The present study is designed to experimentally investigate the flow field past a circular cylinder in a uniform stream. The circular cylinder was set across an open test section of water channel. Experiments were conducted with Reynolds number based on a cylinder diameter of 120 to 800, and the flow field was visualized by the hydrogen-bubble method and visualization of the flow velocity pattern in the lateral direction was made by using an ultrasonic velocity profiler (UVP). Fluctuating velocity fields, v(y, t), were measured. Changes in the deflection amount and width of the separated shear layer are indicated from intensity distributions of velocity fluctuation. According to the analysis based on the POD, it is clarified that the present flow fields may be represented by the reduced order model of the vortex structure. Integral of v(y, t) with respect to y is related with the averaged vorticity, and its time history indicates a stability of vortex shedding mode.

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