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.