本研究利用閉迴路循環垂直式水洞進行實驗, 在壓克力材質的水洞測試段內安裝一圓柱, 觀察單一圓柱自由端嵌入4個不同直徑尺寸的圓蓋在3個不同流速中的流場特性, 進行12組( 4 × 3 ) 的實驗觀測。4個不同直徑大小的圓蓋分別為1D (6.4 mm)、2D (12.8 mm)、3D (19.2 mm) 和4D (25.6 mm), 圓蓋厚度為3.2 mm。實驗的3個雷諾數 (ReD) 分別為250 ( V∞ = 40 mm/s )、560 ( V∞ = 87 mm/s ) 和1080 ( V∞ = 170 mm/s )。單一圓柱的長度 (L) 為150 mm, 直徑 (D) 為6.4 mm, 固定的展弦比 (AR) 為23.44。本實驗採用流場可視化觀察流場分布的結構, 再藉由質點影像速度儀 (Particle Image Velocimetry, PIV) 進行速度的估算, 以及利用質點追蹤速度儀 (Particle Tracking Velocimetry, PTV) 與質點影像速度儀的結果相互比較, 流場觀測面分為縱截面 (X-Y 平面) 和橫截面 (Y-Z 平面)。具有不同直徑尺寸之圓蓋圓柱的縱截面 (X-Y 平面) 流場可視化結果皆顯示, 當流體流經具有圓蓋之自由端時, 將在圓蓋頂部加速, 並在圓蓋後方形成下洗流。具有1D、2D 和3D 圓蓋圓柱的縱截面 (X-Y 平面)之流場可視化結果在近尾流域呈現再循環區(recirculation zone), 其中接近圓蓋部分的流速較遠離圓蓋部分的流速來的快, 具有4D 圓蓋圓柱之流場可視化結果於低雷諾數時, 則是在近尾流域位於 (X/D, Y/D) = (-1.3, -3) 的位置出現源點, 其源點會隨著雷諾數的增加而逐漸消失, 導致具有4D 圓蓋圓柱的低雷諾數之流場結構不同於其他三個圓蓋圓柱之流場。PIV瞬時流場在橫截面 (Y-Z平面) 上的分析結果顯示, 當X軸方向靠近圓蓋的距離 (X/D = -1.5), 可以觀察到具有1D 和2D 圓蓋圓柱之流場的近尾流域內呈現一對反向渦旋, 具有3D 和4D 圓蓋圓柱的流場, 則是開始呈現渦旋逸放。PIV和PTV 瞬時流場在橫截面 (Y-Z 平面) 上分析呈現PTV的剪切層相比PIV較符合流場可視化所觀測到的結果。利用PIV 分析4種圓蓋圓柱之流場於雷諾數250在縱截面(X-Y 平面)上的紊流強度, 其結果顯示具有4D 圓蓋圓柱之流場, 其圓蓋後方X/D = 1 ∼ −1.2, Y/D = −3.5 ∼ −10的區域, 出現一明顯的紊流擾動區, 使其流場結構不同於其他圓蓋圓柱的流場。PIV平均速度的渦度場在橫截面 (Y-Z 平面) 上X軸靠近圓蓋之距離 (X/D = -1.5) 的結果呈現隨著圓蓋直徑增大, 渦度影響範圍出現擴大的現象。比較4種圓蓋圓柱之平均速度的渦度場, 結果顯示具有4D 圓蓋圓柱之流場的渦度為最弱。

The study investigates the flow characteristics of the flow field by flow past a finite-length cylinder with a cap of different sizes. The finite-length cylinder with the aspect ratio (AR) is 23.44, the length (L) is 150 mm and the diameter (D) is 6.4 mm, which is mounted in the test section of a vertical closed-loop water tunnel. The thickness of each cap is 3.2 mm and the diameters of the caps are 1D (6.4 mm), 2D (12.8 mm), 3D (19.2 mm) and 4D (25.6 mm). There are 12 (4×3) experiments which were conducted with Reynolds numbers as 250 ( V∞ = 40 mm/s ), 560 ( V∞ = 87 mm/s ) and 1080 ( V∞ = 170 mm/s ). The flow structure of the experiments is observed and analyzed by flow visualization, Particle Image Velocimetry and Particle Tracking Velocimetry. The flow field is observed in two main planes, one is the transverse-section plane (X-Y plane), and another is the cross-section plane (Y-Z plane). The results of the flow visualization on X-Y plane show that the flow is accelerated at the free end and becomes the downwash behind the cap. In the 1D, 2D and 3D cases, the recirculation zone is formed in the near wake region. In the 4D case, there is a source point which appears in the near wake region at ReD = 250 and it disappears as Reynolds number increases. The PIV instantaneous velocity fields on Y-Z plane (X/D = -1.5) show that the 1D and 2D cases have a pair of the counter vortex in the near wake region, but the 3D and 4D cases begin to present the vortex shedding. The PTV instantaneous velocity fields are more close to the original image results to observe the shear layer than the PIV results. The turbulence intensity fields are analyzed by PIV on X-Y plane. The results of turbulence intensity show that the fluctuation region appears behind the cap at X/D = 1 ∼ −1.2, Y/D = −3.5 ∼ −10 in the 4D case. The time-averaged vorticity fields are analyzed by PIV on Y-Z plane. Comparing the four caps diameter, the effect region of the vorticity expands as the diameter of the cap increases and the vorticity of the 4D case is the weakest.

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