Interaction of an off-surface cylinder with separated flow from a bluff body leading edge

•Phase locking between cylinder and bubble shedding while flapping not affected.•Cylinder pressure gradient eliminates separation bubble.•Cylinder shedding Strouhal number is reduced due to increased blockage from bubble.•When cylinder near the wall bubble flapping modes dominate over shedding modes.

Time resolved PIV measurements are performed on a cylinder of diameter D   placed in the vicinity of the leading edge of an elongated bluff body at ReD=2.6×104ReD=2.6×104. The off-surface cylinder is placed at three different wall-normal positions and two streamwise positions, upstream and downstream the bluff body leading edge. Topological, statistical and transient analyses are performed on the resulting flowfield. The streamwise position of the cylinder relative to the bluff body leading edge significantly affects the mean flow topology and the vortex shedding dynamics of both the cylinder and the bluff body separation bubble. When the cylinder is positioned upstream of the bluff body leading edge, the cylinder shedding becomes phase-locked to the bluff body bubble shedding, while the low frequency bubble fluctuation is not significantly affected. When the cylinder is positioned downstream the bluff body leading edge, the pressure gradient eliminates the separation bubble and regular cylinder vortex shedding occurs. The StDStD number of the cylinder shedding is reduced when the separation bubble is eliminated as the latter causes blockage that reduces the effective cylinder diameter. POD analysis captures the separation bubble and cylinder shedding dynamics, demonstrating that when the cylinder is placed near the wall, low frequency fluctuation modes associated with the separation bubble dominate over the cylinder shedding modes.