Large Eddy Simulation of Near-Field Dispersion Within and Above Forest Canopies

Numerical simulations were conducted to investigate the feasibility of predicting near field concentrations of a tracer gas within and above forest canopies. The current research is geared towards providing forest managers with a tool for developing anti-aggregation techniques to control the bark beetle. Several field experiments have been conducted in different forest canopies linking tracer gas concentration fields with meteorological and canopy parameters. Field experiment results are site and situation specific. Numerical simulations are far less expensive and allow for variation in virtually all flow parameters such as atmospheric stability, wind speed and direction, and turbulence intensity. As a first step, a CFD simulation has been used to study dispersion in a generic lodgepole pine forest canopy based on leaf area index (LAI) and stem density. Steady Reynolds Averaged Navier Stokes (RANS) solutions were computed using the k-ε and Reynolds Stress Model (RSM) turbulence closure models. These solutions provide insight into in-canopy dispersion; however they do not fully capture the dynamics of the flow. The current work uses large eddy simulation (LES). LES resolves large flow dominated eddies while modeling smaller eddies using a sub grid scale model. Unsteady LES, can be used to capture the dynamics of flow within a canopy, including large rolling eddies above the canopy, bursting and sweeping within the canopy, multiple shear layers, and drainage flows.