Abstract: The interaction between a pair of incident vortices and a circular cylinder has been simulated numerically by using a fast vortex method in present paper. A secondary and a tertiary vortices separated from the cylinder surface are, induced by the incident yortex, observed. Because of pairing with the secondary one, the incident vortex changes its trajectory. The 'rebounding' phenomenon of the incident vortex is attributed to the effect of secondary vortex. This phenomenon has been vertified by flow visualiz...