Image Analysis and Control for Unsteady Structure of Necklace Vortices

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The aims of present investigation are to clarify the structure of necklace vortices produced in the junction flow fields around a long cylinder or a rectangular plate mounted on the flat wall, and to know the effects of the Reynolds number and the boundary layer thickness on the necklace vortices structure by means of the flow visualization techniques and the Image analysis method.In the case of the visualization of necklace vortices around the effectively long cylinder, the Reynolds number based on the cylinder diameter was varied in the range between 300 and 2300, and the cylinder was mounted in three different positions from the reading edge of flat plate to vary widely the boundary layer thickness. As the results, the pattern of the necklace vortex system changes keeping laminar flow from one vortex system to three vortex system at the Reynolds number of about 500. Then the necklace vortex pattern changes from three vortex system to five vortex system at the Reynolds number of abou … More t 2000, in this Reynolds number the vortex flow seemed to become unsteady. It will be predicted from the present experiment that the unsteady phenomena was brought from the original vibration of the five necklace vortex system or from the unsteady flow by the transition of laminar boundary layer on the flat wall. In addition, the effect of the necklace vortex on the flow around a circular cylinder was investigated by using the wind tunnel testing. It was known from the results that in the juncture region the necklace vortex made the separation line on the cylinder move back, and made the base pressure behind the cylinder increase and made the drag decrease.In order to clarify the detail of necklace vortices structure, it was tried to increase the relative scale of necklace vortices to the mounted body size. Therefore, the rectangular plate which have various aspect ratios w/h was picked up as the object body instead of a cylinder. The aspect ratio, which was considered to indicates the various shape of the mounted body, of rectangular plate was varied to w/h=0.5, 1, 2, 3, 5 and 7. As the results, it was known that the necklace vortex pattern was strongly dependent on the aspect ratio of plate, but almost independent apart from Reynolds number in this experiment, and the relative scale and the structure of necklace vortices as to the rectangular plate with the same aspect ratio was decided by the ratio of the plate height to the boundary layer thickness. Less