The passive scalar transport across a sediment-water interface (SWI) is of fundamental importance to river environments. A model for scalar transfer from water to organic sediment bed is developed. The bed is formed by packed particles so that combined effect of permeability and roughness can be considered. Large-eddy simulation is used to get spatial and temporal information. The numerical results are compared to data from laboratory experiments, which stress the importance of considering bed roughness to better characterize the turbulent diffusion across the SWI. From the smooth regime to the transitionally regime, the turbulent diffusion exceeds the molecular diffusion and the dominating role at the interface changes from molecular diffusion to turbulent diffusion. In the completely rough regime, the molecular diffusive flux can be ignored compared with the turbulent flux. Moreover, turbulent Schmidt number is found to be higher near the SWI and lower around the middle of flow depth, ranging from 0.5 to 1. The results will be helpful to design the parameterization for the scalar transport over streams, rivers, or coastal areas where flow near the bed surface is relatively high and hyporheic exchange is affected by roughness and bed permeability.
- scalar transfer,roughness Reynolds number
- sediment-water interface
- turbulent flux