Wave-Related Fluctuations in the Airflow Above Natural Waves

Abstract

Simultaneous observations of wave heights and velocity fluctuations at two levels above the waves are analyzed to examine properties of the wave-related fluctuations in the allow. Results are obtained from spectral and joint probability density function, conditional mean function (JPDF-CMF) analyses. Results are examined with respect to predictions from potential flow theory and recent theoretical formulations for wind-wave coupling. Of interest are recent formulations which allow interaction between the wave-induced motion and turbulence in the airflow, the so-called ?turbulence? models. Cospectral results exhibit features which are predicted by theoretical formulations with regard to height variations of the wave-related momentum transfer. These features include the oscillatory variations predicted by recent turbulence models and also enhanced transfer at both levels as predicted by the quasi-laminar model. JPDF-CMF analyses are used to obtain phase-amplitude information for those variables examined in the spectral analyses. For a period in which the presence of the ?critical level? could have been a factor, the phase relation between the wave-related vertical velocity and the wave height agrees with the quasi-laminar prediction. For periods in which only the turbulence in the airflow would be expected to influence the wave-induced motion, phase and amplitudes of the wave-related fluctuations differ from the potential flow predictions. It is concluded that the interaction between the wave-induced motion and airflow turbulence had a significant effect on the observed wave-related fluctuations. Another conclusion is the assertion of the value in using JPDF-CMF analyses for examining wave-related fluctuations.