Sidewall effects in flow over a backward-facing step: Experiments and numerical simulations

We present detailed experimental and numerical results of three-dimensional laminar flow over a backward-facing step with an expansion ratio of 2 and a downstream aspect ratio of 20. Experimental results for transitional and turbulent flow are also reported. We focus on the effects of sidewalls on flow structure. Experiments were based on both digital and stereoscopic particle image velocimetry; a spectral element method was used for the simulations. For laminar flow, both experimental and computational results illustrate the presence of a wall-jet at the channel lower wall, directed toward the channel midplane, in agreement with previous observations. The development of self-similar wall-jet profiles with increasing distance from the sidewalls is demonstrated. The intensity of this secondary flow increases with Reynolds number in the laminar regime, and decreases in the transitional and turbulent regimes. The effect of sidewalls on the primary and upper wall recirculation zones is demonstrated, and explains the literature discrepancies between experiments and two-dimensional simulations.