本論文探討垂直不可壓縮流經過頂端具有圓蓋之有限長水平圓柱的近場尾流特性, 實驗於垂直循環式水洞中進行, 圓柱外徑, D, 為0.64 cm, 置入測試段的長度, L, 為15 cm, 以不同直徑的圓蓋將實驗分為三組, 分別於圓柱的頂端裝置直徑為2D (1.28 cm)、3D (1.92 cm)、4D ( 2.56 cm)的圓蓋, 每一組的雷諾數ReD = 250, 560和1080。本研究採用流場可視化(Flow visualization) 的方法觀測流場型態, 以及使用質點影像速度儀(PIV) 量測速度資料。不同直徑的圓蓋有不同的下洗效應範圍; 當有限長圓柱頂端具有4D直徑圓蓋時, 在沿著圓柱方向, 離圓柱頂端1倍直徑, 1D,位置的尾流區域開始有渦旋逸放(vortex shedding) 的現象; 當圓柱具有3D直徑圓蓋時, 在離圓柱頂端2D位置開始有渦旋逸放現象; 圓柱具有2D直徑圓蓋時, 則在離圓柱頂端3D位置開始有渦旋逸放現象。流體流過圓柱的頂端後有顯著的下洗效應, 在圓柱的尾流區域形成分離線, 從時間平均速度場結果得知, 當有限長圓柱的頂端具有4D直徑圓蓋時, 其分離線在尾流區域, 距離圓柱3倍直徑, 3D, 的位置; 當圓柱具有3D直徑圓蓋時, 其分離線在距離圓柱2D的位置; 當圓柱具有2D直徑圓蓋時, 其分離線在距離圓柱1.5D的位置。時間平均渦度和時間平均剪應變等位圖結果顯示, 隨著減小圓柱頂端圓蓋直徑的尺寸, 最大的平均渦度和最大的平均剪應變值跟著增加, 當圓柱不具圓蓋時, 相較於具有圓蓋時, 有較大的最大平均渦度和最大平均剪應變值; 而且這兩個物理量隨著雷諾數的增加而增大。

This research studies characteristics of the near wake region by the vertical incompressible flow past a finite-length horizontal cylinder with a cap at the tip. The diameter, D, of the cylinder is 0.64 cm and the length, L = 15 cm, of the cylinder is placed into the test section of the water tunnel. The sizes of three caps are 2D (= 1.28 cm),3D (= 1.92 cm) and 4D (= 2.56 cm) respectively. Experiments were carried out in a vertical closed-loop water tunnel with the Reynolds numbers of 250, 560 and 1080. Flow structures were observed by the flow visualization method and Particle Image Velocimetry (PIV) technique was used to measure velocity fields. Experimental results show that when the size of the cap increases, the region which is influenced by the downwash effect decreases. The vortex shedding starts to take place in the wake region at the location with a distance of 1D, 2D or 3D from the cylinder tip along the cylinder length direction for the cases with the cap size of 4D, 3D or 2D respectively. The bifurcation line forms in the wake region and is away from the horizontal cylinder about the distance of 3D, 2D or 1.5D for the cases with the cap size of 4D, 3D or 2D respectively. The maximum values of average vorticity and average shear strain, which are estimated by using the time-averaged velocity field, both increase with higher Reynolds number, or with the smaller cap size.

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