Aerodynamic modeling of trees for small scale wind tunnel studies

In building and environmental aerodynamics, wind tunnel studies of natural vegetative structures like bushes or trees have to be modelled. From a fl uid mechanical point of view, these structures differ essentially from man-made constructions because of their structural fl exibility and permeability. Whereas the small-scale modelling of technical structures is widely known, a lack of knowledge exists in an adequate modelling of vegetative structures. In this article, aerodynamic characteristics of infl exible model trees with crowns of varying porosity are presented. Drag force and fl ow fi eld measurements were performed in a uniform or boundary layer fl ow, respectively. The force measurements delivered drag coeffi cients in the range of 0.8-1.2. This is in good accordance with natural trees for low wind velocities (<10 m/s). Flow field measurements show typical wake characteristics of permeable structures with recirculation zones being extended or even detached when compared with impermeable bodies. By means of spectral analysis, an energy shift from lower to higher frequencies, i.e. a breakdown of larger eddy structures into smaller eddies, in the crown wake region was found.