振荡潜流带沉积层?水界面污染物输运的研究

潜流带中污染物质交换与输运特性是影响水资源环境的重要问题之一. 本文对底部为高渗透沉积层的三维槽道振荡流高Schmidt数传质问题进行了大涡模拟研究. 采用动力学亚格子模型来封闭滤波后的三维不可压缩Navier-Stokes方程以及污染物输运方程, 同时采用修正的Darcy-Brinkman-Forcheimer模型来描述沉淀有锌离子污染溶质的可渗透沉积层. 通过对沉积层内外流场和浓度场的统计特性以及瞬态结构的分析, 探究了上覆水体中振荡流驱动作用对污染物输运的动力学影响以及扩散率随振荡周期和振荡角的变化规律. 研究结果表明, 浓度通量中的湍流浓度分量在垂向物质交换中起主导作用, 流向、展向速度, 湍流强度和污染物浓度的波动跟随振荡驱动力呈现准周期变化, 同时发现沉积层?水交界面处的湍流浓度通量与法向湍流强度两者之间的变化具有明确的相关性. 并且在较大振荡角和低频振荡的情况下, 沉积层?水交界面处的有效扩散率增大, 这主要是来自于沉积层?水交界面处流体的猝发行为有效促进了湍流混合和物质交换, 进而增强了污染物的垂向输运. 

STUDY ON CONTAMINANT TRANSPORT AT THE SEDIMENT-WATER INTERFACE IN OSCILLATING FLOW

The exchange and transport characteristics of pollutants in the hyporheic zone are one of the important issues affecting the water resources environment. In this paper, the high Schmidt number mass transfer phenomenon of a channel oscillatory flow with a highly permeable sediment layer at the bottom is numerically investigated by using large eddy simulations. A modified Darcy-Brinkman-Forcheimer model describes the bottom highly permeable sediment layer using the volume-averaged Navier-Stokes equation and convective diffusion equation for the flow and transport of metal ion contaminants within the sediment layer. We explore the statistical characteristics and the instantaneous structure of flow field and concentration field, and the dynamic influence of oscillating flow inside and outside the sediment-water interface on the transport of pollutants. In addition, the variation of the effective diffusivity at the sediment-water interface with oscillation period and oscillation angle is also studied. The results show that the turbulence component of concentration flux plays a dominant role in vertical mass transport, and the streamwise and spanwise velocity, the fluctuations of turbulence intensity and pollutant concentration follow the quasi-periodic variation of periodic oscillation driving force. At the same time, it is found that there is a clear correlation between the variation of turbulent concentration flux at the sediment-water interface and the intensity of normal turbulence, and the effective diffusivity at the sediment-water interface increases at larger oscillation angles and low-frequency oscillations, which mainly comes from the burst behavior of the fluid at the sediment-water interface that significantly promotes turbulent mixing and material exchange, and then the concentration scalar is acted on by a convection-diffusion mechanism, which in turn enhances the vertical mass transport of pollutants.