Numerical simulation of complex turbulent Flow over a backward-facing step

A statistical and topological study of a complex turbulent flow over a backward-facing step is realized by means of Direct and Large-Eddy Simulations. Direct simulations are performed in an isothermal and in a stratified two-dimensional case. In the isothermal case coherent structures have been obtained by the numerical simulation in the mixing layer downstream of the step. In a second step a thermal stratification is imposed on this flow. The coherent structures are in this case produced in the immediate vicinity of the step and disappear dowstream for increasing stratification. Afterwards, large-eddy simulations are carried out in the three-dimensional case. The subgrid-scale model is a local adaptation to the physical space of the spectral eddy-viscosity concept. The statistics of turbulence are in good agreement with the experimental data, corresponding to a small step configuration. Furthermore, calculations at higher step configuration show that the eddy structure of the flow presents striking analogies with the plane shear layers, with large billows shed behind the step, and intense longitudinal vortices strained between these billows