Large-eddy simulation of a forest-edge flow

The animation displays the development of coherent turbulent structures above a forest canopy downstream of a clearing-to-forest transition. Animation data were derived using the parallelized large-eddy simulation model PALM (http://palm.muk.uni-hannover.de/), simulating a neutrally stratified forest-edge-flow with a mean flow from left to right and a 10-m wind of 6m/s above the clearing. The forest, as surrounded by the green isosurface, is modeled in PALM as a porous viscous medium that decelerates the mean flow and damps the turbulence. Only a part of the total PALM domain is presented, and the vertical direction is stretched by a factor of 1.5 for better visualization. Turbulence structures and intensities are visualized by the rotation of the velocity vector (absolute value), with highest values in pink and lowest values in yellow. The animation spans over the last 180 seconds of a 3-hr simulation with a time-lapse factor of 3.6, and it was created with VAPOR (www.vapor.ucar.edu). The total PALM domain had a size of 768 x 384 x 128 grid points in streamwise, spanwise and vertical direction, with a uniform grid spacing of 3m in each direction. In total, the simulation required 18 hours of CPU time using 512 CPUs on the SGI Altix ICE of the North-German Supercomputing Alliance (https://www.hlrn.de/). The approaching flow is turbulent with different scales of turbulence being randomly distributed. Entering the forest volume, 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. They form a layer of high turbulence activity, a so-called internal boundary layer, within the flow adjusts to the abrupt change of the surface conditions at the clearing-to-forest transition.