Large-Eddy Simulation Study of Log Laws in a Neutral Ekman Boundary Layer
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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The characteristics of wind profiles in a neutral atmospheric boundary layer and their dependence on the geostrophic wind speed U-g, Coriolis parameter f, and surface roughness length z0 are examined utilizing largeeddy simulations. These simulations produce a constant momentum flux layer and a log-law layer above the surface characterized by a logarithmic increase of wind speed with height. The von Karman constant derived from the mean wind profile is around 0.4 over a wide range of control parameters. The depths of the simulated boundary layer, constant-flux layer, and surface log-law layer tend to increase with the wind speed and decrease with an increasing Coriolis parameter. Immediately above the surface log-law layer, a second log-law layer has been identified from these simulations. The depth of this upper log-law layer is comparable to its counterpart in the surface layer, and the wind speed can be scaled as uufh34, as opposed to just u in the surface log-law layer, implying that in addition to surface processes, the upper log-law layer is also influenced by Earths rotation and large-scale conditions. Here u is the friction velocity at the surface, and h is the boundary layer depth. An analytical model is proposed to assist in the interpretation of the log laws in a typical Ekman boundary layer. The physics and implications of the upper log-law layer are discussed.
- Fluid Mechanics