and surface drift currents. In strong contrast, ot her authors have more recently observed very substantial normal stress contributions on the air side. The availability of PIV and associated image technology now permits a timely re-examination of the results o f Okuda et al., which have been influential in shaping present percept ions of the physics of this dynamically important region. The PIV tech nique used in the present study overcomes many of the inherent shortco mings of the hydrogen bubble measurements, and allows reliable determi nation of the fluid velocity and shear within 200 mu m of the instanta neous wind-driven air-water interface. The results obtained in this st udy are not in accord with the conclusions of Okuda et al, that the ta ngential stress component dominates the wind stress. It is found that prior to the formation of wind waves, the tangential stress contribute s the entire wind stress, as expected. With increasing distance downwi nd, the mean tangential stress level decreases marginally, but as the wave field develops, the total wind stress increases significantly. Th us, the wave form drag, represented by the difference between the tota l wind stress and the mean tangential stress, also increases systemati cally with wave development and provides the major proportion of the w ind stress once the waves have developed beyond their early growth sta ge. This scenario reconciles the question of relative importance of no rmal and tangential stresses at an air-water interface. Finally, consi deration is given to the extrapolation of these detailed laboratory re sults to the field, where the present findings suggest that the sea su rface is unlikely to become fully aerodynamically rough, at least for moderate to strong winds.