Induced Global Unsteadiness and Sidewall Effects in the Backward-Facing Step Flow: Experiments and Numerical Simulations

A detailed experimental and numerical study of three-dimensional laminar flow over a backward-facing step with an expansion ratio of 2 and a downstream aspect ratio of 20 is presented. Experimental results for transitional and turbulent flow are also reported. Experiments were based on both Digital and Stereoscopic Particle Image Velocimetry; a spectral element method was used for the simulations. The focus of the present work was two-fold: (i) A passive control scheme for inducing global unsteadiness, suggested in previous work, was implemented, and the controlled flow was studied in the laminar and turbulent regimes. (ii) In the uncontrolled flow, the effects of sidewalls on the flow structure were studied at laminar, transitional, and turbulent Reynolds number values. The control scheme consisted of external flow recirculation due to suction at the step wall and blowing at the lower wall. The problem geometry was fixed and, thus, for a given Reynolds number, the flow dynamics depended only on the recirculation volume flow rate. For laminar flow, periodic vortex shedding from the step edge was observed within a range of recirculation flow rates, both in experiments and two-dimensional simulations. Global stability analysis calculations support the present simulation results. The experimental laminar flow was found to be strongly three-dimensional, exhibiting streamwise vortices in the shear layer regions. For turbulent flow, the present control scheme significantly increased mixing in the region immediately after the sudden expansion. This led to a decrease in reattachment length of the order of 70% in comparison to the non-manipulated flow. The control scheme was found to be robust to variations in the suction/blowing velocity boundary conditions. In the present study of sidewall effects, both experimental and computational results illustrated that, for laminar flow, a wall-jet is present at the channel lower wall, directed towards the channel mid-plane, in agreement with previous observations. In the present work, the development of self-similar wall-jet profiles with increasing distance from the sidewalls is demonstrated. The intensity of this secondary flow increased with Reynolds number in the laminar regime, and decreased in the transitional and turbulent regimes. The effect of sidewalls on the primary and upper wall recirculation zones is demonstrated, which explains the discrepancies in the previous literature between experiments and two-dimensional simulations.