在一般情況之下，橋墩與河床交界處上游會產生一、二或三個馬蹄形渦漩，對河床產生刮沙作用，最終導致橋墩坍塌。過去的研究顯示，使用適當設計的半圓錐流動控制器安置於交界處上游可消除圓柱上游的馬蹄形渦漩，降低壁面剪應力，因而改善馬蹄形渦漩對圓柱根部的刮沙效應。本研究在一拖曳式水槽中，安裝一支垂直於一平板之圓柱，以模擬真實河道中水流對橋墩根部之沖刷情況。將一半圓錐流動控制器安置於圓柱與平板交界處之上游，改變安裝角度，使用實驗方法探討受半圓錐控制之圓柱與壁面交界處上游的流場特徵，討論此半圓錐流動控制器對圓柱與平板交界處刮沙效應的抑制效果。使用雷射光頁輔助質點軌跡流動可視化技術觀測圓柱與壁面交界處上游垂直面與水平面的流場特徵行為，再以質點影像測速儀量測速度場，轉換成流線，並計算壁面剪應力、紊流強度以及紊流擾動動能。當半圓錐的長度大於1倍圓柱直徑時，在半圓錐旋轉偏移角與雷諾數的域面上，圓柱與壁面上游交界處的流場呈現四種特徵模態，分別為「順向流動」、「逆向流動」、「渦旋」及「不穩定渦旋」。順向流動模態時，靠近壁面處，流體呈現順向流動行為；逆向流動模態時，靠近壁面處，流體呈現逆向流動行為；渦漩模態的流場特徵為數目一、二或三的馬蹄形渦漩；不穩定渦漩模態的流場特徵呈現數目不固定的馬蹄形渦旋。在順向流動與逆向流動兩種模態時，壁面剪應力不會出現特別大的數值；當圓柱上游形成馬蹄形渦漩時，相對於馬蹄形渦漩處，壁面會產生相當大的剪應力，因此對河床產生刮沙作用。當雷諾數小於4500，半圓錐的旋轉偏移角小於10度時，四種流動模態的區域範圍會與旋轉偏移角等於0度時相近，意即當旋轉偏移角小於10度時對流動的控制效應不會減弱；當旋轉偏移角大於10度時，順向流動與逆向流動模態所佔的範圍變窄，意即流動控制的有效區域變小。當雷諾數大於4500，旋轉偏移角小於2度時，流場呈現順向流動模態，意即流動控制仍舊有效；旋轉偏移角大於2度時，流場呈現不穩定渦漩模態，意即流動控制器失去效用。

The upstream area around the juncture of a pier and river bed usually suffered from the scour effect induced by horse-shoe vortices. Present work employed a circular cylinder mounted normal to a flate plate to simulate the juncture of the pier and rever bed. The circular cylinder and flate plate were immersed in a towing water tank to perform experimental studies. A semi-cone flow controller was installed at the upstream corner of the juncture to modulate the flow status. The laser light-sheet assisted particle-tracking flow visualization technique was used for flow visualization. A particle image velocimeter (PIV) was applied to measure the velocity field. The streamlines, turbulence intensity, and wall shear stresses were obtained by processing the measured velocity data. At the semi-cone length to circular diameter ratio greater than unity, depending on the semi-cone rotation angle and Reynolds number, the flow around the upstream area of the juncture displayed various characteristic modes: forward flow, reverse flow, vortical flow, and unsteady flow. The wall shear stress presented relatively small values at the forward and reverse flow modes. However, the wall shear stress revealed significantly large values around the locations corresponding to where the vortices appeared at the vortical flow modes. At Reynolds numbers less than 4500 and semi-cone rotation angles smaller than 10o, the regimes of the characteristic flow modes were similar to those of the flow at zero semi-cone rotation angle, i.e., small semi-cone rotation angle would not deteriate the effect of flow control. At semi-cone rotation angles greater than 10o, the regimes of the forward and reverse flows reduced, which indicated a reduncion in effective flow control. At Reynolds numbers greater than 4500 and semi-cone rotation angles smaller than 2o, forward flow mode was observed, indicating an effective flow control. However, at Reynolds numbers greater than 4500 and semi-cone rotation angles larger than 2o, unsteady vortical flow mode appeared and therefore the flow control became ineffective.

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