Numerical simulation of the vortex couples in the far wakes of behind circular cylinder

The present research on the structure of vortex couples in the far wakes of circular cylinder was carried out numerically. The unsteady and unstable flow field is evaluated with the discrete vortex method. Based on the obtained flow field, the pattern of vortices and the contours of vorticity are then solved simultaneously. The transition process from the primary Karman vortex street to the secondary vortex street is clearly distinguished by the numerical results. The discrete vortices in the far wakes have the trend of becoming to be the couples of two and three vortices. The numerical results tell the essence of the vortex structure in the flow field. The formation of the secondary vortex couples and the Karman street is arosen due to the dynamic instability of the fluid itself, because the primary incoming flow field is homogenous and undisturbed. Each of the couple of two vortices has equal vorticity, but their directions of vorticity are reversed. In the couple of three vortices, one has the same amount of vorticity of the other two vortices, but their directions are different too. The unstable flow is produced in the process of flow separated from the blunt body. From the results, the far wakes and the structure of vortex couples can be understood clearly.