Large Eddy Simulation of an Inhomogeneous Atmospheric Boundary Layer under Neutral Conditions

Ching-Long Lin Department of Mechanical and Industrial Engineering and IIHR Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa

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John W. Glendening Naval Research Laboratory, Monterey, California

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Abstract

Flow structures in an inhomogeneous neutrally stratified atmospheric boundary layer flow, obtained from large eddy simulation, are analyzed and compared with homogeneous case counterparts. The inhomogeneity is imposed in the streamwise direction by using two different surface roughness heights zo, each covering a streamwise distance of 4.8 km, to produce internal boundary layers. Adjustments of the mean velocity profiles are primarily confined to the lowest 140 m of the overlying boundary layer, with parcels being accelerated and decelerated over the respective smooth and rough surfaces. For large fetches downwind of the surface roughness discontinuities, the mean velocity profiles close to the surface are approximately logarithmic, but fitting to Monin–Obukhov similarity profiles using the zo and stress of the underlying surface requires that the von Kármán constant be k = 0.4 over the smooth surface and k = 0.37 over the rough surface. Much of this difference is attributed to velocity accelerations created by locally induced pressure gradient forces within the boundary layer, requiring k to be adjusted when horizontally homogeneous similarity expressions are utilized. Quadrant analysis indicates that ejection and sweep intensities differ from those of homogeneous surface cases but the occurrence frequencies are similar. Budget analysis of momentum flux indicates that the shear production and pressure destruction terms roughly balance, which is consistent with previous homogeneous surface findings. Flow visualization and conditional sampling demonstrate that these two terms are physically associated with ejections, sweeps, and vortical structures.

Corresponding author address: Dr. Ching-Long Lin, Dept. of Mechanical and Industrial Engineering and IIHR Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa 52242-1527. Email: ching-long-lin@uiowa.edu

Abstract

Flow structures in an inhomogeneous neutrally stratified atmospheric boundary layer flow, obtained from large eddy simulation, are analyzed and compared with homogeneous case counterparts. The inhomogeneity is imposed in the streamwise direction by using two different surface roughness heights zo, each covering a streamwise distance of 4.8 km, to produce internal boundary layers. Adjustments of the mean velocity profiles are primarily confined to the lowest 140 m of the overlying boundary layer, with parcels being accelerated and decelerated over the respective smooth and rough surfaces. For large fetches downwind of the surface roughness discontinuities, the mean velocity profiles close to the surface are approximately logarithmic, but fitting to Monin–Obukhov similarity profiles using the zo and stress of the underlying surface requires that the von Kármán constant be k = 0.4 over the smooth surface and k = 0.37 over the rough surface. Much of this difference is attributed to velocity accelerations created by locally induced pressure gradient forces within the boundary layer, requiring k to be adjusted when horizontally homogeneous similarity expressions are utilized. Quadrant analysis indicates that ejection and sweep intensities differ from those of homogeneous surface cases but the occurrence frequencies are similar. Budget analysis of momentum flux indicates that the shear production and pressure destruction terms roughly balance, which is consistent with previous homogeneous surface findings. Flow visualization and conditional sampling demonstrate that these two terms are physically associated with ejections, sweeps, and vortical structures.

Corresponding author address: Dr. Ching-Long Lin, Dept. of Mechanical and Industrial Engineering and IIHR Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa 52242-1527. Email: ching-long-lin@uiowa.edu

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