Large eddy simulation of turbulent flow across a forest edge.

摘要

A Large Eddy Simulation (LES) has been used to investigate the turbulent flow across a forest edge. The statistics from the simulated flow are examined to reveal edge effects and to compare with results from field, wind tunnel and other numerical studies. The flow evolution demonstrates the growth of an internal boundary layer (IBL) and the establishment of an equilibrium layer (EL) downwind of the edge. Regardless of the limited domain size upstream of the edge, the flow statistics produced by the LES in this study generally agree well with field and wind tunnel observations.; Momentum and turbulence kinetic energy (TKE) budgets are studied to understand the flow controlling mechanisms. At the edge, the pressure gradient force and streamwise advection are found to be more significant than vertical turbulence diffusion. For the variance and TKE budgets, the important processes at the edge include production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and strong canopy dissipation.; Bending moments at the base of the trees are estimated and analyzed to address the issue of tree damage. Comparison of mean and maximum bending moments at different locations downwind of the edge show that trees in the equilibrium region tend to suffer more from occurrences of wind gusts, in a relative sense, than in the transition region. Correlation analyses and detections of high wind values demonstrate that the pressure gradient and the streamwise advection of upstream momentum are highly correlated with, and induce the extreme values in bending moments at the forest edge. In the equilibrium region, however, vertical turbulence diffusion of momentum plays the role of exposing the trees to large aerodynamic forces.; Coherent structures at different locations downwind of the edge are identified and analyzed. From the ensemble-averaged quantities, an increasing level of structure organization is observed with increasing distance from the edge. The ensemble-averaged structures in the equilibrium flow at x/ h = 14 contain all the typical features observed in homogeneous canopy flow. In the transition region, the structure outlines can be identified but are generally vague and do not clearly exhibit the well-defined patterns known to exist in homogeneous canopy flow. Structures identified in the equilibrium flow have smaller horizontal dimensions (or shorter durations), but carry more momentum and make a greater contribution to the local momentum flux than in transition flow. Although sweeps are stronger than ejections in terms of their relative contributions to momentum flux in all regions, the largest contrasts between them are found in the equilibrium region.