本研究探討壁掛式有限長圓柱具有不同尺寸圓蓋之周圍流場分佈, 實驗在直立式水洞中使用質點影像速度儀 (PIV) 進行速度場量測。 設置圓柱凸出於水洞測試區壁面, 圓柱直徑 (D) 為6.4 mm, 圓柱長度 (L) 為150 mm, 有限長圓柱展弦比 (AR) 為23.44。四種圓蓋尺寸分別為1D (6.4 mm)、 2D (12.8 mm)、 3D (19.2 mm)、 4D (25.6 mm),在圓柱的頂端分別安裝不同尺寸圓蓋, 實驗在每一個尺寸的圓蓋分別測試雷諾數 (Re)為250, 560與 1080。 三維的下洗流與卡曼渦流在靠近圓柱頂部發生交互作用, 實驗主要研究圓柱具有圓蓋的前端流場。 實驗數據拍攝所選取的範圍從圓柱前端 X/D = 2, 圓柱具有圓蓋的自由端到圓柱長中段 X/D = − 13 的位置。 流場由兩個主要觀測平面所建立, 分別為橫截面 ( X-Y 平面) 和剖面 ( Y-Z 平面), 在橫截面 ( X-Y 平面) 放大流經圓蓋前端區域的速度場, 觀測剖面 ( Y-Z 平面) 為數個不同觀測位置, 距離圓柱自由端不同遠近的位置。 實驗結果包括平均速度和紊流強度, 在觀測橫截面的速度結果中,隨著圓蓋尺寸增加而削弱 X 方向的速度。 X方向速度隨著離圓柱頂部距離增加而消失,在 Y 方向紊流強度介於近尾流區和遠尾流之間域有較高的數值。 在圓柱具有4D圓蓋,雷諾數為250和 560, 在圓蓋下方的近尾流區域有源點發生。

The flow fields of a wall-mounted finite circular cylinder with a round cover cap having different sizes were studied experimentally in a vertical water tunnel by using Particle Image Velocimetry (PIV). A cylinder of the diameter (D) as 6.4 mm and the length (L) as 150 mm, giving the aspect ratio (AR) as 23.44, was mounted on the wall plane of the test section. Four round cover caps having the sizes of 1D (6.4 mm), 2D (12.8 mm), 3D (19.2 mm) and 4D (25.6 mm) were respectively mounted at the tip of the cylinder. Experiments were conducted for each cover cap size with Reynolds numbers as 250, 560 and 1080. This research studied the 3-dimensional downwash flow interacting with von Karman vortex near the tip of the cylinder. The experimental study focused on the flow fields around the tip of the cylinder with the cap. The range of experimental data was from the tip of the cylinder, X/D = 2, to the location of X/D = −13. The velocity fields of the wake flow were observed in two main planes (the transverse section, X-Y, plane and the cross section, Y-Z, plane). The local amplified velocity fields of the upstream flow passing the cover cap were observed on the transverse section, X-Y, plane. The cross section planes were observed at several different locations, which were near and away from the tip of the cylinder. The flow fields on the transverse section plane show that the X velocity component decreases in both the near and far wake regions close to the cylinder tip as the cap size increases. Accompanying the X velocity component disappearing away from the cylinder tip, high turbulence intensity of the Y velocity component takes place around the border region between the near and far wake regions. A singular source point was found in the wake region near the cap trailing edge for the cases with a 4D size cap and Reynolds numbers as 250 and 560.

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