The transitional backward-facing step flow in a water channel with variable expansion geometry

Abstract The backward-facing step flow is investigated experimentally and numerically at moderate Reynolds numbers. Different channel expansion ratios ( ER  = 1.43, 2, 2.5, and 4) and inlet flow conditions (steady and pulsatile) are applied with the aim to analyze the structure and stability of flow behind the step. Electrodiffusion technique is used to measure the wall shear rate along the experimental water channel. Direction sensitive sensors detect the near-wall extent of different flow-recirculation regions (primary recirculation and secondary corner, roof, and bottom eddies). The results of 2D numerical simulations performed in commercial CFD software FLUENT provide additional information on global flow rearrangement caused by the change of operation parameters. As the channel expansion ratio is increased, the steady recirculation pattern observed in the laminar flow regime becomes more complex. The obtained experimental and numerical data suggest possible scaling for the reattachment length and roof eddy size. In the transitional regime the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations. The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step. A significant reduction of the reattachment length and an intensification of pulsatile back flow can be achieved by applying an appropriate forcing at frequencies close to that of the global flow instability.