Shelter effects of porous multi-scale fractal fences

Rows of trees have been naturally used as wind and snow fences to reduce strong winds, to prevent soil erosion and mitigate snow/sand drifts. Trees, composed of multi-scale fractal structures, modify the airflow through and around them in a complicated manner. This research investigates how engineered porous fences, designed using a typical multi-scale fractal structure, would change the wake characteristics and subsequently reshape the wind shelter effects. A two-dimensional (2D) fractal fence composed of cross-grid type fractal struts, along with an one-dimensional (1D) fractal fence, were tested using a planar particle image velocimetry (PIV) method in a boundary-layer wind tunnel. Wakes and shelter effects of two fractal fences are compared to those of the non-fractal fence model of the same height, length, bottom gap and porosity of 50%. Results of the 2D fractal fence show a clear favorable shelter effect - the extended sheltered area of 1.5-4 times the fence height with the shelter parameter of 0.4, about 70% longer than that of the non-fractal fence provides. Multi-scale fractal fences may potentially be adopted to transform the porous fence design by allowing tuning of the arrangement of struts, an additional set of parameters besides fence height, length and porosity.