Analysis of Coherent Structures Within the Atmospheric Boundary Layer

Abstract

Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy
simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve
the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation
to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures
with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with
direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric
boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures
arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows.
It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical
plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from
the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise
direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale
structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing
the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of
the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic
energy more efficiently than the vorticity.