近床二維有序流結構對非均勻砂礫啟動之影響

Abstract

本研究探討二維紊流突暴對非均勻砂礫啟動之影響，研究中採用二維Gram-Charlier聯合機率密度函數描述近床紊流速度之分布。本研究利用前人實驗資料分析近床有序流結構，包括水流方向與垂直方向速度之二階、三階動差與動差交錯項(紊流強度、偏態係數與擴散係數)，將此參數代入二維Gram-Charlier三階聯合機率密度函數，並配合顆粒啟動之機制，計算顆粒啟動機率，並與前人之研究結果進行比較。本研究納入底床幾何條件之隨機特性，與非均勻顆粒間的遮蔽效應，預測非均勻顆粒在部分運移與全運移兩種情況下之啟動機率，並利用實驗數據驗證計算結果。本研究並探討二維流況中紊流四事件所佔的比例，預測結果與實驗數據相當吻合，顯示本模式可以準確描述近床區域之紊流速度分布，並可預測紊流突暴四事件對顆粒啟動之貢獻。

In this study we investigate the effect of near-bed 2D coherent structures on the entrainment of mixed-size sediments. The 2D Gram-Charlier joint probability density function is employed to describe the near-bed velocity. The experimental data is used to analyze the relation between the near-bed coherent flow structures, including second-, and third-order moments of velocity fluctuation (i.e. turbulent intensity, skewness, diffusion), and the roughness Reynolds number required in the 2D third-order Gram-Charlier joint probability. The entrainment probabilities of mixed-size sediments are computed and compared with previous results. The random mean bed surface is parameterized with exposure height and friction height, and the hiding effect of mixed-size sediments is taken into consideration for prediction of the entrainment probabilities of sediments under partial- and full-transport conditions. In this work we present the quadrant analysis for turbulent. The results obtained coincide well with the published data. The present model is able to describe the velocity distribution in the near-bed region, and to predict the contribution of four bursting events on sediment entrainment.