Effect of Reynolds number on the separated-reattached flow upstream of a fence

The flow development upstream of a rigid fence is investigated experimentally for three Reynolds numbers 18 000, 36 000, and 54 000. Time-resolved, planar particle image velocimetry is used to capture the instantaneous velocity fields with high spatio-temporal resolution in key areas, while smoke wire flow visualization is used to obtain a global description of the flow. The results reveal a complex separating-reattaching flow region two to three fence heights upstream of the fence. In addition to a local flow separation in the immediate vicinity of the fence, a laminar separation bubble is formed upstream and its mean extent decreases with increasing Reynolds number. Coherent structures are shed from the upstream separation region significantly affecting flow development downstream and exhibiting a strong dependence on the Reynolds number.