Flow across a forest edge

Large-eddy simulation of a forest-edge flow (2013)

Project: High-resolution LES studies of the turbulent exchange processes between forest and atmosphere in a forest-edge flow regime

Responsible: Farah Kanani?, Björn Maronga?, Helge Knoop?

Description: The first sequence shows the development of coherent turbulence structures above a forest canopy downstream of a forest edge. A neutral wind-tunnel-like flow is simulated with the mean flow direction from left to right, perpendicular to the forest edge. The animation spans over 180 seconds of simulated time and runs at an accelerated pace. Only a part of the model domain is shown. The forested part of the domain is surrounded by the light green iso surface. The turbulence structures are visualized by the absolute value of the three-dimensional rotation, with highest values in pink and lowest values in yellow. The effect of form and viscous drag of the forest on the turbulent flow is modeled by PALMs canopy model. Upstream of the forest, the 10-m wind is approximately 6m/s.

The approaching flow is turbulent with different scales of turbulence being randomly distributed. Entering the forest volume, the flow is decelerated and turbulence is efficiently damped by the forest drag. Above the forest, turbulence is effectively generated due to the strong velocity shear near the forest top. With increasing distance from the forest edge, the developing turbulence structures grow in size and strength, forming a layer of high turbulence activity, within the flow adjusts to the new surface conditions. This layer represents an internal boundary layer.

Model Setup
Total domain size (x|y|z):	2304m x 1152m x 384m
Grid spacing (x|y|z):	3m x 3m x 3m
Number of grid points (x|y|z):	768 x 384 x 128
Forest size:	1000m x 1152m x 30m
Simulated time:	3h
CPU-time:	18h 20m
Number of CPUs:	512
Machine/ processor type:	SGI Altix ICE at ​HLRN / Intel Xeon Gainestown
Visualization software:	​VAPOR
DOI:	​10.5446/14297

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Large-eddy simulation of the scalar transport in a forest-edge flow (2013)

Project: High-resolution LES studies of the turbulent exchange processes between forest and atmosphere in a forest-edge flow regime

Responsible: Farah Kanani?, Björn Maronga?, Helge Knoop?

Description: The second sequence visualizes the transport of a passive tracer, which was homogeneously released at the domain surface. The data is from the same simulation as in the first sequence, but a smaller selection of the domain is shown and stretched in the vertical direction by a factor of 6. A certain range of lower tracer concentrations is represented by different shades of white, while a range of higher concentrations is drawn in shades of red. The colored surface represents near-surface values of the streamwise (edge-perpendicular) velocity component. Blue colors illustrate a flow from left to right and green colors mark the opposite flow direction. The arrows additionally illustrate direction and strength of the flow.

A region of tracer accumulation can be detected inside the forest, at a certain distance from the forest edge. This localized accumulation can be attributed to the horizontal convergence of the flow, resulting from the flow deceleration inside the forest. A reversed flow is even visible in this area, which enhances the convergence effect. Such a flow reversion sets up in dense forests, where the strong streamwise flow deceleration leads to a strong upward mass transport and hence creates a low pressure zone inside the forest. This in turn forces the flow to recirculate in order to compensate the mass loss. Looking at the tracer concentration reveals that the tracer distribution is fairly variable downstream of a forest edge, same as the tracer transport between forest and atmosphere. This adds high complexity to the interpretation of concentration and flux measurements near forest edges.

Model Setup
Total domain size (x|y|z):	2304m x 1152m x 384m
Grid spacing (x|y|z):	3m x 3m x 3m
Number of grid points (x|y|z):	768 x 384 x 128
Forest size:	1000m x 1152m x 30m
Tracer source strength:	0.2µgm-2s-1 (homogeneous surface source)
Simulated time:	3h
CPU-time:	18h 20m
Number of CPUs:	512
Machine/ processor type:	SGI Altix ICE at ​HLRN / Intel Xeon Gainestown
Visualization software:	​VAPOR
DOI:	​10.5446/14311

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