Visualizations and fluctuation measurements of the spanwise unsteady motions over a backward-facing step under periodic perturbations

The spanwise unsteady characteristics induced by the development of large-scale vortex structures along the shear layer over a backward-facing step with Re=9630 was investigated. The flow was perturbed periodically using a synthetic jet at separation. Structures were visualized from top and side views using a smoke-wire, while the induced velocity fluctuations were measured with a movable spanwise hot-wire sensor array. Comparisons were made for perturbation frequency varying from StA=fAH/Uo=0.04 to 0.39, and StA=0.22 was found to be the optimal frequency, as forming spanwise heterogeneous three-dimensional shedding structures with the maximum turbulent kinetic energy. In optimal cases, structures were paired in a staggered span-wise order, causing a doubling period. Structures became smaller and showed weaker interactions under superoptimal perturbation, while a large-scale two-dimensional low-speed bubble was observed at suboptimal perturbation, StA=0.04.