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

Abstract

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.