Comparative study of flow field and drag coefficient of model and small natural trees in a wind tunnel

The influence of trees in urban areas is typically assessed using urban microclimate models. These models rely on wind tunnel experiments using small-scale tree models to verify and validate their predictions of the flow field. However, it is not known sufficiently to which extent small model trees used in wind tunnel studies can recreate the behavior of large trees found in cities. In the present study, the drag coefficient and the turbulent flow downstream of model trees are compared with the ones of natural trees of a similar size to determine whether both types of tree provide similar aerodynamic characteristics. Therefore, measurements of the drag force and the flow field, using particle image velocimetry, are performed. The aerodynamic characteristics of the small trees are compared with the ones measured on larger mature trees from previous studies. The present study shows that the drag coefficients of model and natural trees are similar only if both types have a similar aerodynamic porosity and if the model tree can undergo an aerodynamic reconfiguration similar to that of a natural tree. Such reconfiguration implies the reorientation of the branches and leaves due to wind. A study on the influence of seasonal foliar density variation shows that the foliage configuration plays a critical role on the drag coefficient and the flow field. A defoliated tree, such as a deciduous tree in winter, is shown to have a substantially lower drag coefficient and a negligible influence on the flow.