A DNS of an incompressible fluid flow over a Backward Facing Step (BFS ) with an Expansion Ratio (ER ) equal to 2, ER = H/(H - h), has been performed. Where h and H represent the step and the outlet heights, respectively. A turbulent channel flow at Ret = 395 has been used as an inflow. The DNS data has been compared with the results obtained by a DDES-SA turbulence model, to assess the non-zonal model accuracy. Particular attention has been paid to the Grey Area (GA ), where the effects of the temporal scheme have been analysed considering simplified tests (without turbulence model, i.e. coarse-DNS ). This study intends to shed light on the strong damping suffered by the resolved turbulence, which is severely affected by the Courant number (Co) and the time integration scheme. Moreover, the suitability of the implicit schemes in comparison of the explicit ones is also questioned, as Co < 1 are commonly typical in Hybrid RANS-LES simulations. Although the DDES-SA is the unique Hybrid model treated in this paper, the conclusions extracted from this study could also be applied to other Hybrid turbulence models.

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