PIV analysis on the shelter effect of a bank of real fir trees

In agriculture, windbreak forests have been used as a natural wind fence to protect crops. The effects of windbreaks on the shelter zone are directly related to the flow structure modified by the forests. Because wind flow through porous three-dimensional heterogeneous vegetative structure is highly complex, majority of previous research employing laboratory experiments, field studies, or numerical simulations seldom revealed any quantitative flow information. In the present study, flow around a bank of small white fir trees placed in a wind tunnel test section was quantitatively visualized using the particle image velocimetry (PIV) field measurement technique. The effect of windbreak on the flow structure and shelter effect was investigated as well. For a single tree, the flow velocity funneled underneath the canopy is high enough to generate an upwelling vortex under the canopy. Compared to the single tree, a bank of trees was observed to be more effective in modifying the flow characteristics and shelter effect in the studied area. Because, the bank of trees functions as a kind of flow resistance that blocks some amounts of oncoming wind and induces uprising wind in the upstream. In addition, the neighboring trees reduce turbulent velocity fluctuation in a bank of trees. This low turbulence characteristic makes a positive effect on the shelter effect.