Data taken beneath wind‐ruffled, mechanically generated water waves in both the fixed‐probe and the wave‐following coordinate systems were analyzed. A phase‐averaging technique was used to define the component associated with the monochromatic, mechanically generated waves and to separate that component from the fluctuating one which is associated with ripples and turbulence. When observed in the wave‐following frame, maxima of the fluctuating normal stresses occur near the wave crest; the fluctuating shear stress is positive and possesses a peak on the windward face of the mechanically‐generated wave. The data in fixed‐probe and wave‐following measurements are consistent when an appropriate transformation is used between the two. The orbital velocities of waves were computed by our implementation of the nonlinear theory of Dean. Then, the modulated mean current was obtained by subtracting the orbital velocity of the wave and the time‐averaged velocity from the phase‐averaged velocity. The results show strong modulation in the current boundary layer. The horizontal component of the modulated mean current is positive in the trough of the carrier‐wave profile and negative near the crest. The range of fluctuation in a wave profile decays with depth.

Wave-turbulence interaction may contribute to momentum and energy transfer in the nearsurface region beneath water waves. However, the nature and extent of this mechanism have not been well understood. A wave separation technique was developed to study quantitatively the momentum and energy transport due to wave-turbulence interaction. In this technique a nonlinear stream function representation of the wave motion was determined so as to satisfy the kinematic and dynamic boundary conditions at the water surface. Wave-induced velocities were then deduced from the stream function and subtracted from measured velocity components to obtain the turbulent flow field. This technique was applied to laboratory data taken beneath wind-generated water waves. Coherence spectra indicate high correlation between turbulence and wave-induced motions at some frequencies. Time-averaged correlations between wave and turbulent velocities increase with wind speed and decrease with increasing distance away from the interface. Although there is no significant momentum transfer due to wave-turbulence interaction, this mechanism results in significant energy transfer among the mean, wave-induced, and turbulent flow fields. BACKGROUNDhave been processed by filtering the wave-induced motions from the total velocity signals. Sufficiently far away from A mean shear current, surface waves, and turbulence are the dominant wave frequency, the turbulent velocity spectra generated in water when wind blows over the water surface. show an f-sis dependence, but with an apparent increase However, the precise mechanisms by which momentum and in spectral density at high frequencies. They proposed that energy are transferred from air to water, and the complicated these features occur as a consequence of wave kinematics in interactions among the mean, wave, and turbulent fields in which frozen turbulence is convected bodily by the combined the water are not fully understood.wave-induced and drift velocities. This frozen-turbulence by-In their laboratory wind wave data, Lin and Gad-eI-Hak pothesis produces the noted features without a requirement [1984] observed a spectral peak in the streamwise velocity at for wave-turbulence interaction. the frequency of the dominant wave. They concluded that in Although many researchers have studied the physics of the region where waves affect the turbulent motions (i.e., at wave-turbulence interaction, a number of questions remain depths less than half the dominant wave length), the vertical unanswered. For example, can one discern a direct intermomentum transport is affected not only by turbulence but action between wave-induced motions and turbulence? How also by the orbital motion of waves themselves. However, important is this interaction? Unfortunately, these questions they did not separate the wave-induced and purely turbulent cannot be addressed by the spectral decomposition technique motions.of Benilov and Filyushkin [1970] because their method tac-Many researchers have attempted to decompose the mea-itly as...