水库异重流的三维数值模拟及影响因素分析

不同水沙条件下异重流的潜入位置以及潜入后流速和含沙量的垂向分布规律是研究异重流运动规律的关键要素,对优化水库异重流的排沙调度、提高水库运用效益等至关重要。本文利用SCHISM三维水沙数学模型对水库异重流水槽试验进行了数值模拟,结果表明模型能够得到符合实际情况的垂线流速和含沙量分布,且异重流潜入后的平均厚度、平均流速和平均含沙量均与实测值吻合较好。对异重流模拟结果的分析发现,入口流量和含沙量的增加造成异重流运动速度越快,而入口含沙量的减小和流量的增加会造成异重流厚度增加。异重流垂向流速和含沙量的无量纲化分布受到入口水沙条件的影响,表现在其与断面Froude数的相关性,Froude数越大,则最大流速点与河床的距离越小,最大流速越大;含沙量致密层的厚度越小,对应的含沙量也越大。此外,潜入点位置也受入口水沙条件的影响,表现在潜入水深与入口水深之比随入口Froude数增加而增加的定量关系,由此可以利用入口水沙条件预报异重流潜入的位置。研究结果不仅验证了SCHISM模型用于模拟水库异重流运动的优势,而且丰富和完善了关于异重流潜入点位置及流速和含沙量垂向分布受入口水沙条件影响的变化规律。

Three-dimensional numerical simulation of density current on a slope and its influencing factors

The plunge points and vertical profiles of the velocity and sediment concentration under different inflow conditions are both key factors of the water and sediment transport rules of the density currents, which are fundamental to optimize the operation and improve the efficiency in the reservoir. A three-dimensional numerical model named SCHISM was adopted to simulate the laboratory experiments of density currents on a slope. The calculated depth-averaged thickness,velocity and sediment concentration of the density currents agreed well with the measured results in experiments. And the simulated vertical profiles of the velocity and sediment concentration in the density current also fit the measured profiles. Model results under different inflow conditions are compared to find out that the higher inflow discharge and sediment concentration both result in the higher velocity of the density current. And the lower inflow sediment concentration and higher discharge cause the increasing thickness of the density current. Further analysis indicates that the dimensionless vertical profiles of the velocity and sediment concentration are also related to the inflow conditions, which can be represented by densimetric Froude number. With the higher Froude number, the height of the velocity maximum is lower and the denser layer is thinner,but the corresponding velocity peak and the sediment concentration of the denser layer are both higher. In addition, the inflow conditions also influence the plunge points. A power function is proposed that the ratio of the plunge depth to inlet depth increases with the increase of the inlet Froude number. This relationship can be used to predict the plunge depth based on the inflow conditions. In this paper,SCHISM is successfully applied to simulate the laboratory-scale density currents and can provide advice for further application in the density currents in the large-scale reservoirs. The original conclusions from the experiments are also extended with the deepening understanding of the effects of the inflow conditions on the density currents.