A NUMERICAL INVESTIGATION OF THE COHERENT VORTICES IN TURBULENCE BEHIND A BACKWARD-FACING STEP

This paper presents a statistical and topological study of a complex t urbulent flow over a backward-facing step by means of direct and large -eddy simulations. Direct simulations are first performed for an isoth ermal two-dimensional case. In this case, shedding of coherent vortice s in the mixing layer is demonstrated. Both direct and large-eddy simu lations are then carried out in three dimensions. The subgrid-scale mo del used is the structure-function model proposed by Metais & Lesieur (1992). Low-step computations corresponding to the geometry of Eaton & Johnston's (1980) laboratory experiment give turbulence statistics in better agreement with the experimental data than both Smagorinsky's m ethod and K- epsilon modelling. Furthermore, calculations for a high s tep show that the eddy structure of the flow presents striking analogi es with forced plane mixing layers: large billows are shed behind the step with intense longitudinal vortices strained between them.