On the three-dimensional aerodynamic structure of shelterbelts

Shelterbelts function by reducing wind speed and altering turbulence fields in the sheltered area. The influence of a particular shelterbelt is dependent on its structure. The more complex the structure becomes the more difficult it is to accurately describe the flow fields associated with the shelterbelt. In order to improve the ability to predict the turbulence field and the resulting protection around the shelterbelt for a given structure, a structural description of the shelterbelt and an understanding of the air motion as determined by shelterbelt structure are required.;It is proposed that the 3-dimensional aerodynamic structure of a shelterbelt is best described by its external structure: height, width and cross-sectional shape; and its internal structure: the amount and arrangement of vegetative surface area and volume as well as the geometric shape of the individual elements. This 3-dimensional structure can then be described by the spatial functions of vegetative surface area density (vegetative surface area per unit canopy volume) and cubic porosity (void volume per unit canopy volume). A model addressing the role of both structural descriptors in determining the air motion in a shelterbelt was developed. The model indicates that the drag force of a shelterbelt on the air flow is determined by vegetative surface area, volume and their spatial arrangement and that air divergence and convergence in a shelterbelt depend on both turbulent velocity and element arrangement. A method to estimate the spatial functions in the model was developed using field measurements. For actual estimation, the 3-dimensional spatial functions were decomposed into a combination of the definable equations describing the amount and arrangement of surface area and volume. Equations were derived and statistically defined for the tree trunk, branches, leaves, and seeds. The application of these equations to the estimation and description of green ash shelterbelt structure was demonstrated. Using the estimates, the structural terms in the model can be determined for the simulation of the turbulence field around the shelterbelt. The developed spatial functions and equations facilitate further research on the prediction of the turbulence flow around a shelterbelt with a given design.