| Description |
A large-eddy simulation study was conducted to characterize the bulk momentum and turbulence features of flow around an isolated deciduous tree in the atmospheric boundary layer under neutral stratification. In addition to characterizing the flow field, a simplified model that contained the essential features of flow around an isolated tree was constructed. The flow structure around the tree consisted of a region of high velocity defect and high turbulent kinetic energy behind the tree with accelerated flow around the edges. The region of high velocity defect was found to form near the location of maximum leaf area density of the tree and to then spread and dissipate downstream approximately following a power law. The region of heightened turbulent kinetic energy was found to be dominated by the streamwise velocity variance. The turbulent stresses around the tree were found to have peaks near the edges of the tree with similar structure to axisymmetric and plane wakes. The similarities between the tree wake flow and canonical axisymmetric wakes was used to motivate a simplified empirical model. To agree with the LES data, the empirical model had two important differences to a standard axisymmetric wake model. First, the plume spread was found to be nonsymmetric between the vertical and horizontal directions and therefore, an angular plume dependence was introduced. Second, the isolated tree wake was found to dissipate within a shorter downstream distance than a standard axisymmetric wake model. To address this second difference, the empirical model breaks the wake up into two regions: a near wake region where plume statistics evolve linearly with downstream distance and a far wake region that exhibits power-law behavior. The model was found to accurately reproduce the turbulent stresses, turbulent kinetic energy, and mean velocity for deciduous trees. When tested for sensitivity to the location of maximum leaf area density, the model was found to overpredict the turbulent kinetic energy and streamwise velocity defect at lower locations. |
