The unsteady interaction of gust with forest edges

点击翻译按钮获取中文摘要

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.