Modulated flows beneath wind‐ruffled, mechanically generated water waves

Abstract

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.