Large eddy simulation of dispersion in free surface shear flow

Abstract

Wastewater and waste heat are frequently discharged into ambient water and affect the water quality there. An accurate evaluation of the turbulent mixing and dispersion processes is one of the key factors for assessing the environmental impact of these discharges. To achieve this objective a threedimensional numerical model incorporating the method of Large Eddy Simulation has been developed. In this method the large scale turbulence is computed explicitly and the subgrid scale turbulence is modelled. The empiricism incurred for the specification of the dispersion and turbulent mixing coefficients is thus reduced to minimal. The governing equations are split into three parts in the finite difference solution: advection, dispersion and propagation. The advection part is solved by a characteristics-based scheme. The dispersion part is solved by the central difference method and the propagation part is solved implicitly by using the Gauss-Seidel iteration method. The model has been applied to simulate a continuous line source in free surface shear flow. The computed results demonstrate the existence of the non-Fickian diffusion and dispersion region close to the source. Further downstream the transverse diffusion process obeys Fick's Law and the transverse diffusion coefficient is in agreement with the empirical value measured in laboratory. The initial non-Fickian diffusion and dispersion region is further analysed based the coherence of the velocity data. The dispersion coefficient is found to follow the Okubo's 4/3 power law, but with a much larger coefficient owing to the shear effect.