Turbulent Kinetic Energy Budgets from a Large-Eddy Simulation of Airflow Aboveand Within a Forest Canopy

摘要

Large eddy simulation output was used to study the terms of the resolved scaleturbulent kinetic energy budget. The computation produces a three dimensional, time dependent simulation of the airflow above and within a plant canopy where the lowest third of the domain was occupied by a drag layer and heat sources that represented a forest. Shear and buoyant production of TKE computed from the simulated resolved scale flow fields were principal sources in the upper canopy; the destruction of TKE due to canopy drag effects and the transfer to subgrid scales (dissipation) occurred primarily in the upper half of the forest where the foliage density was large; and turbulent transport showed a loss at the canopy top and a gain within the canopy. These general features have been found in various plant canopies in field experiments, higher order closure models, and wind tunnel studies, but in such studies, there is a lack of information on the pressure transport term. Previous studies, typically incorporated pressure effects into a residual term due to the difficulty in directly measuring turbulent pressure fluctuations. In the present LES study, the pressure was calculated directly; thus, the pressure transport term could also be calculated. Above the canopy, pressure transport appeared to balance approximately one third of the turbulent transport, while near the canopy top and below pressure transport was the same sign as turbulent transport showing a sink near the canopy top and a source below. The transport terms accounted for over half of the TKE sink at the canopy top, and in the lowest two thirds of the canopy the transport terms were the dominant source terms in the budget. The pressure transport was the largest source of turbulent kinetic energy in the lowest levels of the canopy. These results indicate that pressure transport is important in the plant.