Stably stratified airflow over a waved water surface. Part 1: Stationary turbulence regime.

Stably stratified turbulent boundary-layer flows over both a waved water surface and a flat smooth surface are investigated through direct numerical simulation (DNS) for the bulk Reynolds numbers, Re, from 15 000 to 80 000. DNS expose the following basic properties of the flow. A statistically stationary turbulent regime is sustained if the turbulent Reynolds number, ReL, based on the Obukhov length-scale and friction velocity, is larger than 10&#178. At ReL <10², turbulence over a flat surface degenerates completely, but over a waved surface it survives in the form of residual fluctuations, which are weaker for smaller wave slopes. In the stationary turbulent regime, at ReL >10&#178, vertical profiles of the mean-flow velocity and temperature have a log-linear shape, as predicted by the Monin-Obukhov similarity theory, with the same empirical dimensionless constants as in laboratory and field experiments. The velocity and temperature roughness lengths, vertical turbulent fluxes of momentum and heat, and root mean square turbulent velocity and temperature fluctuations increase with increasing slope of the surface waves. At the same time, vertical profiles of the mean velocity and temperature keep the self-similar shape predicted by the Monin--Obukhov theory, irrespective of the wave slope. [ABSTRACT FROM AUTHOR]