방풍림의 방풍효과에 대한 실험적 연구

Abstract

Windbreak forests have been used for various purposes including reduction of wind speed, pollutant diffusion, improvement of environment, or increase of crop yields. These functional effects of windbreaks are directly related to the reduction of oncoming wind speed. However, studies on the shelter effects of windbreaks have been performed only recently. Because most of previous studies are base on laboratory experiments, field studies, or numerical simulations, quantitative flow information are seldom revealed. This results from the fact that wind flow through porous three-dimensional heterogeneous vegetative structures is highly complex. Although the systematic study for a real tree has been a long-standing problem, no attempt was tried yet.In this thesis, the flows around real tree models, which are placed in a closed-type wind tunnel test section, are quantitatively visualized using a PIV (particle image velocimetry) velocity field measurement technique. Several fundamental studies are investigated to find a novel means to evaluate the sheltering effects of a windbreak forest. The flows around tree with two different optical porosities (β ≈ 0.6 and 0.7) are investigated. The effect of the two optical porosities on the reduction of oncoming wind speed was not so significant. However, the effect of porosity on the turbulence structure and shelter effect was great. The effect of tree leaves on the flow structure and shelter zone formed behind the real tree is also studied. The porosity of a real tree largely depends on the abundance of leaves. The wind-speed reduction coefficient of a real tree is considerably greater than that of artificial fences having similar porosity, regardless of leaf abundance. Tree leaves not only reduce optical porosity, but also increase the wind-speed reduction coefficient and turbulence kinetic energy in the leeward region behind the tree canopy. General flow characteristics of wind around a real tree are consistent with the previous results for artificial porous fences. The leaves of tree canopy induce updraft toward the top region and downdraft toward the bottom gap in the upstream region of the tree model. These vertical wind motions generate flow characteristics different from those of wind flowing the leafless tree and two-dimensional artificial porous fences. The extensive reduction in mean velocities and turbulence intensities of wind flow around a tree is mostly attributed to tree leaves which provide a good shelter zone in the leeward region.The flow around a small white fir tree is also investigated with varying the length of the bottom trunk (hereafter referred to as bottom gap). Three different flow regions are observed behind the tree due to the bottom gap effect. Each flow region exhibits a different flow structure as a function of the bottom gap ratio. For the case of the low gap ratio, a downwelling vortex is observed. This vortex is consistent with the previous studies for windbreaks of low porosity. However, an upwelling vortex is formed in the leeward of the canopy when the gap ratio is high. This phenomenon has not yet been reported before. At a high gap ratio, the downdraft is increased in the upstream region, the flow velocity funneled underneath the canopy is high enough to generate the upwelling vortex under the canopy. Depending on the gap ratio, the aerodynamic porosity (α) of the tree is changed and different turbulence structure is induced. As the gap ratio increases, the maximum turbulence intensity is increased as well. However, the location of the local maximum turbulence intensity is nearly invariant. These variations of the flow and turbulence structures around a tree according to bottom gap significantly affect the shelter effect of the tree. The shelter effect of a bank of trees is also examined in the present study. To investigate the effect of neighboring trees on the shelter effect, the flow characteristics of a bank of trees is compared with those of a single tree. Compared to a single tree, a bank of trees is found to be more effective in enhancing shelter effect with modifying the flow characteristics around windbreaks. Because, the bank of trees acts as a kind of flow resistance that blocks some amounts of oncoming wind and induces an uprising wind in the upstream. In addition, the neighboring trees in the bank of trees are helpful reducing turbulent velocity fluctuations. This low turbulent velocity fluctuations make additional contribution to the shelter effect.The flow information obtained in this study would be helpful for understanding the shelter effect of a windbreak forest and for validating numerical predictions. This understanding about the sheltering ability of real trees also can be used for better design and installation of artificial wind fences.