DOI: 10.1615/TSFP5
DIRECT AND LARGE EDDY SIMULATIONS OF A BOTTOM EKMAN LAYER UNDER AN EXTERNAL STRATIFICATION
RÉSUMÉ
A steady Ekman layer with a stratified outer layer and an adiabatic lower wall is studied with direct numerical simulation (DNS) and large eddy simulation with a near-wall model (NWM-LES). As the flow evolves from a uniformly stratified initial state, a mixed layer forms near the wall, separated from the outer layer by a strongly stratified thermocline. The thickness of the mixed layer is strongly limited by the outer layer stratification. Observations from the DNS are used to evaluate the LES. The mean velocity profile, mean temperature gradient, and boundary layer height from the LES agree well with the DNS. However, while the LES that is considered here resolves more than 80% of the turbulent kinetic energy away from the wall, it is not able to sufficiently resolve the overturning scales responsible for entrainment at the top of the mixed layer. As a result, the turbulent heat flux and rate of change of the temperature in the mixed layer are significantly underestimated in the LES.