The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results |
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Authors: | M Cassiani G G Katul J D Albertson |
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Institution: | (1) Department of Civil and Environmental Engineering, Duke University, 121 Hudson Hall, P. O. Box 90287, Durham, NC 27708, USA;(2) Nicholas school of the Environment, Duke University, Durham, NC 27708, USA |
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Abstract: | The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an
active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem
from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by
an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation
in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to
ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region
close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics
within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments.
In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial
ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest
edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled
the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up
downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone
can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing
the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation
zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the
mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined
by LAI was explored. |
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Keywords: | Back-facing step flow Boundary layer Exit flow Forest edge Large-eddy simulations Recirculation |
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