Flow structure and unsteady fluctuation with separation over a two-dimensional backward-facing step

The flow over a backward-facing step (BFS) is a typical separation and reattachment flow. Its flow structures and unsteady mechanisms are still not well explored. In this paper, the global velocity fields of a BFS are obtained by a synchronous particle image velocimetry (PIV) system with Re-h = 5345 (Reynolds number) and E-r = 2 (expansion ratio). Flow structures are distinguished and defined by the fraction of the negative velocity (u(pn)). The reattachment zone (L-r) is quantitatively defined as (u(0.9),u(0.1)) on the bottom wall. Spatial distribution of the large-scale vortices couples well with the divided flow structures and their temporal evolution presenting four stages (forming, developing, shedding and redeveloping) when travel downstream. The unsteady motions with various low frequencies are well explained by the coherent vortices and flow structures. Among the unsteady low frequency motions, the Kelvin Helmholtz (KH) vortices and the oscillation of X-r (OX) come likely from the free shear layer. The KH vortices contribute to the unsteadiness of the temporal flow, and the OX is the primary response to the vortical fluctuations.