On Wind Forces in the Forest-Edge Region During Extreme-Gust Passages and Their Implications for Damage Patterns
On Wind Forces in the Forest-Edge Region During Extreme-Gust Passages and Their Implications for Damage Patterns
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DOI：
10.1007/s10546-018-0348-4
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发表时间：
2018-03
影响因子：
4.3
通讯作者：
C. Gromke;B. Ruck
中科院分区：
文献类型：
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作者：
C. Gromke;B. Ruck
在一段时间的强风之后，偶尔会发现一种破坏模式，显示出森林内一片受损的树木，它们位于一片完整的树木后面，直接位于迎风边缘。为了了解导致这种破坏模式的机制，在风洞中使用比例森林模型研究了不同特征的极端阵风通过时森林边缘区域的风荷载。瞬态极端阵风与静止大气边界层（ABL）作为背景气流在森林边缘的相互作用导致冠层顶部形成涡旋。这种涡旋在向下游移动时加剧，随后使树冠上方的高动量空气向下偏转，导致树冠上的风负荷增加。在这种条件下，与静止ABL接近流相比，顺流方向风荷载的减少相对较弱。而离林缘有顺流距离的树木的阻力，则是在定流条件下对风荷载的适应性生长的结果，在向风缘后2 ~ 3树高范围内，阻力迅速下降。对于一些已实现的极端阵风，在森林边缘后大约三棵树的高度处发现风荷载超过树木的阻力，这表明上述破坏模式可能是由短暂的极端阵风与静止的ABL流相互作用造成的。
A damage pattern that is occasionally found after a period of strong winds shows an area of damaged trees inside a forest stand behind an intact stripe of trees directly at the windward edge. In an effort to understand the mechanism leading to this damage pattern, wind loading in the forest-edge region during passages of extreme gusts with different characteristics are investigated using a scaled forest model in the wind tunnel. The interaction of a transient extreme gust with the stationary atmospheric boundary layer (ABL) as a background flow at the forest edge leads to the formation of a vortex at the top of the canopy. This vortex intensifies when travelling downstream and subsequently deflects high-momentum air from above the canopy downwards resulting in increased wind loading on the tree crowns. Under such conditions, the decrease in wind loading in the streamwise direction can be relatively weak compared to stationary ABL approach flows. The resistance of trees with streamwise distance from the forest edge, however, is the result of adaptive growth to wind loading under stationary flow conditions and shows a rapid decline within two to three tree heights behind the windward edge. For some of the extreme gusts realized, an exceedance of the wind loading over the resistance of the trees is found at approximately three tree heights behind the forest edge, suggesting that the damage pattern described above can be caused by the interaction of a transient extreme gust with the stationary ABL flow.