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研究生：	吳昫穎 Wu, Thu-Yin
論文名稱：	從水動力觀點探討樁柱鄰近地形淘刷行為之特性 Scour hole development around piles - in the perspectives of hydrodynamics
指導教授：	楊瑞源 Yang, Ray-Yeng
學位類別：	碩士 Master
系所名稱：	工學院 - 水利及海洋工程學系 Department of Hydraulic & Ocean Engineering
論文出版年：	2020
畢業學年度：	108
語文別：	英文
論文頁數：	53
中文關鍵詞：	海洋結構物、淘刷、Flow-3D 、淘刷數值模式
外文關鍵詞：	Scour, Flow-3D, Scour development, numerical model
相關次數：	點閱：163 下載：19
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海洋結構物與離岸風機基座常受到波浪和海流作用之影響。在台灣大多數開發中的離岸風場之風機基礎或海洋結構物的水深都超過15m。在這種水深下，海洋結構物基礎周圍的沖刷主要受到海流與波流交互作用之影響，將會導致基礎損壞或坍塌問題。

本研究主要研究圓柱結構物受海流作用下的淘刷行為，於數值軟體Flow-3D中重現Chen et al (2019) 於成大水工試驗所風波流水槽中進行的實驗，比較了RNG與k-ε，紊流模型以及不同輸沙模型之間的差異並以實驗數據加以驗證。再由數值軟體計算渦度、剪力等等較難由實驗中精準測得之數據。同時本文也比較了將兩根圓柱布並列排放在較小的距離下(G/D < 1.5)對淘刷坑以及剪力、渦度等的影響。
數值結果顯示剪力會集中於圓柱的兩側以及下游淘刷坑的上坡部分，而當兩根圓柱接近，流速所造成的剪力會由於流線限縮而增大。本文將渦度分為在YZ平面上旋轉的ω_x以及在XZ平面上旋轉的 ω_y進行討論。在流體流經圓柱後會形成上下成對方向不同之渦漩，此渦漩會隨著遠離圓柱而減弱，而在兩根圓柱較為接近的狀況下ω_x會被限縮在圓柱後方而形成一深而窄之淘刷坑。在沙面上會有一較強之ω_y出現，當淘刷更深之後此較強之渦流便會消失，可以推測淘刷坑的加深與ω_y有密切的關係。淘刷坑在結構物附近成長時ω_x 、ω_y皆集中於其成長位置，而剪力則因邊界層邊界過細而無法在此模型中精確描述。

本文以數值軟體Flow-3D模擬水流流經對圓柱的淘刷行為雖然就網格設定上有一定的不足之處，但對淘刷坑附近的渦度行為有更加清楚的了解。希望在增進網格設計之後能進而對波浪作用下甚至對波流交互作用下的淘刷特性進行研究以助於海工結構物的設計規劃。

Scouring is a phenomenon that fluid brings sediment away at the bottom of the structure. It is an important issue in subsea and marine engineering, it may affect the stability of offshore structure. The paper presents the numerical modelling of current induce scour around monopile and piles group in a side by side arrangement, using the computational fluid dynamic software Flow-3D. The software solves Navior-Stoke equation, RNG model and k-ε model are test in order to choose a better turbulent model to simulate the initial progress of scour. A numerical water tank with chosen sediment is established for a simulation. The geometry and development process of scour hole were verified with the model test in Chen et al (2019).
Vortex and shear stress are two variables of scour hole development. In order to determine the influence of each variable on scouring, shear stress and vortex are separate studied in different position. The result using 2cm uniform grid indicate vorticity is a reason induce scour around cylinder(s) but the Reynolds stress is unable to observed in such grid size. The mechanism of vorticity in different direction are also studied for a deeper understanding for local scour. The integration of vortex and shear stress effect will cause the scour hole locates at downstream of piles. The reason of scour hole geometry difference between gape ratios are caused by the vortex concentration after piles, the narrower gape will induce a stronger vortex group after pile, will form a deeper but narrower scour hole after piles group with a wider mound out of the scour hole.

ABSTRACT…………………………………………………………………I
中文摘要…………………………………………………………………………………II
ACKNOWLEDGEMENT………………………………………………………III
CONTENTENTS………………………………………………………………………IV
LIST OF TABLES………………………………………………………………V
LIST OF FIGURES………………………………………………………VI
LIST OF SYMBOLS………………………………………………………… IX

Chapter 1 introduction………………………………………………………………1
1.1 Research motivation and purpose………………………………………1
1.2	Literature review…………………………………………………………………2
1.2.1	Scour phenomenon is steady current……………………………………2
1.2.2	Numerical study for scouring…………………………………………….4
1.3	Outline of the study………………………………………………………………6
Chapter 2 Methodology………………………………………………………………7
2.1	Numerical model…………………………………………………………………7
2.1.1 Introduction of Flow-3D……………………………………………………7
2.1.2 Governing equation used in Flow-3D………………………………………7
2.1.3 Turbulent transport model…………………………………………………12
2.1.4 Sediment scour model……………………………………………………14
2.2	Model setup……………………………………………………………………18
2.2.1 Verification………………………………………………………………22
Chapter 3 Result and Discussion…………………………………………28
	3.1 Amplification factor……………………………………………………………29
	3.2 Vortex mechanism………………………………………………………………33
	3.2.1 Vortex mechanism in YZ plane…………………………………………...33
	3.2.2 Vortex mechanism in XZ plane………………..37
	3.2.3 Comparison of X, Y vorticity.……….………….39
	3.3 Scour hole development………………………………………………………….43
3.3.1 scourhole development around structure………………………….43
3.3.2 scourhole development after piles group…………………………………44
Chapter 4 Conclusion and Suggestion…..……………………………….48
References……………………………………………………………………………50

                                    

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