Structure Analysis and Control of Necklace Vortices Generated between Two Circular Cylinders Forming a Separating Cross

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The three dimensional flow field area and the necklace vortices structure around two cylinders forming a separating cross were visualized using a dye injection method and a tracer particle method under the Reynolds number between 500 and 2000, and the surface pressure distribution on two circular cylinders was measured under the Reynolds number between 1000 and 8000 in detail to clarify the necklace vortex structures in uniform flow and the effects of the necklace vortex behavior on the fluid force into and on the separation flow from the cylinders. In addition, the relationship between the necklace vortex structure being made change by the gap distance and the pressure distribution on the surface of the cylinders was investigated.It is found that the various patterns of the necklace vortices system are produced, depending on the gap distance "G" between two cylinders. In the case of the gap distance ratio "G/d" between 0 and 0.1, the boundary layer on the surface of the upstream cylinder separates and rolls up as a small necklace vortex in the front of the downstream cylinder, and the vortex system with 4 vortices is produced in the symmetrical plane of y/d=0. In the case of G/d=0.2-1.0, a pair of main necklace vortices grows in the gap space between two cylinders, and the 8 vortices system as total is produced. In the case of G/d=1.0-1.5, the pair of main necklace vortices starts vibrating as well as twin vortices behind a cylinder in the low Reynolds number. In the case of G/d=1.5-2.0, the behavior of necklace vortices behind the upstream cylinder is similar to that of Karman vortices under the low Reynolds number.