明渠中悬移质的弥散-对流方程及悬浮机理

THE CONVECTION-DISPERSION EQUATION AND THE MECHANISM OF SUSPENSION IN TURBULENT OPEN-CHANNELS

悬移质泥沙通常构成冲积河流总输沙量的主体,研究悬移质的悬浮机理具有重要的意义.以双流体模型为基础,通过引入弥散速度的概念,建立了悬移质泥沙的输沙方程以及泥沙扩散系数的本构关系.应用该方程分析了二维明渠均匀流中悬移质泥沙浓度垂向分布规律,并与Einstein和Chien的泥沙浓度实验资料及经典扩散理论进行了对比.以此为基础,分析了紊动扩散、颗粒自身的紊动、颗粒碰撞应力对泥沙悬浮的影响在垂向上的变化,以及浓度、粒径等对这些因素的影响.结果表明,泥沙颗粒在明渠紊流中的扩散是浑水的紊动扩散、颗粒自身的紊动、颗粒碰撞应力3部分不同机制共同作用的结果,把泥沙颗粒的悬浮简单归因于水流的紊动是不全面的.

The suspended sediment usually occupies a large portion of the total load transported by streams in large fluvial rivers. Therefore, it is of crucial importance to study the mechanism involved in the suspension of suspended sediment. In this paper, we derive an equation for suspended sediment transportation and the sediment diffusion coefficient based on the two-fluid model by introducing the concept of dispersion velocity, and apply the theory to calculate the concentration profiles of uniform two-dimensional open channel flows. Comparisons with the experimental data from Einstein and Chien and classical diffusion models are also presented, which show that present theory agrees well with the experimental data. In addition, we analyze the variations of influences of different effects contributing to the suspended sediment in the vertical direction. It is found that the suspension of the sediment in open channels is the result of three different actions, which are the turbulence diffusion of mixture, particle turbulence, and particle collisional stress. The result also shows that it is insufficient to ascribe the sediment in suspension solely to the turbulence of the flow.