An experimental study was conducted to investigate large-scale coherent structures induced by a bio-inspired flapping actuator in a turbulent backward-facing step flow. The flow field velocity dataset in the horizontal-vertical plane was obtained by planar particle image velocimetry. The flapping actuator was implemented over the step edge and it was driven to vertically oscillate, generating periodic small perturbations into the separated shear layer. As a result of active flow control, time-averaged reattachment length was reduced by 31% and Reynolds stresses as well as in-plane turbulent kinetic energy in the separated shear layer were considerably increased. In proper orthogonal decomposition analysis, the first two modes are found to be phase-correlated in the phase portrait of the coefficients. Therefore, reduced-order coherent structures are reconstructed by the first two modes, showing the phase evolution of the vortex shedding process. The extracted reduced-order coherent structures provide a better understanding of the underlying flow physics and are beneficial to effective flow separation control.
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