Marine Atmospheric Turbulence Coupled to Time-varying Three-dimensional Surface Water Waves: Results from LES

Large-eddy simulations (LESs) are used to examine the couplings between turbulence in the marine atmospheric boundary layer with an externally imposed time-dependent spectrum of 3D surface water waves. The wave field is built from a sum of linear plane waves with random phases and amplitudes chosen to match the empirically measured Donelan et al (1985) wave height spectrum. The computational grid used in the LES is non-orthogonal and translates vertically to follow the motion of the underlying waves. Fine meshes of 512x512x128 (grid spacings 2.3 m in the horizontal and 1 m in the vertical near the surface) are used. Our LES experiments and subsequent analysis focus on turbulent flow in the surface layer in the wind speed range [10-20] m/s. Conditionally sampled flow fields are obtained using the technique of linear stochastic estimation (Adrian et al, 1989) using wave height as the detection field. These results provide insight into the average 3D flow fields in and around the surface waves and their dependence on wind speed and wave age. As in previous investigations the wave induced pressure field changes markedly with wave age and the presence of swell. Also, we attempt to compare the surface drag from LES with empirically determined drag laws deduced from field observations.