指導教授：周逸儒臺灣大學：應用力學研究所陳聖堯Chen, Saint-YaoSaint-YaoChen2014-11-302018-06-292014-11-302018-06-292014http://ntur.lib.ntu.edu.tw//handle/246246/264094本研究利用大渦流模式(Large eddy simulation)模擬含沙異重流(hyperpycnal plume)於不同斜坡上之行為，其斜坡之斜率分別為0.005及0.01。為了解含沙異重流中的潛入點(plunging point)、本體(main body)以及前端區域(front region)之動力行為，模擬結果將分為兩個部份進行討論。 首先計算捲水係數(entrainment coefficient)以及福祿數(Froude number)在這三種不同區域中之變化，探討含沙異重流與外圍流體之間的動力行為。研究結果顯示，不同斜率下在潛入點附近都有捲水之現象產生，且界面拖曳力(interfacial drag)隨斜率增加而增加，造成陡坡的捲水係數高於緩坡。而在前端區域時，兩種坡度呈現不一樣的結果，緩坡的前端區域顯示大部分的質量被捲出至外圍流體，而陡坡的前端區域則顯示外圍流體被捲入至含沙異重流中。 接著，本文分析含沙異重流之縱向平均速度分佈、泥沙濃度分布以及密度場分布，探討不同斜率下三種不同區域之變化情形。分析結果顯示，下坡的重力隨斜率增加而增加，造成陡坡之平均速度高於緩坡，另外，因混合現象發生於前端區域，造成泥沙濃度及密度在此區明顯低於其他兩區。最後，紊流能量收支平衡(turbulent kinetic energy budget)的分析顯示，紊流主要形成於含沙異重流與外圍流體之界面上。除此之外，潛入點附近有很強之向下的垂直速度，這說明了為何在潛入點附近會有捲水現象之發生。另外，比較兩者不同斜率下之含沙異重流行進距離，發現因下坡的重力增加造成含沙異重流在陡坡的行進距離長於緩坡。In the present study, large eddy simulation (LES) is employed to study the behavior of the hyperpycnal plume under different bottom slope conditions. The bottom slopes of 0.005 and 0.01 are chosen. In order to understand the dynamics of the undercurrent in regions of the plunging point, main body, and front point, the results are divided into two parts for the discussion. First, we estimate the entrainment coefficient of the ambient fluid and the Froude number to investigate the dynamic behavior between the hyperpycnal plume and their surroundings. Both of the gentle and steep cases show that the entrainment of the ambient fluid occurs near the plunging region. The interfacial drag increases as the slope increases, which results in high entrainment coefficients on the steep slope. The entrainment coefficient of the gentle slope in the front region shows that considerable mass of the hyperpycnal plume is detrained into the ambient fluid, while the steep case indicates that the ambient fluid is entrained into the hyperpycnal plume. Second, the vertical distribution of the mean velocity, the sediment concentration, and the density field are investigated. The results indicate that the mean velocity of the steep slope is higher than that of the gentle slope, which is because the downslope gravitational force increases as the slope increases. Moreover, lower sediment concentration in the front region indicates occurrence of mixing. The TKE budget analysis shows that turbulence is mainly generated at the interface between the clear-water and sediment-containing layer. Additionally, the strong downward velocity in the plunging region explains why the entrainment of the ambient fluid occurs. Furthermore, comparison of the run-out distance between two different bottom slopes shows that the steep slope generates larger downslope gravitational force that drives a longer run-out distance.口試委員會審定書 i Acknowledgments ii 中文摘要 iii Abstract iv Contents vi List of Figures ix List of Tables xiv Chapter 1 Introduction 1 1.1 Background 1 1.1.1 Laboratory experiments 2 1.1.2 Numerical models 3 1.2 Motivation and Objectives 4 1.3 Outline 8 Chapter 2 Turbulence Model 9 2.1 Introduction 9 2.2 Mathematical Formulation 10 2.3 Boussinseq Approximation 11 2.4 Filtered Governing Equations 12 2.5 Governing Equations in Curvilinear Coordinates 13 2.6 Density Stratification 15 2.7 Model Description 15 2.7.1 LES code 15 Chapter 3 Model Validation 17 3.1 Introduction 17 3.2 Review of Turbulence Scales and TKE Spectra 18 3.2.1 Energy cascade 18 3.2.2 The Kolmogorov hypotheses 18 3.2.3 The energy spectrum 20 3.3 The Fluctuating Velocity Spectra Analysis 22 3.3.1 Case I 22 3.3.2 Case II 25 3.4 Vertical Profiles 27 3.4.1 Simulation parameters 27 3.4.2 The simulation results for the saline current 28 3.4.3 The vertical profiles of the saline current 29 3.4.4 The simulation results for the turbidity current 30 3.4.5 The vertical profiles of the turbidity current 31 3.5 Summary 32 Chapter 4 The Dynamics of Turbidity Currents 34 4.1 Introduction 34 4.2 Computational Setup 34 4.2.1 Simulation domain and parameters 34 4.3 The definition of undercurrent boundary 36 4.4 Entrainment Evaluations 39 4.4.1 Gentle case (S=0.005) 40 4.4.2 Steep case (S=0.01) 40 4.5 Summary 43 Chapter 5 The structure of turbidity currents 44 5.1 Introduction 44 5.2 Vertical Profiles 44 5.2.1 Turbulent kinetic energy budget (TKE) 44 5.2.2 Components of the subgrid-scale (SGS) stress 45 5.2.3 Gentle case (S=0.005) 46 5.2.4 Steep case (S=0.01) 57 5.3 Summary 68 Chapter 6 Conclusions and Suggestions 69 6.1 Conclusions 69 6.1.1 Dynamics 69 6.1.2 Vertical distributions 70 6.2 Suggestions for Future Study 70 Reference 722781177 bytesapplication/pdf論文公開時間：2019/03/21論文使用權限：同意有償授權(權利金給回饋學校)含沙異重流大渦流模式捲水係數福祿數紊流能量收支平衡thesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/264094/1/ntu-103-R00543082-1.pdf