本研究以實驗的方式探討均一速度分布之自由流撞擊有限長圓柱後產生之下洗效應與上洗效應對尾流區流場的影響，以及分析不同流動和空間參數的實驗影像，以瞭解不同流動和空間參數對尾流區渦旋逸放的影響。本研究使用流場可視化和質點影像測速儀 (PIV) 方法觀察不同雷諾數於各觀察平面上的尾流區流場。實驗觀測於閉迴路循環直立式水洞中進行，將直徑為6.4 mm之有限長圓柱水平安裝在水洞的透明測試段之垂直壁面上，進入測試段的圓柱長度為160 mm，圓柱展弦比為25，並在圓柱頂端嵌入直徑為6.4 mm、厚度為3.2 mm之圓蓋作為自由端。本研究使用高速照相機蒐集實驗流場影像，以雷射光頁照射兩個垂直切平面方向：圓柱縱向切平面 (XY平面) 與圓柱橫向切平面 (YZ平面) ；圓柱縱向切平面 (XY平面) 僅有一個觀測平面，為Z/D = 0；圓柱橫向切平面 (YZ平面) 共有五個觀測平面，分別為X/D = -1、-5、-10、-15及-20：每個觀測平面上皆有六個不同雷諾數 (ReD) 的結果，分別為195、250、360、560、880及1080。XY平面上之流場可視化結果顯示下洗效應與上洗效應對尾流區的影響範圍，YZ平面上之流場可視化結果顯示尾流區在不同位置的流動型態差異。頻譜分析顯示不同雷諾數於各觀察平面上之尾流的功率譜密度函數中之主頻率及其對應之功率。PIV瞬時結果顯示尾流區隨時間變化之流場特性，即由於渦旋逸放效應，流動呈現規律地左右擺動，及渦旋結構跟隨自由流向下游移動並離開近尾流區。PIV平均結果顯示尾流區多周期之平均流場特性，即平均速度場和平均渦度場之分布對稱於垂直方向上的圓柱中心線。接近圓柱自由端 (X/D = -1、-5) 或接近圓柱底部 (X/D = -20) 之流動型態強烈地受到下洗效應或上洗效應造成的縱向方向速度影響，呈現出三維流動的特徵。遠離圓柱自由端和底部 (X/D = -10、-15) 之流動型態主要受到渦旋逸放效應的影響，顯示相對有規律地渦旋逸放，並趨於二維流動的表現。

This experimental study investigates the downwash and the upwash effects on the wake flow fields of a finite-length cylinder immersed in a uniform free stream flow. To understand the influence of several parameters on the vortex shedding in the wake flow, the study analyzes the experimental images of different flow and spatial parameters. Flow visualization and Particle Image Velocimetry (PIV) techniques are used to observe the wake flow fields on different observation planes with different Reynolds numbers. The experiments are conducted in a closed-loop vertical water tunnel. A finite-length cylinder with a diameter of 6.4 mm is mounted horizontally to a vertical wall of the transparent test section of the water tunnel. The length of the cylinder entering the test section is 160 mm, which gives the cylinder an aspect ratio of 25. A cap with a diameter and thickness of 6.4 and 3.2 mm, respectively, is embedded into the top of the cylinder as its free end. The movements of particles inside the water tunnel are captured by a high-speed camera with the illumination of the laser light sheet. A longitudinal plane (XY-plane), located at Z/D = 0, and five transverse planes (YZ-plane) located at X/D = -1, -5, -10, -15, and -20 are selected as observation planes. Six Reynolds numbers of 195, 250, 360, 560, 880, and 1080 are used to observe the different wake flow fields. The flow visualization results on XY-plane show the three-dimensional wake flow, and the downwash and the upwash effects are observed in this plane. The flow visualization results on YZ-plane show the wake flow patterns at different locations. The spectrum analysis results show the dominant frequency and its corresponding power in the power spectral density function of the wake flow on different observation planes with different Reynolds numbers. The instantaneous results of PIV show the characteristics of the time-varying flow field of the wake, i.e., the flow oscillates from side to side regularly due to the vortex shedding effect, moreover, the vortex structures move downstream with the free stream flow and leave the near wake region. The mean results of PIV show the characteristics of the multi-period time-average flow field of the wake, i.e., the mean velocity field and the mean vorticity field distributions are symmetrical along the center line of the cylinder in the vertical direction. The flow patterns near the cylinder free end (X/D = -1 and X/D = -5) or near the cylinder base (X/D = -20) are strongly affected by the downwash or upwash effect, i.e., the dominant longitudinal direction velocity, and present characteristics of the three-dimensional flow. The flow patterns away from the free end and base of the cylinder (X/D = -10 and X/D = -15) are mainly affected by the vortex shedding effect, display relatively rhythmic shedding of vortices, and tend to performances of the two-dimensional flow.

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