The unsteady interaction of gust with forest edges

木本植物生态系统对地球气候起着重要作用。由风暴事件引发的森林风阻和防风林每年给德国造成数百万欧元的损失（1999年的飓风“洛萨”吹毁了大约2900万棵树，2007年的飓风“基尔”吹毁了大约1500万棵树）。观察表明，风阻通常从森林边缘附近开始，逐渐进入林分。显然，高度不稳定阵风与多孔森林边缘的相互作用在这方面起着关键作用。这一流体力学现象已在第一个资助期进行了深入的研究。利用高科技流量测量技术（TR-PIV），可以详细地阐明森林边缘强风的动态及其对破坏性动量通量的贡献。研究结果与文献中基于开尔文-亥姆霍兹不稳定性理论的主流知识不一致。研究发现，强风必须被认为是罕见的、速度最高的单一事件，在大多数实验或“数值”风洞模拟中不会发生。第一个资助期的研究表明，近缘冠层的动量输入是由横向流动现象引起的，这种现象只存在于森林边缘。受影响的冠层面积的长度明显取决于阵风的长度。这意味着近缘冠层的破坏机制是特征阵风尺度的函数。在第一个资助期揭示了过程的现象学之后，第二个资助期将在风洞中进行定量系统研究，以揭示阵风特性与近缘森林破坏之间的关系。总体目标是为林业提出建议，以提高林分对风暴事件的稳定性。

Woody plant ecosystems play an important role for the earth's climate. Initiated by storm events, windthrow and windbreak in forests causes in Germany multi-million euro losses every year (Hurricane "Lothar" threw / broke in 1999 approximately 29 million, Kyrill in 2007 approximately 15 million trees). Observations show that the windthrow often starts near the forest edge and advances progressively into the stand. Apparently, the interaction of highly unsteady gusts with porous forest edges play a key role in this respect. This fluid mechanical phenomenon has been thoroughly investigated in the first funding period. By using high-tech flow measurement techniques (TR-PIV), the dynamics of strong wind gusts on forest edges and their contribution to the destructive momentum flux could be clarified in detail. The results do not coincide with prevailing knowledge in literature, which is based on Kelvin-Helmholtz instabilities theory. It was found that strong wind gusts must be considered as rare, singular events of highest velocity and do not occur in most experimental or "numeric" wind tunnel simulation. As the studies of the first funding period have shown, the momentum input into the near-edge canopy is caused by a cross flow phenomenon, which exists only at forest edges. The length of the affected canopy area depends obviously on the length of the gust. This means that the damaging mechanism at the near-edge canopy is a function of characteristic gust scales. After revealing the phenomenology of the process in the first funding period, quantitative systematic studies in the wind tunnel shall be performed in the second funding period in oder to reveal the relation between gust properties and near-edge forest damages. The overall goal is to make recommendations for the forestry, in oder to increase the stand stability against storm events.