設計水面之下的跨河管線以及海底纜線時，必須考量水流之衝擊力，以避免管 線之損壞。本研究整合大渦流模式與 VOF 法探討水面下的圓柱體之尾流及圓柱體 受水流之衝擊力。模擬結果先與水槽實驗量測得之自由水面比較、驗證，以增加數 值模式的可信度。再利用數值模式針對圓形斷面之柱體進行一系列的模擬，檢視福 祿數、沉沒比、阻滯比等參數對圓柱體的阻力與升力之影響。數值模擬結果顯示: 因為水深限縮流場的關係，造成水流加速通過圓柱，使得水流施予圓柱的阻力係數 隨著阻滯比變大而增大。當福祿數 FrD = 0.45 時，水面沒有明顯地起伏地流過圓柱 上方;但當福祿數 FrD - 0.68 和沉沒比 h* = 0.75 時，圓柱上方的水面壅高，並在圓 柱後方發生水躍現象。研究結果亦發現:當沉沒比下降(圓柱接近水面)時，因為圓 柱上方受自由水面之影響，發生圓柱上下表面的動壓力不對稱的現象，導致圓柱受 到向上的升力。當圓柱沉沒比 h* > 2.0 時，圓柱表面的動壓分佈就不再受水面影響， 圓柱表面上下壓力對稱，升力係數趨於零。本研究之成果可應用在淹沒於水中管線 的結構設計。;The hydrodynamic loadings on the pipelines are essential parameters for pipeline design, especially when the pipelines are submerged in the river flow during flood events. This study focuses on the interaction between the free surface flow and a submerged cylinder with circular cross-section. This study integrates a Large Eddy Simulation (LES) model and the Volume of Fluid (VOF) method to examine the effect of free surface on the hydrodynamic loading of a submerged circular pipeline. The simulation results are verified by the results of flume experiments. Then the numerical model is utilized to investigate the influences of the Froude number, submergence ratio and blockage ratio on the flow field and the force coefficients of the circular cylinder. The simulation results reveal that the drag coefficient of the cylinder increases as the blockage ratio increases. When the Froude number FrD < 0.45, the water surface is smooth and undisturbed; when FrD > 0.68 and submergence ratio h* = 0.75, the water surface is elevated by the submerged cylinder and dropped behind the cylinder to create a forced jump. The simulation results indicate that the dynamic pressure on the upper side of the cylinder is affected by the water surface and the pressure distribution became asymmetric when the cylinder is very close to the water surface. This leads to the lift coefficient deviated from zero when the submergence ratio h* 2.0. The results of this study can be used for the structure design of submerged pipelines.
