本研究在拖曳式水槽中使用實驗方法，探討圓柱與壁面交接處附近的流場特性。以雷諾數與邊界層位移厚度(boundary layer displacement thickness)為參數，對圓柱上、下游之流場特徵行為及速度場特性進行研究。使用質點軌跡視流法，觀察圓柱周圍的流場特徵;並利用質點影像速度儀量化圓柱周圍流場的結構。藉由分析圓柱與壁面之交接處上游的中心垂直面流場可視化照片，在不同的雷諾數與邊界層位移厚度範圍內，流場可區分為四種特徵模態：單渦漩、雙渦漩、三渦漩及不穩定模態。在圓柱上游接近圓柱底部之水平面上，馬蹄形渦漩與自由流分別有向上游流動及向下游流動之速度分量，兩個速度分量交會於平面上一點，接著由兩側分開向下游運動，因此形成一個四向鞍點。在圓柱與壁面之交接處下游的中心垂直平面上，長時間之平均速度及流線分佈圖中主要的流場特徵為分歧線及源點。在圓柱下游水平面上，長時間之平均速度及流線分布圖中主要的流場特徵為兩顆反向旋轉之渦漩。利用質點影像速度儀方法，分析尾流區流速隨時間之變化，研究不同流場模態對尾流渦漩逸放頻率之影響，結果發現接近圓柱底之水平面上觀察不到渦漩逸放之現象。

The effects of Reynolds number and boundary layer displacement thickness on the flow field characteristics near the junction between the circular cylinder and the wall were experimentally investigated in a water towing tank. The qualitative and quantitative characteristic flow behaviors near the junction of the circular cylinder were measured by particle tracking flow visualization method and particle image velocimetry (PIV), respectively. By observing the flow patterns of horseshoe vortex appearing in the vertical symmetry plane upstream the circular cylinder, four characteristic flow modes were identified in the domain of Reynolds number and boundary layer displacement thickness—single-vortex mode, dual-vortex mode, triple-vortex mode, and unsteady-vortex mode. Near the wall surface upstream the circular cylinder, the horseshoe vortex and freestream in the horizontal plane induced two flows approaching each other and then departed away from a specific point so that a four-way saddle is formed. In the downstream area, a bifurcation line appeared in the vertical symmetry plane and two counter-rotating recirculation bubbles exhibited in the horizontal plane. The frequency characteristics of unsteady flow structure at various characteristic flow modes in the wake region were examined by extracting the time-series velocity data from PIV measurement. No peak frequency was detected near the wall surface.

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