Numerical Study of Flow Past a Conifer Tree
行道樹 ； 數值模擬 ； 風洞試驗 ； Street trees ； Numerical simulation ； Wind tunnel experiment
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在建築環境風場的考量上，行道樹的設置對行道風場有重要的減風效果。在風力評估初步規劃的階段，設計者常採用數值模擬的方式進行風場問題的分析。然而由於樹木複雜的枝葉分布難以在數值模式中作直接的表達，為研擬出一套便於風場數值模擬的解析模式，本研究沿用前人提出適用於群樹之控制方程式，進而建議出樹木相應枝葉參數之變化關係，以為離散樹木風場數值預測之用。 本研究之主要探討對象為針葉樹。研究中首先以風洞試驗的方法，量得固定厚度與疏密度樹體模型下游之平均與擾動風速剖面，再以試誤的方法進行數值模擬，藉計算與試驗量測結果間之比對以率定出在該厚度與疏密度情況下數值模式中最適切之樹體參數值。繼針對不同厚度與不同疏密度之樹體模型，獲得樹體參數之變化關係。 在研究範圍中，研究結果顯示大體上樹體參數均隨著樹體厚度與疏密度之增加而增加，但後者之影響較為顯著。此外，建議之樹體參數關係應用於獨立三維樹後風場模擬有不錯之預測結果。但在二維樹體的情況，建議之樹體參數需作程度上之折減以獲得與試驗量測較為吻合之結果。
Planting trees in an open area can generally slow down the near-ground wind speed thus becomes an important way in pedestrian wind planning. At the preliminary design stage, the method of numerical simulations is commonly used in the wind field analysis. However, technical difficulties are encountered in reflecting the physical existence of leaves and the tree body due to their geometric complexity. Accordingly, the major goal of the study is to explore a relationship to properly describe the physical effect of trees that can be handily adopted in the governing equations proposed by previous researchers to predict the wind field with discrete trees. The study concentrates on the problem of coniferous trees. At the beginning, wind experiments were performed to measure the velocity profiles at the cross-sections downstream of a tree panel with a prescribed tree body density and width in the longitudinal direction. By using a trial-and-error method, the base-fit tree body parameter, which reflects the effect of tree characteristics, was then determined. Afterwards, the variation of the tree parameter was obtained as a function of the tree body density and width. Finally, the calibrated relationship of the tree parameter was further used in simulating the problems of uniform wind past two-dimensional and three-dimensional isolated tree to examine its validity. Within the range of test program, results show that basically the tree parameter increases with increases of both the tree body density and width. The former effect appears more significant than the latter. The application of the proposed tree parameter in the three-dimensional tree case leads to downstream wind velocity profiles in good agreement with the measured ones. However, the parameter needs some extent of reduction to obtain an agreeable downstream wind field in the two-dimensional tree case.
工程學 > 土木與建築工程