The addition of a small amount of surfactant into water induces viscoelasticity. In turbulent channel and pipe flows, the wall friction significantly decreases with the surfactant additive, but a heat-transfer reduction also occurs. On the other hand, a phenomenon is reported that the instantaneous main flow broadly meanders in space and time under a certain condition in the surfactant solution flow past a backward-facing step (BFS); thus, the mean Reynolds shear stress remarkably increases. The influence of such a phenomenon cannot be neglected in thermal fluid equipment with usually complicated configurations. Therefore, understanding this phenomenon in detail is important both academically and industrially. In this study, particle image velocimetry measurements were carried out in a viscoelastic BFS flow with meandering phenomenon at high Reynolds number. We focused on the turbulent structures in the flow fields and investigated the interrelationship among the spatial scales of the eddy structures, Reynolds shear stress, and meandering motion using a spatial two-point correlation function or conditional averaging. In the meandering condition, we revealed that the Reynolds shear stress due to the low-speed fluid that departs from the upper wall opposite the step across the main flow contributes the largest to the mean Reynolds shear stress value. Then, a secondary recirculation region appears near the upper wall, in addition to the primary recirculation region. A reverse flow is apt to occur when rapid deceleration and pressure rise happens due to the sudden expansion of the channel cross section. Therefore, flow separation occurs at the upper wall, and a large-scale circulation appears there. Such flow is caused by the relationship between the pressure rise and the momentum transfer between the flow and the wall. We can conclude that the condition where an unstable motion occurs is influenced by the concentration of the surfactant solution because the surfactant additive acts on the shear stress.

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