Investigation of Coherent Stuctures in Active Flow Control over a Backward-Facing step by PIV

Abstract

An acoustic tube was designed and implemented in turbulent shear flow over a backward-facing step at Re = 2.0×10^4 with respect to the step height. As an active flow control device, the acoustic tube generated periodic small perturbations at 100 Hz which educed coherent structures into the shear layer to reduce the size of the flow separation. Standard particle image velocimetry (2D-2C- PIV) was employed and the region of interest was 310×70 mm2 in x- and ydirection respectively, which covered the turbulent boundary layer in the step region, the separated shear layer and the reattachment area. In the controlled case, it is shown that the flow separation was suppressed and the reattachment length was reduced by 43.0%. The consecutive phaselocked results presented the generation, entrainment and dissipation of shedding vortices and the development of periodic motions in the turbulent shear layer. Besides, two-point spatial crosscorrelations revealed the shape of growing shear layer and the coherent features. The Reynolds shear stress was increased correspondingly where coherent structures were educed, entrained and dissipated. The associated organized structures extracted by Proper Orthogonal Decomposition (POD) based on their kinetic energy evolve as traveling waves. These coherent structures eventually convey kinetic energy from the mean flow to the fluctuating motions and consequently enhance the momentum transfer in the separated shear layer.