Modeling transverse turbulent mixing by using an eddy viscosity approach

Although transverse mixing is a significant process in river engineering when dealing with the discharge of pollutants from point sources or the mixing of tributary inflows, no theoretical basis exists for the prediction of its rate, which is indeed based upon the results of experimental works carried on in laboratory channels or in streams and rivers. The chapter presents the results of a numerical study undertaken to simulate the transverse mixing of a steady-state point source of a tracer in a two-dimensional rectangular geometry. This geometry is that of Lau and Krishnappan, who collected turbulent mixing data for a shallow flow. In the numerical study an approach based on the Reynolds Averaged Navier-Stokes (RANS) equations was applied, where the closure problem was solved by using turbulent viscosity concept. Particularly, the classical twoequations k-ε model was used. Two methods were applied to the model results to evaluate the turbulent transverse mixing coefficient. The effect on transverse mixing of two different grids located upstream and in the section of the tracer source was also compared and studied, confirming that RANS-approach may provide acceptable results.