LES modeling and experimental measurement of boundary layer flow over multi-scale, fractal canopies

Abstract

In many regions the atmospheric surface layer is affected substantially by vegetation canopies. Most previous work has focused on effects of vegetated terrain characterized by a single length scale, e.g. a single obstruction of a particular size, or canopies consisting of plants, often modeled using a prescribed leaf-area density distribution with a characteristic dominant scale. It is well known, however, that typical flow obstructions such as canopies are characterized by a wide range of length scales, branches, sub-branches, etc. Yet, it is not known how to parameterize the effects of such multi-scale objects on the lower atmospheric dynamics. This work aims to study boundary layer flow over fractal, tree-like shapes. Fractals provide convenient idealizations of the inherently multi-scale character of vegetation geometries, within certain ranges of scales. Preliminary results from a large-eddy simulation (LES) and experimental study of a fractal tree canopy in a turbulent boundary layer are reported. The LES use Renormalized Numerical Simulation (Chester et al., 2007, J. Comp. Phys.) to provide subgrid parameterizations of drag forces from unresolved small-scale branches. Experiments aiming at understanding drag forces acting on fractal trees are performed in a water tunnel facility. Drag force measurements are obtained on a set of “pre-fractal” trees containing 1-5 branch generations.