细颗粒悬沙浓度分布的影响因素分析

Analysis on the factors affecting the vertical concentration distribution of fine sediment

从颗粒运动的PDF(概率密度分布函数)输运方程出发建立颗粒相的质量、动量和脉动速度二阶矩方程。对明渠二维恒定均匀流,利用垂向动量方程导出了修正的泥沙扩散方程,表明颗粒脉动强度梯度、升力、有效重力和紊动扩散都影响悬沙运动。理论和数值分析了细颗粒泥沙的密度、粒径、浓度,挟沙水流的水深和剪切流速等因素对泥沙扩散系数和浓度分布形状的影响,并定量分析了细颗粒试验资料,表明传统扩散方程在粒径、剪切流速相对较大、水深和水沙密度比较小的流动中存在较大误差,需要修正。

Equations of mass, momentum and second moments for particulate phase are derived from a particle PDF (probability distribution function) equation in dilute turbulent sediment-laden flows. The equation of momentum in the vertical direction for two-dimensional steady and uniform open-channel flows is used to develop a modified sediment diffusion equation, which shows that the vertical gradient of particle velocity fluctuation and lift force contribute to the movement of suspended sediment as well as the effective gravity and turbulent diffusion. The influences of the density, diameter and average concentration of fine sediment, and those of the water depth and friction velocity on sediment diffusion coefficient and the concentration distribution patterns are analyzed theoretically and numerically. Experiment data of fine sediment is also analyzed quantitatively. It shows that the traditional diffusion equation brings remarkable error under flow conditions where friction velocity, particle diameter and sediment-water density ratio are relatively large and the water depth is small.