A LARGE-EDDY SIMULATION OF TURBULENT FLOW WITHIN A PROGRESSIVELY THINNED LOBLOLLY PINE FOREST
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Forest thinning has been going on for decades for reasons such as removing dead trees, controlling potential wildfires and controlling invasive bark beetle infestations. However more needs to be done in the way of examining the effects of the thinning on canopy flow. This research presents the results of a large-eddy simulation (LES) analysis of flow within and above a loblolly pine forest. The simulations represent an actual forest where the US Forest Service engaged in a project to measure canopy flow properties while progressively thinning the forest vegetation. The LES model is compared to an eddy-viscosity model as well as data from the forest thinning. It was found that the number of large coherent structures decreased as the canopy density decreased. The spacing between the structures increased with decreasing canopy density. As the forest was thinned the velocity in the lower canopy increased but the average velocity profile continued to show an inflected shape. The height of the inflection remaining unchanged with the change in canopy densities examined. Zero displacement height tracked linearly with the percentage of density reduction and it also proved to be a good reference point for validating reestablishment of transitioning flow. Studies of canopy edge flow where the flow transitions from dense forest to open ground indicate a distance of approximately 15 canopy heights before the flow completes its transition and behaves as if there had been no canopy present. When the flow transitions to a thinned canopy instead of bare ground the presence of the canopy has a measureable effect on the distance required to reestablish steady state flow. The denser the canopy the more quickly the flow is reestablished. The thinner the canopy, the higher the downward sweeps of velocity occur to reestablish the flow. Research into flow transitioning from canopy to bare ground found a recirculating zone at the transition edge for some canopy densities. No recirculation zone was found at the transition edge from the fully dense canopy to the three different thinnings examined here.