| 摘要: | 本研究採用大渦模擬(Large Eddy Simulation, LES)模式，結合流體體積法(Volume of Fluid, VOF)，分析狹窄渠流中圓柱形橋墩周圍的流場，數值模型的準確性經由渠流實驗結果比對予以驗證。並透過一系列數值模擬阻滯效應與入流福德數(Froude no.)在狹窄渠道條件下，探討對圓柱周圍的流場變化、渦流結構與橋墩的受力之影響。研究結果顯示：當圓柱直徑與渠道寬度的阻滯比(Blockage ratio, Br) 超過 20% 時，阻滯效應會造成圓柱周圍渦流結構、圓柱體表面壓力、阻力係數、橫向力係數變化幅度以及史徹荷數(Strouhal number)皆產生顯著的影響。較高的阻滯比會增強近壁渦流的生成，並強化圓柱周圍的剪切層。垂直向(Ωy)、順流向(Ωx)與橫向(Ωz)的渦度分量均會隨著Br增加而變大，使得近底部的渦流結構更為強烈且具廣泛空間分布，並形成水平方向延伸的渦流結構。在較高阻滯比下，順流向的渦度往下游衰減速度變慢，使得有序渦流結構得以延伸至更下游位置。這些變化凸顯狹窄渠道中阻滯效應會放大流場不穩定性方面的關鍵角色，對橋墩結構的受力與河床局部沖刷之發展具有重要意涵。
本研究亦探討在阻滯比 Br = 30% 狀況下，入流福德數(Froude no.)對橋墩受力及渦流場的影響。模擬結果顯示：隨著福德數增加，上游水面會因為阻滯效應而壅高，流經圓柱體時，流速增加形成超臨界流，柱體下游產生局部性水躍後又恢復成亞臨界流。隨著福德數改變，圓柱體的阻力係數 CD ≈ 2.11，橫向力係數的均方根值及史特勞哈數幾乎不變。渦度分析顯示：會加強圓柱–河床交界處的垂直渦度，強化逆向旋轉的順流渦，並增強沿剪切層的橫向捲吸現象。這些變化使渦度幅值增大、渦心位置偏移，並產生更複雜的三維流場結構。在下游區域，低福德數會形成較弱且快速消散的對稱渦旋；而高福德數則產生更強、持續且混亂的波動。研究結果有助於狹窄渠道中圓柱結構或兩個橋墩十分靠近時水動力與渦度場的瞭解。
;This study employs a Large Eddy Simulation (LES) model and the Volume of Fluid (VOF) method to investigate the flow field around a circular cylinder in narrow open channels. The accuracy of the numerical model is validated through comparison with experimental results of a flume study. A series of numerical simulations are conducted to examine the effects of blockage and inflow Froude number, focusing on their influence on flow field characteristics, vortex structures, and the hydrodynamic forces acting on the circular cylinder. The simulation results indicate that when the blockage ratio, Br, defined as the ratio of the cylinder diameter to the channel width—exceeds 20%, the blockage effect significantly enhances the vortex structures around the cylinder, surface pressures, drag coefficient, lateral force coefficient, and the Strouhal number. A higher blockage ratio intensifies the near-wall vortices and strengthens the shear layers around the cylinder. The vertical (Ωy), streamwise (Ωx), and spanwise (Ωz) vorticity components all increase with Br, leading to stronger and more spatially extensive vortex structures near the bed. The downstream decay rate of streamwise vorticity becomes slower under higher Br conditions, allowing coherent vortex structures to persist farther downstream. These findings underscore the critical role of blockage effects in amplifying flow instability in narrow channels, with important implication for hydrodynamic loading on bridge piers and the development of local scour on the riverbed.
This study also examines the impact of inflow Froude number on pier-induced forces and vortex dynamics under a blockage ratio of Br = 30%. Simulation results demonstrate that with increasing Froude number, the water surface upstream of the cylinder elevates due to the blockage effect. When flowing through the cylinder, the flow velocity increases, resulting in supercritical flow. A localized hydraulic jump occurs downstream of the cylinder, before the flow returns to subcritical flow. As the Froude number changes, the drag coefficient of the cylinder (CD) remains approximately 2.11, while the root-mean-square value of the lateral force coefficient and the Strouhal number remain virtually unchanged. Vorticity analysis shows that the vertical vorticity at the cylinder-bed interface is enhanced, counter-rotating downstream vortices are strengthened, and lateral entrainment along the shear layer is intensified. These changes increase the vorticity amplitude, shift the vortex center, and produce a more complex three-dimensional flow field structure. In the downstream region, low Froude numbers produce weak, rapidly dissipating symmetric vortices, while high Froude numbers produce stronger, persistent, and chaotic fluctuations. The research results help to understand the hydrodynamic and vorticity fields when cylindrical structures or two bridge piers are closely spaced in narrow channels. |
