含沙異重流之紊流解析模擬

Abstract

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.