The effect of inlet pulsations on the backward-facing step flow

This experimental study of backward-facing step flow is focused on the transient flow regime (RehReh from 30 to 1800). The electrodiffusion technique is applied to measure the wall shear rate in a water channel with the most common expansion geometry (ER=2ER=2). The direction-sensitive wall probes are able to recognize the individual regions of flow separation and reattachment behind the step. The bottom and roof wall shear rate profiles are obtained under steady and pulsatile flow conditions at the inlet. The near-wall extent of primary (mean reattachment length) and secondary (corner and roof eddy location) flow-recirculation zones is determined from these profiles. The high level of wall shear rate is observed inside the reverse flow regions even at moderate Reynolds numbers. The spectral analysis of natural flow fluctuations reveals that the low frequencies characterized by Sth∼0.15Sth∼0.15 dominate the reattachment region. The inlet flow pulsations affect strongly the overall flow structure behind the step. Up to 80% reduction of the reattachment length is achieved by applying flow pulsations at the most effective frequency. Two flow instability modes are found to be important to control this frequency: the “step mode” dominates the laminar flow regime, whereas the “shear layer mode” prevails at the transient flow conditions. The frequency of “step mode” coincides with the scaling recently suggested for the global instability of backward-facing step flows.