Influence of porous material on the flow behind backward-facing step - experimental study

We investigate effect of porous insert located upstream of the separation edge of backward-facing step (BFS) in early transitional regime as a function of Reynolds number. This is an example of hydrodynamic system that is a combination of separated shear flow with large amplification potential and porous materials known for efficient flow destabilisation. Spectral analysis reveals that dynamics of backward-facing step is dominated by spectral modes that remain globally coherent along the streamwise direction. We detect two branches of characteristic frequencies in the flow and with Hilbert transform we characterise their spatial support. For low Reynolds numbers, the dynamics of the flow is dominated by lower frequency, whereas for high enough Reynolds numbers cross-over to higher frequencies is observed. Increasing permeability of the porous insert results in decrease in Reynolds number value, at which frequency cross-over occurs. By comparing normalized frequencies on each branch with local stability analysis, we attribute Kelvin-Helmholtz and Tollmien-Schlichting instabilities to upper and lower frequency branches, respectively. Finally, our results show that porous inserts enhance Kelvin-Helmholtz instability and promote transition to oscillator-type dynamics. Specifically, we observe that amplitude of vortical (BFS) structures associated with higher frequency branch follows Landau model prediction for all investigated porous inserts.