Lagrangian Visualization of Vortex Evolution in the Wake of a Backward-Facing Step

We carry out experimental study on turbulent backward-facing step flow at the step Reynolds number of Re = 1.0 × 10³ by planar particle image velocimetry (PIV). Based on the time-resolved velocity fields in the central vertical plane, we analyze the vortex growth as well as the convection within the separated/reattaching shear layer. By visualizing the fluid transport of non-inertial color-dyed flow tracers in the field of view, we find the characteristics of vortex growth governed by the two distinct mechanisms, i.e., the shear-layer mode in the initial part of the shear layer and the wake mode in the middle of the recirculation region. The shear-layer mode of instability causes the vortex rolling-up and pairing process while moving fast downstream, whereas the wake mode governs the further vortex growth to the scale of the step height while the vortex core remains stationary approximately in the middle of the reattachment length. Strong reverse flow is clearly visualized by the vortex-induced flow tracers beneath the vortex core, as the key to the onset of the absolute instability in the wake flow. As a result, large-scale, unsteady and vertical flapping motions occur in the latter half of the shear layer.