Shelter effect of a fir tree with different porosities

Windbreak has been used for centuries as a natural wind-fence. The functional effects of windbreaks are directly related to the flow structure. In the present study, the flow around a real fir tree placed in a wind tunnel was quantitatively visualized using PIV (particle image velocimetry) technique. The effects of tree porosity and leaves on the shelter effect were investigated as well. Tree leaves not only reduce the porosity, but also increase the wind-speed reduction and turbulence kinetic energy in the leeward region behind the tree canopy. The leaves of tree canopy induce updraft toward the top region and downdraft toward the ground in the upstream region of the tree. The momentum difference of these two flows generates peculiar upwelling flow characteristics. The large reduction in mean velocities and turbulence intensities of flow around a tree is attributed to tree leaves providing a good shelter zone in the leeward region.