防風林風口對風速影響之風洞試驗

Abstract

台灣風害頻仍，防風林在防範風害方面扮演著不可或缺的角色，與國土保安有著密不可分的關係。但隨著經濟開發，許多穿越林帶的林內道路將會損害防風林原有之功效，但目前國內少有關於此類之研究；有鑑於此，本研究選擇折角開口，搭配75°、60°及45°三種角度，以及斜交型態開口搭配90°、75°、60°及45°四種角度，加上用來對照之完整林帶共8種林帶配置，以10行林帶為基準，進行風洞實驗，並計算相對風速及保護指數，以期能獲得林內道路對於減風效果之影響。試驗結果顯示減風效果與開口型態之關係較小，但與開口角度有著密切的關係，當開口角度較小時，有較佳之減風效果，甚至在開口角度為45°時，不論是折角或是斜交形態，減風效果均接近於完整林帶配置，而垂直形態之開口則毫無減風效果，因此，若需要在保安林內開闢道路的情況下，當以45°開口為優先考量。

In Taiwan, the windbreak forests play a significant part in protection against wind hazard and are crucial to homeland protection. Many roads that pass through the forest bands as economy develops would compromise the effects to the windbreak forests, in which, however, little research based on this problem has been made. This study thus conducts wind tunnel experiments on eight types of forest band layouts, which are a selection of the angular type gaps at the degrees of 75�, 60� and 45�, and the oblique crossing gaps at the angles of 90�, 75�, 60� and 45�, plus a complete forest band as the control, each arranged in the basis of ten rows. Relative wind speeds and protection indices are calculated, hoping to determine the effects of in-forest roads on the wind speed reduction. The experimental results indicated that the wind speed reduction has little to do with the type of gap but is greatly related to the angles of gap. Smaller gap angles result in better wind speed reduction, so much so that at a 45� degree of the gap, regardless of angular type or oblique crossing type, the forest bands have wind speed reduction all nearly as good as the complete forest band layout. The perpendicular gap is not helping at all in wind speed reduction. Therefore, if to cut through a protection forest to build roads, the optimal option should be a 45� gap.