The flow field downstream of a backward facing step with Reynolds numbers of 1050–3890 were studied using stereoscopic particle image velocimetry (PIV) with field-of-views perpendicular to the incoming flow. It was found that the separated shear layer underwent a flapping motion with a frequency fH/Uo ≲ 0.04, much smaller than the shedding frequency (fH/Uo ≈ 0.1) for Reynolds number larger than 2000. Here H is the step height and Uo is the freestream velocity. The low frequency flapping motion appeared to be two dimensional, i.e. the motion was in-phased in the spanwise direction. The cause of the flapping motion is still not clear.

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