Unsteady behavior of back-facing step flow

In this paper, a contribution to the study of the unsteady behavior of a back facing step flow is reported. The main interest is devoted to low-frequency motions. A probe for skin friction measurements is employed to acquire time hystories from the step to downstream the the reattachment region. Signals are analyzed in the physical space and in the frequency domain. The obtained results, associated with flow visualization in a companion experiment in water, support a model of cyclic motion of growing and successive breakdown of the secondary recirculating bubble. The frequency of this quasi-periodic motion is comparable to the flapping frequency of the whole separated regio, reported in the literature.