Numerical Modeling of Sedimentation Tanks

Abstract

The numerical simulation of a sedimentation tank involves determining the velocity field and the sediment concentration distribution for the tank. In this study, the vertical velocity field is established by a finite difference model of the vorticity‐transport stream function equations with a constant eddy viscosity. The eddy viscosity is obtained with the aid of a physical model. The numerical model is verified experimentally. The vorticity transport is treated by the Alternating Direction Implicit (ADI) method. A variable grid is used to maximize the effectiveness of the computational nodes. A weighted centereddifference‐upwind scheme is used for the convective terms. The two‐dimensional sediment transport (convective‐diffusion) equation is solved by the same numerical approach used for the vorticity transport equation. The bottom boundary condition permitted re‐entrainment of settled solids. The model can be used for steady or unsteady solids transport. The model is applied to study the effect of reaction baffle submergence on solid removal. The model output is also compared with the Camp‐Dobbins method of determining solids removed in turbulent flow. The removal predicted by the numerical model is consistently less than that predicted by the Camp‐Dobbins approach.