近年來全球氣候變遷，世界各地洪災頻傳。以過去洪水頻率設計之橋梁，易錯估現實洪水流量而造成橋面板淹溢於洪流中，進而發生橋面板被沖毀之事件。本研究探討無橋墩之矩形斷面橋面版受水流衝擊的問題，研究中使用三維大渦流模式與VOF方法來探討矩形柱體周圍之自由水面及流場之流動情形。模擬結果先與Malavasi & Guadagnini (2007)之實驗結果及本研究水槽實驗之自由水面比較、驗證，以增加數值模式的可信度。再利用數值模式針對矩形斷面之柱體進行一系列的模擬，研究雷諾數、福祿數、沉沒比、阻滯比等參數對柱體的阻力與升力之影響。研究結果顯示：雷諾數對柱體的阻力與升力系數影響不大。當福祿數從亞臨界流進入穿臨界流時，水面發生水躍，柱體的阻力係數變大，而升力係數會出現負值則是因為柱體上方的自由水面影響了柱體上表面的分離剪力層的發展，使得柱體上下表面的動壓分佈不對稱而造成總力向下。數值模擬結果亦發現：當柱體的阻滯比增加時，阻力係數也會隨之增加，這是因為阻滯效應造成水流加速通過矩形柱體，導致水流施予柱體的阻力增加。而升力係數則會因為柱體接近水面(沉沒比下降)而變小，這是因為柱體上方的壓力隨之變大，使得柱體受到向下的總力。

In recent years, the numbers of floods have increased due to the global warming. The bridge engineers used the historical flood levels to design, so it may underestimate the water level during the future flood. This study focuses on the interaction between the free surface flow and a submerged cylinder with rectangular cross-section. The turbulent flow and pressure distribution around the cylinder were computed by the Large Eddy Simulation (LES) model with the Volume of fluid (VOF) method to track the water surface. The experimental results of Malavasi & Guadagnini (2007) and water-surface measured in this study were compared with the simulation results to verify the numerical results.
Then the verified LES model was utilized to examine the influences of Reynolds number, Froude number, blockage ratio and submergence ratio on the drag and lift coefficients. The simulation results demonstrated that the drag coefficient is dependent on the Froude number, rather than the Reynolds number. For sub-critical flow (deck Froude number FrD < 0.52), due to the blockage effect, the drag coefficient increases as the blockage ratio increases. For trans-critical flow (FrD  0.78), because of the wave-induced drag, the drag coefficient is larger than that in sub-critical flow with the same blockage ratio. On the other hand, the lift coefficient is a function of the deck Froude number and the submergence ratio h*. The separation shear flow on the upper side of the cylinder was constrained by the water surface when h* = 2.0, and resulted in an asymmetric pressure distribution on the upper and lower sides of the cylinder, subsequently generate a downward force on the cylinder.

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