Turbidity Current in a Settling Tank

本文描述了矩形沉淀池中的浑浊流。当含沙水进入沉淀池时，由于水比周围的水重，它会向下游流动。当浊流发展到很大程度时，它会将沉淀物从罐底重新悬浮或卷起进入主体流中，在水面上可能会产生珠状水区。这些现象降低了沉淀池的效率。在宽0.2m、长5m、深1.5m的模型沉淀池中进行了实验研究。测量了不同水流条件下水池内的流速分布和含沙量分布，在理论分析中采用了流体和泥沙的质量守恒定律和流体运动方程。对于密度差的处理，采用了Businesque近似，并假定槽内的湍流扩散系数为常数。在这些假设下，浊流只能用密度弗劳德数FDC和无量纲运动湍流扩散系数K两个参数来表示，在确认理论方程的数值解可以预测实验结果的基础上，利用理论模型对浊流进行了讨论。给出了浊流的最大流速Um与Fdc和k的函数关系，并给出了出现死水区的临界值。这些结果表明，浊流是随着Fdc和K值的减小而发展的。为了便于实际应用，给出了Um、Fdc和K值之间的简单关系式。作为这一课题的补充工作，还介绍了其他一些工作，如含悬浮泥沙的水平管道流和泥沙处置引起的浑浊流。

This paper describes the turbidity current in rectangular settling tank. When sediment-laden water flows into settling tank, it flows downstream as the water is heavier than the ambient one. When the turbidity current developed to the large extent, it resuspends or picks up settled particles from tank bottom into the bulk flow and the bead water zone may be produced at the water surface. These phenomena decreases in the efficiency of the settling tank. The objective of this work is make clear the basic properties of this kind of density current.The laboratory experiment are conducted in a model settling tank, which has 0.2m width, 5m length and 1.5m depth. Flow velocity profiles and sediment concentration distributions in a tank are measured under various flow conditions.In the theoretical analysis, the mass conservation for both fluid and sediment, and equation of motion of fluid were employed. To the treatment of the density difference, the Businesque approximation is used and the turbulent diffusivity is assumed to be constant through the tank. Under these assumptions, the turbidity current can be expressed by only two parameters, Fdc : The densimetric Froude number and K : nondimensional kinematic turbulent diffusibility.After confirming that the numerical solution of the theoretical equation can predict the experimental results, the turbidity current are discussed by using the theoretical model. The maximum velocity of turbidity current Um are given as a function of both Fdc and k, and critical values from which the dead water zone appears are presented. These results show that the turbidity current develops with decrease in the value of Fdc and K. For practical use, an approximate simple equation is given which gives the relationship between Um, Fdc and K.As an additional work on this topic, some other work, such as horizontal pipe line flow with suspended sediment and the turbidity current induced by sediment disposal are also presented.