方形截面建築紊流流場之數值探討

Abstract

矩形鈍體在風域中的受風效應之分析一向為建築研究領域裡重要的一環，在風工程的設計規範中往往以此為標準外型而提出各種之相關規定。為的獲取建築物承受之風荷重，以為推算其相應動態反應之依據，現行的方法多採用簡化狀況的風洞試驗配合實場量測資料而得之風函數。但此類風函數並不足以提供實際情況中複雜幾何與流力狀況下準確之風荷重數據，如建築物之造型特殊、風場中存在著其他建築物、或當來流屬非典型風況時，應用此類函數於設計中將導致誤差，甚至造成不保守的結果。 本研究旨在以微可壓縮流加大渦模擬的方法，計算地面上三維矩柱建築之鄰近紊流流場。研究中，流場來流為冪級數邊界層流但流場為紊流流場，邊界層厚度為14倍之矩柱寬，並改變矩形鈍體不同之高寬比和不同之雷諾數，以探討並比較在不同高寬比及雷諾數下平均阻力係數、平均昇力係數、擾動阻力係數和擾動昇力係數及其相應速度場之關係，其藉流場之模擬系統化地探討矩形結構物受到風力作用所引起的氣動行為。 研究結果指出，高寬比對於流場的速度場及風力係數的影響大於雷諾數的影響。當流場雷諾數大於105後，流場已具備不變性。平均阻力係數、擾動阻力係數隨高寬比的增加而增加，但是隨雷諾數變化無明顯變化。擾動昇力係數隨高寬比及雷諾數增加均呈現增加的趨勢。

Investigation on flow around a rectangular bluff body has been an important subject in building research. To evaluate the dynamic response of a structure, the flow effects on the building (such as the variation and mean values of lift and drag) and related flow characteristics are necessary during the design stage. So far, there are mostly obtained by wind tunnel model testings. However, the accuracy of the experimental results usually suffers from scale effect. Besides, the achievement of the complete flow data is generally costly. Therefore, the application of high-speed computation in flow simulation then becomes an important way of the analyses. The objective of the study is to investigate numerically the turbulent flow around a three-dimensional surface-mounted rectangular cylinder. In the research, a weakly-compressible-flow method incorporated with a space-average large-eddy-simulation technique is adopted. By varying the aspect ratios of the building (the ratio of vertical dimension from the surface to the across-wind dimension), the resulting flow and wake characteristics are analyzed. Results show that the major influence of the flow characteristics is the aspect ratio. As the aspect ratio increases, the mean drag coefficient; the variations of the lift and drag coefficient also increase. The general characteristics of the flow remains unchanged as Reynolds number exceeds about 105. Finally, the root-mean-square value of the drag coefficient increase as aspect ratio and the Reynolds number increase.