Investigation of spanwise coherent structures in turbulent backward-facing step flow by time-resolved PIV

•The separated shear layer loses its two-dimensionality and begins to evolve spanwise variation due to instability.•High- and low-momentum fluid is entrained through the shear layer by large-scale vortices.•The spanwise coherent structures occur in the latter half of the shear layer and they are highly correlated with the vertical pumping and flapping motions of the shear layer.•The spanwise coherent structures have the influence of dragging the convection velocities of the coherent structures slower than the local mean flow. We experimentally investigate spanwise coherent structures in the turbulent shear layer downstream of a two-dimensional backward-facing step (BFS). The incoming free-stream flow separates from the backward-facing step edge and then reattaches to the downstream wall surface, resulting in a separated/reattaching shear layer and a recirculation region behind the step. This separated/reattaching shear flow is measured by time-resolved particle image velocimetry (PIV) in one streamwise-vertical plane and three parallel streamwise-spanwise planes, respectively. As a result, the multiple cross-sectional flow diagnosis reveals that the separated shear layer remains two-dimensional from the step edge to approximately the half of the reattachment length. Further downstream, the shear layer begins to flap vertically and it evolves spanwise-oriented structures with alternating high- and low-speed streaks. The near-wall vortex tube beneath the shear layer simultaneously evolves unsteady spanwise variation as well. The unsteady shear layer as well as the vortex tube causes entrainment of the high-momentum fluid from the shear layers downwards into the near-wall vortex tube, and entrainment of the low-momentum fluid upwards in the opposite way. By proper orthogonal decomposition and dynamic mode decomposition, the spanwise coherent structures are characterized as large in size as 2 step heights and they have the same order of frequencies as the vertical pumping and flapping motions, both of which contribute a major part of the turbulent kinetic energy in the shear layer. Further analysis by spatial-temporal cross-correlation function shows that the spanwise coherent structures have influence on the convection velocities of the coherent structures in the latter half of the shear layer.