Observations of the Enhancement of Kinetic Energy Dissipation Beneath Breaking Wind Waves
Most attempts to characterize the kinetic energy dissipation in the upper 20 metres of the water column revert to simple wall layer scaling — proportional to the cube of the friction velocity u ✱ and inversely proportional to depth, ∈αu ✱ 3 z −1With a concomitant logarithmic velocity profile, this is consistent with a total kinetic energy flux from the wind of 3 ✱ . However, in fully rough flow and strongly forced waves the energy input may be one to two orders of magnitude greater. Where does this energy go? Why is it not reflected in most of the upper layer measurements? This paper attempts to answer these questions and to demonstrate that there are two regimes of kinetic energy dissipation in the near surface layers under breaking waves. Near the surface, the dissipation rate is very high and scales with the wave characteristics. At greater depths the dissipation rate drops quickly and reverts to wall layer scaling. In the intermediate region the dissipation decays more rapidly than z−1. This may be viewed as a transition region between the deeper shear layer and the near surface region, with intense patches of breaking-imposed turbulence. In the absence of density stratification, dissipation in near surface region is analogous to that due to grid-generated turbulence and decays as z−4
KeywordsDissipation Rate Significant Wave Height Wind Wave Wall Layer Velocity Spectrum
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