Review of backward-facing step flow and separation reduction

This article reviews the development of research work on backward-facing step flow and separation reduction techniques. Firstly, the effects of inflow conditions and geometry parameters on the bubble length, pressure coefficient, skin friction coefficient, velocity profiles and profiles of Reynolds stresses of the backward-facing step flow are identified. Secondly, we find that the unsteadiness of backward-facing step flow is dominated by shear layer mode and shedding mode, the former including Kelvin-Helmholtz instability, vortex pairing and step mode. Furthermore, the lower-frequency characteristics are consist of shear layer flapping and reattachment oscillating, resulting from the interaction between the coherent structures in the shear layer and the wall. Finally, the flow control practice on the backward-facing step flow has also been discussed. It is found that the recirculating bubble length can be significantly reduced with appropriate excitation frequency in the harmonic excitation practice. The optimum excitation frequency, Sth, is of about 0.2, and a physical explanation is given by the authors.