Flow separation without a wall: laminar shear-wake–cylinder interactions

This study investigates necklace-vortex systems forming when a laminar shear-wake, generated by two streams merging at the trailing edge of a splitter plate, interacts with a circular cylinder placed downstream in the wake. Hydrogen-bubble flow visualisations were employed in a water channel capable of producing laminar shear-wake flows. In the absence of the cylinder, oppositely signed vorticity in the shear-wake undergoes mutual annihilation. The introduction of the cylinder interrupts this evolution, promoting off-wall flow separation upstream of the cylinder and vortex roll-up. The study primarily focuses on two non-dimensional parameters, the Reynolds number $ \textit{Re}_m$ and the shear ratio $ \textit{SR}$ , and presents a mapping of the observed vortex regimes. Increasing $ \textit{Re}_m$ promotes either the formation of additional vortices or unsteadiness. Increasing $ \textit{SR}$ generally suppresses vortex formation or attenuates unsteadiness, except near $ \textit{SR}\approx 0$ at low to moderate $ \textit{Re}_m$ , where the two-vortex system is unstable to additional vortex generation. Observed configurations range from no-vortex states to one- or two-vortex systems at low Reynolds numbers, and to three-, four- and five-vortex systems at larger Reynolds numbers, with unsteadiness becoming prominent beyond the three-vortex regime and predominant in four- and five-vortex systems. Beyond regime mapping, we delve into the structure of a steady two- and three-vortex system at low to moderate $ \textit{Re}_m$ . This provides insights into the emergence and evolution of the vortex system, which is analysed in the context of the vorticity-transport equations.