A NONREFLECTING OUTLET BOUNDARY-CONDITION FOR INCOMPRESSIBLE UNSTEADY NAVIER-STOKES CALCULATIONS

The goal of this work is to adapt a nonreflecting outlet boundary condition, derived from a wave equation, to the numerical solution of the full incompressible Navier-Stokes equations, for an elliptic unsteady free shear flow. The numerical results show that a significant improvement is achieved with this nonreflecting boundary condition, in comparison with the results obtained by using free boundary layer type conditions. The physical phenomena studied concern the onset of the Kelvin-Helmholtz instability in the free (non-forced) shear layer and certain 2D characteristics of transition towards turbulence. These phenomena are simulated naturally, without imposing perturbations. The frequency of the organized vortices and the spread of the mixing layer are correctly predicted. The performances of the method are shown through comparison with the physical experiments. Owing to the nonreflecting boundary conditions, the feedback noises are inhibited effectively, so that the computation domain can be reduced and the dynamic characteristics of the flow are maintained up clearly to the outlet boundary. © 1993 Academic Press, Inc.