Experimental Real-Time observation of very low-frequency phenomena downstream of a Backward-Facing Step

The Backward-Facing Step (BFS) flow is considered and used as a relevant benchmark case for shear flows and separated flows. Its relevance lies in its simple geometry, a downward step of height h, with a sharp edge, producing very complex fluid structures such as a shear layer, vortex rolling/paring/shedding, and formation of coherent structures, superimposed to a large recirculation bubble. In this study, Real-Time velocity field data are recorded for several hours and various Reynolds numbers by means of a 2D2C Real-Time Optical Flow Particle Image Velocimetry (RT-OF PIV) system. The signal computed and recorded by the RT-OF system is the time-evolution of the instantaneous recirculation area A_R(t) of the separated flow, together with multiple local velocity probes distributed downstream of the BFS. The observation and recording of this information over such a long time (hours of recording) have allowed us to explore high energy low-frequency phenomena (of the order of 10^-3 Hz) downstream of the BFS. To our knowledge, this is the first experimental evidence of very low-frequency of large-scale structures in separated flows.

The 2D2C RT-OF system used in these experiments has been developed in collaboration with Photon Lines Inc.. It allows a higher resolution of the velocity fields and long-time recording of instantaneous characteristics of the natural BFS flow than in previous studies.