Vortex interactions in a two side-by-side cylinder near-wake

The flow behind two side-by-side circular cylinders is experimentally investigated based on laser-illuminated flow-visualization, particle image velocimetry and hot-wire measurements. The flow is classified as three regimes: single street (the cylinder center-to-center spacing T/d < 1.2), asymmetrical flow (1.2 < T/d < 2.0) and two coupled street (T/d > 2.0). Special attention is given to the regime of 1.2 < T/d < 2.0, which is characterized by one narrow and one wide wake. It is found that the flow structure and its downstream evolution are closely linked to the phase relationship between the gap vortex in the wide wake and that in the narrow wake. When the gap vortex in the wide wake leads in phase, the two opposite-signed vortices in the narrow wake are typically engaged in pairing, which yields a relatively low-pressure region between them, thus drawing in the gap vortex along with fluid in the wide wake. This vortex interaction may act to stabilize the gap flow deflection. When lagging behind in phase, the gap vortex in the wide wake fails to merge with the vortices in the narrow wake. Interactions between vortices in the two wakes lead to the changeover of the gap flow deflection from one side to another. It is further noted that, for L/d > 2.0, the flow structure change from the anti-phase to in-phase mode starts with a phase shift between gap vortices. The dynamical role of gap bleeding between cylinders for L/d < 1.2 is also examined.