Experimental study of self-sustained spanwise streaks and turbulent mixing in separated shear flow

Abstract

The self-sustained streaks in turbulent backward-facing step flow are experimentally studied at a step height Reynolds number of 1.0 × 103. As one of the featured dynamic behaviors, evolution of the spanwise streaks plays an important role in the large-scale, aperiodic and vertical flapping motions of the separated/reattaching shear layer. By time-resolved particle image velocimetry, we measure the velocity vector fields in the streamwise-spanwise and streamwise-vertical planes downstream of a two-dimensional backward-facing step. The two cross-sectional velocity fields show that the shear layer remains two-dimensional along the span in the initial half of the shear layer, and then three-dimensional structures start to evolve in the latter half, leading to an unsteady reattaching shear layer and a shifting reattachment point on the downstream wall. By analyzing the finite-time Lyapunov exponent in the spanwise field of view, we find that low-speed fluid elements are vertically entrained upwards into the shear layer. As a result, the entrained fluid elements produce transient and localized low-speed fluid patches in the streamwise-spanwise plane, which move and decay downstream with strong straining and stretching motions along their boundaries. Furthermore, the large-scale streak-type and flap-type flows are clearly extracted and reconstructed by the energy-containing proper orthogonal decomposition modes.