以三維賓漢流數值模式模擬海嘯沖刷坑之發展

3D Numerical Simulation on the Scouring Problem Induced by Tsunami Flood

陳孟志(Meng-Zhi Chen)

指導教授 ： 吳祚任

國立中央大學/地球科學學院/水文與海洋科學研究所/碩士(2011年)

摘要

本文將三維NS-VOF模式結合賓漢雙黏性流模式，以探討海嘯湧潮與颱風洪水造成結構物基礎沖刷之問題，模式以兩黏性參數以及降伏應力表示泥沙受水流沖刷之運動行為，其中 為牛頓流體運動行為與 為泥沙固態之特性， 表示泥沙被水流帶起之臨界應力值。在進行研究之前，本文先進行模式驗證。自由液面驗證方面，本研究設計潰壩湧潮撞擊結構物陣列之實驗，以驗證模式預測自由液面發展之預測準確度，其模擬結果與實驗高速攝影機捕捉之水位十分吻合。在賓漢雙黏性流方面，與 Bird et al.（1983）所推導之解析解驗證，模擬求得之流速與其求得之解析解流速相當一致，確認模式發展無誤。在沖刷坑發展方面，採用Dey and Barbhuiya (2005) 所發表水流對於圓形橋墩之沖刷坑實驗作為驗證對象，其結果表示模式能適切描述底床沖淤之過程與範圍，且能預測最大之沖刷深度，然於下游堆積處有較明顯之差異。本模式對於水流衝擊結構物之墩前壅水現象、向下射流與沖刷坑周圍近似馬蹄型渦流之效應產生皆能確切描述。
模式驗證並瞭解其準確性後，本文以2009莫拉克颱洪之雙園大橋斷橋事件為三維真實尺寸為研究對象，分別探討未補樁之初始橋墩與補樁擴座後之兩種橋墩型態對於沖刷坑變化之影響，並進一步分析基樁之水流壓力與公路橋梁設計規範（交通部，2009）之經驗公式值之適用性。後續並探討洪水挾帶漂流物掛淤於結構物間對於底床沖刷變化之影響，其中掛淤位置分別為浮木掛淤於兩座橋墩間、浮木掛淤於橋墩上游側、浮木掛淤於群基樁間及浮木掛淤於橋墩跨距間之極端掛淤案例四類逐一探討，研究結果發現浮木掛淤於兩座橋墩間、掛淤於上游與掛淤於群基樁間對沖刷坑並無顯著之影響，然而增加浮木掛淤數量則會加速沖刷坑發展且使沖刷坑深度增大。
由以上驗證與案例分析可知，賓漢雙黏性流模式能適切的探討洪流與結構物交互作用之沖刷坑發展過程，且能避免使用過多經驗公式以及經驗參數。研究亦證明本模式能廣泛運用於湧潮或海嘯對於沖刷結構物基礎之沖刷深度預測。

關鍵字

局部沖刷 ； 賓漢流 ； 洪水 ； 海嘯湧潮 ； 沖刷坑 ； 體積分率法(VOF)

並列摘要

We integrate the 3D NS-VOF model with Bingham Bi-viscous fluid model to investigate the scouring problems around the structures induced by tsunami bores or typhoon floods. In this model, the scour mechanism is modeled by two viscosities and a yield stress. In which is used to describe Newtonian viscosity of liquefied sediment, and is for the solid-state properties of the sediment, and is for presenting the yield stress induced by the current. Before implementing this model, benchmark tests are performed. For validating the model accuracy on predicting the free-surface kinematics, we designed a case in which a dam-break bore is impinging with a structure array. A high-speed camera is used to record the free-surface movement. Very good agreement between the model prediction and experimental data can be seen. For validating the Bingham Bi-viscous fluid model, we compare the result with the analytical solution derived by Bird et al.（1983）. The comparison shows that the numerical result is almost identical to the analytical solution in terms of the velocity distribution. For validating the profile of scour hold, we simulate the case proposed by Dey and Barbhuiya (2005) in which the current is flowing around a circular pier. The result shows that our model is able to predict the maximum scouring hole and properly describe the scour mechanism and also the scour area around the cylinder. However, larger discrepancy can be found in the accumulation zone at the downstream direction. In this case, the present model can accurately present several phenomena, such as the bow wave, downword jet current, and horseshoe vortex around the cylinder.
After validating the model, we study the event of Shuangyuan bridge failure caused by 2009 Moarkot typhoon flood. We focus the discussion on the shape difference of the scour holes around the origional of bridge plies and the extended bridge piles. We also explore the suitability of using the empirical formula issued by the government (Ministry of transportation and communications R. O. C., 2009) We further study the the blocking effect from the drifting obstacles. Four scenarios are studied for understanding the effect from different stocking locations. The result shows no significant difference for difference stocking locations. However, adding the quantity of the drifting obstacles will increase the scouring rate and increase scour depth.
Overall speaking, Bingham Bi-viscous fluid model can be used to explore the development of scour hold, and avoid using too many empirical formulations and parameters. We also demonstrate that the present model can be widely used to predict the local scour depth induced by floods or tsunami bores.

並列關鍵字

Local Scour ； Volume of Fluid (VOF) ； Scour Hole ； Tsunami Bore ； Bingham Flow ； Flood

參考文獻

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