本研究針對平板置於方柱上游之結構物受到氣流衝擊時之流場，探討平板與方柱之間的距離(L/D)、平板與方柱的寬度比(d/D)及雷諾數的變化，對平板與方柱之間的流場、方柱表面壓力分佈、升阻力特性以及尾流區渦旋逸放之流場結構的影響。在一氣動力風洞中，藉由煙線可視化技術與煙霧可視化技術觀察平板與方柱之間及方柱尾流流場的特徵，使用表面壓力孔與壓力掃描技術量測方柱表面的壓力分佈，並計算升、阻力係數，同時探討壓力分佈及平板與方柱之間流場特徵的相關性。使用熱線風速儀量測平板與方柱之間及方柱尾流區動態結構的頻率、尾流速度場、尾流寬度、尾流雷諾正向應力、統計尾流紊流特性，並將結果與平板與方柱之間的流場特徵相結合。平板與方柱之間流場的模態與平板距離方柱的遠近有緊密的相關性，依平板離方柱距離的不同由近到遠，可以觀察到流體流經平板，在平板與方柱之間產生渦旋逸放(Mode A)，及在平板上下兩側產生剪流層流經方柱(Mode B)兩種模態。在從模態B轉變成模態A的時候，阻力係數皆會上升，在d/D ≥ 0.50時，模態B的阻力係數會呈現負值。根據熱線風速儀量測方柱尾流量化的資料，可以發現在模態B時也會因為平板距離方柱的遠近，而造成方柱尾流速度場、紊流強度、雷諾正向應力有所不同。很明顯的，平板與方柱之間的距離，造成平板與方柱之間的流場改變，而影響了方柱表面的壓力分佈與尾流渦旋逸放及流場的特性。

The flow around the square cylinder is controlled by a flat plate fixed at the upstream area. Fluid forces acting on the square cylinder, vortex shedding frequency and flow patterns are systematically investigated. The flat plate width is varied from 0.25, 0.50, 0.75 and 1.00 times the width of the square cylinder. The perpendicular distance between the flat plate and the square cylinder is varied from 0.5 to 5.0 times the width of the square cylinder. Flow characteristics around square cylinder in a cross-stream at incidence are experimentally studied by smoke-wire technique at Reynolds numbers between 8x102 and 1.5x104. Vortex shedding characteristics and pressure distributions are measured by hot-wire anemometer and home-made linear pressure scanner. The effects of the flat plate on the square cylinder’s flow patterns, pressure distribution on the square cylinder’s surface, and wake properties are revealed and discussed. Two different flow patterns, namely: pattern A and B, are observed around the square cylinder at various positions of the flat plate. Flow pattern A showed vortex shedding formed between two objects. On the other hand, flow pattern B has one of the separated shear layers of the flat plate attached to the surface of the square cylinder. Drag acting on the cylinder obviously increases when the flow pattern is changed from B to A. Different behaviors of pressure distributions, drag, Strouhal number of vortex shedding, turbulence in the wake, and wake width are found in each of the flow pattern.

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