Flows over backward-facing steps with different spanwise widths

The separated and reattached flow over backward-facing steps with different spanwise widths is experimentally investigated in a wind tunnel. The height and streamwise length of steps are fixed at 19 mm and 3.95 m, respectively. Five different spanwise aspect ratios (AR), defined as the ratio of step width to height, 1, 2, 4, 8, and 16, are tested. The Reynolds number based on the step height and free-stream velocity is 11300, and a well-developed turbulent boundary layer is present before the flow encounters the step. Planar particle image velocimetry is employed to measure the velocity field along the centerline over and behind the steps. Results show that the three-dimensional effect becomes more significant as AR increases up to 4 and persists when AR = 8, likely attributed to the corner vortices formed alongside the step, as well as rolled up the sidewall shear layer. Surface oil-film flow visualization shows that the effect of side edges on the central flow remains significant until AR reaches 16. For AR = 16, the step flow is nominally two dimensional and exhibits similar characteristics as a typical twodimensional backward-facing step flow. The mean reattachment length increases with AR and then tends to approach an asymptotic level, while a much larger AR may be required for the saturation in the span of the reattachment zone. For AR = 4 and 8, a distinct flow structure is observed, characterized by jettisoned flow away from the floor after reattachment, and the distributions of Reynolds stresses are therefore not confined within a narrow region below the step height. The highest Reynolds stresses are observed when AR = 4 while the largest-scale turbulence appears when AR = 2. Furthermore, as AR increases, the evolution of vortical structures becomes more complex and the spreading of the shear layer slows down.