Flopping phenomenon of flow behind two plates placed side-by-side normal to the flow direction

Experiments were made for the flow over two side-by-side normal plates for which the gap ratios are in the range 1.4–2.1 and the Teynolds numbers are at 6.6 × 10³ and 1.8 × 10⁴. At low gap ratios, i.e., 1.4–1.6. the gap flow appears always to be biased and flip-flops to the preferred side non-periodically with respect to time. As the gap ratio becomes larger, the percentage of time occupied by the gap flow in the biased state decreases and the non-biased state of the gap flow becomes prevalent. A comparison of the experimental results obtained under five free stream turbulence conditions further shows that the addition of artificial disturbance into the free stream promotes gap flow flopping at low gap ratios.