| 摘要: | 本研究藉由計算流體力學(Computational Fluid Dynamics，CFD)軟體進行住區內不同植栽高度及樹冠體積對區域性空氣污染物的濃度分布的影響，並提出針對空氣污染物濃度影響改善的室外植栽配置要素，用以研討透過植栽配置以改善住區空氣污染濃度的可能性。在研究中，首先針對特定的住區與氣候條件進行選址與現場實測，也對如何在CFD軟體適當地模擬住區風場特性進行相關參數的率定，包括計算域的長度、高度、寬度及固態表面的磨擦係數等，其後針對不同植栽尺寸與配置進行模擬，並分析比較其對空氣污染物的濃度分布情況。
研究成果顯示，住區建築群高度的設計以及佈置於行人風場的植栽規模可以影響高空風場與行人風場的風速差，進而影響行人風場中懸浮微粒濃度改善的效益。藉由CFD模擬與氣流平行之建築高/街道寬度比(H/W) 為0.5的街谷，並變化植栽或樹冠層尺寸以探討其對行人風場的影響。以全域流場而言，距離地面2m高的樹冠層由於氣流可向地面逸散，故樹冠背風側之最大最小風速差小於落地植栽，同時對於周邊空氣風速穩定的效果大於落地植栽，故在樹冠層兩側基於風速穩定衰減，空氣物染物濃度較高。由空氣污染濃度分布指出，單位時間通過量高的區域為建築物或植栽上空的風速較高的區域，而單位時間沉降累積量較高的區域為風速最低同時與物體摩擦力較高的區域，如樹冠周圍、建築壁面周圍等。由風速衰減數據表明，高度1m-3m的落地植栽對於行人風場的風速弱化效果，影響範圍約為植栽後50m，同時於1.5m測點中，植栽高度與通過植栽後的風速回升鋒值成反比同時與其回升至鋒值的距離成正比。最後，0m-3m高度的落地植栽中，依據植栽背風側風速對照圖分析各線段斜率後，及風速衰減統計表指出，於植栽後10m距離約為斜率由高至低(弱風效果降低)的平均範圍，同時風速衰減曲線以及風場流線也指出植栽後10m的距離約為氣流回到不受植栽高度影響的穩定衰退狀態的距離，得出最佳的弱風效果的植栽高度範圍落在高度大於1.5m小於3m的範圍中而理想間隔距離為10m。
由於城市快速的發展，不適當的建築佈局，包含錯誤的高樓選址或是不適當的人工造景，可能會使得該地區環境品質劣質化，包含地表風場、濕度、溫度等環境物理量，甚至直接或間接地造成了住區中部分區域空氣污染物的濃度上升而危害到人體健康。透過計算流體力學的適當建模與計算模擬可以提供規劃設計階段的參考。
A computational fluid dynamic (CFD) package was employed to investigate the influences of planting height in a residential area on the local distribution of suspended particles, and the major influence factor which can improve the air pollution concentration were identified. In this study, the field surveys and evaluations about the microclimate on different residential district was firstly proceeded, the suitable parameter setting to simulate the residential district wind field was calibrated, and then the simulations on different planting heights and layouts were proceeded, the distributions of air pollutant were evaluated.
The results show that, the layout and the height of the buildings of residential district can affect the wind field and cause the difference of wind speed between high altitude and pedestrian area, and also affect the settlement of suspended particle in pedestrian wind field. With the simulation on the street canyon with building height/street width = 0.5 contains various planting height, the variations of pedestrian wind fields were investigated. For the wind field distribution, a tree crown with 2 m above the ground can cause the vortex of wind flow, the down-wind speed is smaller than that in the case of floor plant with uniform width, and the concentration of pollutants near the crown is higher that the adjacent area. From the distributions of pollutants, the concentration of pollutants is proportional to the wind speed distribution, high discharges occurs at high wind zone, and high concentration occurs at low wind zone or high friction coefficient surface, such tree crown or wall. The floor planting of 1 m ~ 3 m height with uniform size can descend the wind speed within 50 m behind the tree, after then the wind speed arose. Within the floor plant with 0 m ~ 3 m high, the optimal weaken wind effect occurs within 10 m behind the tree with 1.5 m height.
Due the rapid development of city, improper construction layout, including the improper site allocation and scale, and landscape design, can decrease the quality of local environments, including wind field, temperature, and moisture, etc., and ascend the concentrations of air pollutants. A suitable modeling and simulation of CFD becomes a powerful tool in planning and design stage. |
