Aerodynamics of two rough circular cylinders placed side by side subject to postcritical flow

This investigation reports on the flow behavior of, and the resulting forces on, two roughened cylinders in the postcritical regime. Specifically, two cylinders of the same surface roughness of ks/D=1.9×10−3, placed apart at a cross-stream pitch (separation) of 1.18≤L/D≤6, are studied at a postcritical Reynolds number of 3×105 using unsteady pressure measurements. The two cylinders behave nearly independently at L/D=6 and the forces on them are the same within experimental uncertainty until the pitch ratio is reduced to L/D=2.25. The gap flow between the two cylinders becomes biased at L/D≤2, with the bias well correlated across the span for 1.33≤L/D≤2. In addition to the conventional narrow-wake (NW) and wide-wake (WW) modes observed at subcritical Reynolds numbers, a less-biased intermittent gap flow is observed for L/D={1.18,1.2}. Intermittency is observed for some spanwise cross sections, thus reducing the spanwise coherence of the flow. Across pitch ratios investigated, the average of drag coefficients for the two cylinders was always larger than for a single cylinder. For L/D={1.33,1.5,1.75}, the cylinder with the larger drag experiences a larger lift. Both of these observations are contrary to previous findings in subcritical flows. On the cylinder halves facing the gap, the boundary layer on the cylinder in the NW mode separates further downstream than for the WW mode. This NW separation angle decreases with increasing pitch ratio while it remains similar for the WW mode across pitch ratios.