Canopy element influences on resolved- and subgrid-scale energy within a large-eddy simulation

Abstract

Large-eddy simulations described here have included the effect of canopy drag through the depth of a plant canopy. Specifically, we have considered the simulation of flow through a forest. Drag forces enter the simulation with the inclusion of form and viscous drag forces in the momentum equation. In addition, we have carried a variable to represent the kinetic energy (KE) associated with the turbulent wakes behind canopy elements. Assigning typical dimensions to canopy drag elements and, hence, to the scale of wake turbulence, we have evaluated wake effects on the dissipation process and on subgrid-scale (SGS) energy arising from the cascade of resolved-scale energy. Despite the fact that the rate of conversion of resolved-scale kinetic energy to wake energy is large, and the observation that wake energy is comparable with SGS energy, an effective diffusivity for wake turbulence can be ignored when calculated from wake-scale kinetic energy and a length scale based on element dimension. Thus, it is unnecessary to carry a wake energy variable in the simulation. On the other hand, it is very important that the process of conversion of SGS energy to wake-scale energy be included in the simulation because the action of wakes is to enhance the dissipation of SGS energy. Viscous drag appears to be of little consequence, at least in our calculations where elements are considered as flat plates and fully exposed to the grid-averaged flow.