Breakdown and rearrangement of regular vortex streets in the wake of a circular cylinder are examined experimentally. The evolution of vorticity field is obtained using digital image processing for visualized flow fields. For 100ÃJRÃ140 (R is the Reynolds number based on the velocity U0 and diameter of the cylinder) a primary vortex street evolves to a parallel shear flow of Gaussian profile due to anisotropic extension of the vortex regions. Subsequently a secondary vortex street of larger scale appears. Its wavelength is within the instability region in the linear stability theory for the above velocity profile. The wavelength is, however, considerably smaller than that of the most unstable mode. Finally it is shown that the vortex regions in the primary and secondary vortex streets move at the speeds 0.15U0~0.19U0 and 0.05U0~0.10U0 respectively, relative to the fluid at infinity.
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