COMPUTATION OF DYE CONCENTRATION IN SHALLOW RECIRCULATING FLOW

The Lagrangian second-moment method is used to compute the dye-concent ration distribution in turbulent recirculating flow generated in a sha llow open channel by a sudden widening of how in the transverse direct ion. The performance of the second-moment method is compared with the Eulerian numerical schemes of Upwind, QUICK, HLPA, and the semi-Lagran gian Hermite scheme. The results show that the Lagrangian second-momen t method is an accurate and a very promising scheme for the computatio n of concentration distribution in flows dominated by advection. It mi nimizes the generation of numerical diffusion, is free from numerical oscillations, and is independent of the value of Courant number. Its e fficiency in terms of computational time is high in flows where the mi xing is confined to a small region of the solution domain and low in h ows where the mixing is extended in the whole solution domain. In addi tion to an accurate numerical scheme, a necessary requirement for the accurate prediction of concentration field is a hydrodynamic model abl e to determine correctly the velocity held and the diffusion coefficie nt in shallow recirculating flows.