# Passive control of coherent structures in a modified backwards-facing step flow

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## Abstract

We study a modified backwards-facing step flow, with the addition of two different plates; one is a baseline, impermeable plate and the second a perforated one. An experimental investigation is carried out for a turbulent reattaching shear layer downstream of the two plates. The proposed setup is a model configuration to study how the plate characteristics affect the separated shear layer and how turbulent kinetic energies and large-scale coherent structures are modified. Measurements show that the perforated plate changes the mean flow field, mostly by reducing the intensity of reverse flow close to the bottom wall. Disturbance amplitudes are significantly reduced up to five step heights downstream of the trailing edge of the plate, more specifically in the recirculation region. A loudspeaker is then used to introduce phase-locked, low-amplitude perturbations upstream of the plates, and phase-averaged measurements allow a quantitative study of large-scale structures in the shear-layer. The evolution of such coherent structures is evaluated in light of linear stability theory, comparing the eigenfunction of the Kelvin–Helmholtz mode to the experimental results. We observe a close match of linear-stability eigenfunctions with phase-averaged amplitudes for the two tested Strouhal numbers. The perforated plate is found to reduce the amplitude of the Kelvin–Helmholtz coherent structures in comparison to the baseline, impermeable plate, a behavior consistent with the predicted amplification trends from linear stability.

## Notes

### Acknowledgements

This work was partly supported by CNPq Grant 444796/2014-2 and also by The Boeing Research and Technology–Brazil through the Boeing University Relations Program. The authors gratefully acknowledge the FENG laboratory for the support in the design, manufacturing and operation of the experimental campaign. We also thank Peter Jordan, Ardeshir Hanifi and Dan Henningson for the useful discussions. Pedro C. Ormonde acknowledges the financial support from CAPES via a M.Sc. scholarship. André Cavalieri was supported by a CNPq research scholarship.

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