In this study, flow behavior behind two side-by-side circular cylinders with unequal diameters, was investigated by using the technique of particle-image-velocimetry (PIV). The experiments were conducted in a re-circulating open water channel within the subcritical Reynolds number regime and a center-to-center pitch ratio T/D range of 1.2 to 3.6. Instantaneous and average velocity patterns, vorticity and Reynolds stress contours acquired with the PIV were used to analyze the flow behavior behind the two cylinders. The effects of the gap ratio and Re on vortex shedding were studied. The results showed that three flow patterns including the asymmetric biased flow, symmetric biased flow and two wake modes with different scales for different gap ratios respectively were observed.

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