Aerodynamic modelling 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 fluid mechanical point of view, these structures differ essentially from man-made constructions because of their structural flexibility 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 inflexible model trees with crowns of varying porosity are presented. Drag force and flow field measurements were performed in a uniform or boundary layer flow, respectively. The force measurements delivered drag coefficients in the range of 0.8-1.2. This is in good accordance with natural trees for low wind velocities (< 10 m s(-1)). 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.