FIV2018 Paper Management System, FIV2018 Conference

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Experimental investigation of three dimensional motions in a separated shear layer over a backward facing step
Nan Gao, Zhuoyue Li

Last modified: 2018-04-25


The low frequency lapping motion of a separated shear layer over a backward facing step received an increasing amount of attentions recently. The frequency of the flapping motion fH/U was typically less than 0.04, an order of magnitude smaller than that of the shear layer frequency, here H is the step height and U is the freestream velocity. The cause of the flapping motion is still unclear despite the amount of investigation emerged recently. In this paper, we are going to present the experimental results of the flow fields downstream of a backward facing step with a Reynolds number of Re_H= 200 to 8000 performed (some parts will be performed) in a water tunnel using a time-resolved stereoscopic PIV system. The field-of-view was perpendicular to the incoming flow to capture the three velocity  components in the yz plane. The laser sheet can be traversed from x/H=0.5 to 8.0. Flow fields at a typical location (e.g. x/H=2.0) for a range of Reynolds numbers will be analyzed in details, including computing the instantaneous location of the shear layer Ys(z,t) and decomposing Ys(z,t) using Proper Orthogonal Decomposition which will give the modal compositions of the spanwise undulating motions in the shear layer. Preliminary measurements of a shear layer with a laminar initial boundary layer and a Re_H of 2000 suggested that the lower POD modes had a characteristic frequency similar to that of the flapping motion reported in the literature, therefore this paper may give an evidence linking the original of the flapping motion to a spanwise instability of the shear layer.


backward facing step; separated shear layer; flapping

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