植栽樹冠形狀對風速衰減之影響

Effect of Tree Cover on Reducing Wind Speed

都市規劃中植栽所扮演的角色多在於點輟都市、塑造良好的視覺景觀、增加綠覆綠面積以及減緩都市熱島效應等。由於植栽具有一定程度的擋風或導風效果，若在適當的配置與應用原則下，不但可以降低風速，減少不良風場的發生機率，其穿透性亦可讓部分空氣穿透而達到通風之效果，進而調節都市微氣候增加環境舒適。本研究為探討不同植栽之樹冠結構變化及配置情形對風環境之影響，運用實際量測與計算流體力學（computational fluid dynamics，簡稱CFD）的方式，探討植栽樹之孔隙率、樹冠形狀變化與配置對於外部風場所產生之影響，進一步提出植栽於風環境控制之設計原則。實驗數據以無因次法解析，並繪製成無因次圖表，透過此圖表，設計者於設計階段量測基地之實際風速（Ur）與配置狀況後，即可得知擋風之成效或初步預估植栽背風處之風速變化。經本研究結果可知，圓形樹冠風遮蔽效果可達1倍樹高（5公尺，d/D＝1），散形樹冠於2倍樹高處（10公尺，d/D＝2）可得明顯之風遮蔽效果，是故散形樹冠風速衰減較圓形穩定且遮蔽效果較佳。

In urban planning, the role of plant cultivation is beautification, creating a pleasant visual landscape, increasing green areas, and reducing the urban heat island effect. Since plant cover has wind breaking or directing effects, it can not only reduce wind speed, but also the occurrence of uncomfortable wind environment, provided that the plants are properly arranged. Wind penetration of such windbreaks can result in ventilation that modifies the microclimate and makes the environment more comfortable. This investigation thus investigates the effects of tree crown porosity, as well as variation of tree crowns and layout, on external wind patterns and further proposes design principles for plant cultivation for wind environment using field measurement and Computational Fluid Dynamics (CFD). The results obtained from the experiments were condensed into simple graphs using the non-dimensional method. Non-dimensional graphs can be employed during the design stage, and users can measure the wind velocity when basic wind data on the location becomes available. The study demonstrates that the round shaped tree crown create a wind shade effect by approximately 1 time of tree height. Better performance is achieved through the spreading of the tree crown such that the shading effect is approximately twice the tree height.