The CFD analysis of flow over a two dimensional backward facing step model having an expansion ratio is done by using Star CCM+. The velocity distribution, recirculation, and reattachment over different models of turbulent and laminar is analysed. In this analysis the velocity is calculated with Reynolds number of Turbulence and laminar. In Turbulence K-epsilon, K-omega, S-A Turbulence models’ computations are performed, and the results are compared in accordance with Navier-stokes equation. The laminar model is also analysed, and comparison of laminar and turbulence flow is Concluded. The Reattachment length is calculated and compared for all the turbulence and laminar models

Keywords : Star CCM+, Reynolds number, Expansion ratio, Recirculation and Reattachment