Study on the Flow and Force Characteristics of the Dam-break Flow on a Bridge Deck

2009年8月莫拉克颱風侵襲台灣，於中南部山區帶來破歷年紀錄之降雨量，引發土石崩塌阻塞形成堰塞湖，在雨水持續作用下造成堰塞湖潰決，潰壩流具有大量水體，並以高速衝擊下游橋梁結構物，造成下游多處橋面版損害。本研究係以FLOW-3D流體動力計算軟體進行模擬潰壩流通過橋面版之流場特性與橋面版受力解析研究。本研究係以利用三維動量方程式為基礎，並以有限差分法配合FAVOR網格障礙物處理技術及流體體積法解析之。透過數值模擬與文獻之實驗比較結果顯示，本數值模式採用以k-?紊流模式進行解析，可準確模擬潰壩流之流場特性與橋面版受力機制。 本研究利用上游儲存一固定高度水位，並設置一指定速度之電動閘門，以利模擬潰壩流之流況，結果顯示於下游渠道無結構物，並改變不同下游水深之情況，潰壩流以t=1.2m/s的速度往下游傳遞，而其渦心則以相對慢的速度往下游傳遞並逐漸消散，比較結果顯示渦心傳遞距離與相對水深比成負相關，且下游水深越淺，潰壩流之流速會越大。再者於下游位置設置橋面版結構物，研究結果可得下游水深越淺，其橋面版周圍產生之渦度值、紊流能量(TKE)以及紊流消散(DTKE)亦相對較大。透過橋面版受力分布發現不同水深對橋面版的水平受力差異不大；而垂直受力有顯著的差異，由比較結果顯示橋面版受力大小主要取決於橋面版與水面之距離，橋面版與水面距離越近，其受力越大。Morakot Typhoon hit Taiwan on August, 2009, brought breaking record of rainfall in central and southern mountainous areas. Rocky collapse caused blockage and a barrier lake, then the barrier lake was broken by continuous rainfall and caused dam-break flow. Dam-break flow with high-speed impact bridge structure at the downstream, and resulted bridge deck damaged. The study used hydrodynamic calculation software called FlOW-3D, which is based on three-dimensional momentum equation and volume of fluid (VOF). In this study, a numerical simulation was investigated the flow and force of the dam-break flow on a bridge deck. Through comparison of numerical simulation and experimental results showed that it's accurate to simulate dam-break flow field and bridge deck force by using k-? turbulence model This study set a fixed water level and an electric gate at the upstream to simulate dam-break flow when the gate was opened. The results showed when there was no structure at the downstream, vortex core would move to the downstream with slow speed and dissipate gradually. And vortex core would transfer far away with lower water depth at the downstream. Then this study set the bridge deck at the downstream. The results showed that when the water depth was lower at the downstream , turbulent kinematic energy(TKE)、dissipation of turbulent kinematic energy(DTKE) and vorticity around bridge deck would be more obvious. Through analysis of force distribution found that force impacted by the dam-break flow depended on the distance from the bridge deck and the initial free surface. That means when the distance is closer, the impact force will larger.目錄 摘要...................................................I Abstract..............................................II 目錄..................................................III 表目錄.................................................VI 圖目錄................................................VII 相片目錄...............................................IX 符號說明................................................X 第一章 前言............................................1 1-1 研究動機............................................1 1-2 文獻回顧............................................1 1-3 本文組織............................................4 第二章 數值模式.........................................6 2-1 FLOW-3D簡介........................................6 2-1.1 質量連續方程式(MASS CONTINUITY EQUATION)...........6 2-1.2 動量方程式(MOMENTUM EQUATION).....................7 2-1.3 紊流模式..........................................8 2-2 FLOW-3D核心技術....................................10 2-2.1 流體體積法.......................................10 2-2.2 FAVOR技術.......................................11 2-3 模擬條件設定.......................................12 2-3.1 物理模型與網格設置................................12 2-3.2 初始條件與邊界條件設定.............................15 2-3.3 數值求解設定.....................................16 第三章 模式驗證........................................17 3-1 網格大小...........................................17 3-2 紊流模式選取.......................................20 3-3 流場驗證...........................................22 3-3.1 自由液面特性.....................................23 3-3.2 速度剖面.........................................26 第四章 模擬結果分析與討論 ...............................29 4-1 潰壩流流場特性......................................30 4-1.1 速度場分析.......................................30 4-1.2 渦度分析.........................................33 4-2 潰壩流通過橋面版之流場特性...........................40 4-2.1 速度場分析.......................................40 4-2.2 渦度分析.........................................48 4-2.3 紊流能量分析.....................................56 4-2.4 紊流消散分析.....................................58 4-3 橋面版作用力分析....................................59 4-4 速度剖面分析.......................................68 第五章 結論與建議......................................71 5-1 結論..............................................71 5-2 建議..............................................72 參考文獻................................................7