It is known that the low frequency fluctuation about 1/20 of the Strouhal frequency exists in the wake behind a two-dimensional circular cylinder. However it's origination is not known yet. Local vortex shedding frequency in spanwise direction is considered to differ from one point to another due to different local Strouhal number caused by flow tree-dimensionality. Therefore, interaction between the different vortex shedding frequencies is considered to be the cause of the long time scale fluctuation. In this study, two different vortex shedding frequencies have been forced to occur at a local spanwise position with the use of a circular cylinder with step and particular phase difference TSC (Trans-Spectral Coherence) analysis was conducted to investigate the presence of nonlinear interaction in the wake. Moreover, by means of flow visualization, it is found that the interaction is caused by the formation of longitudinal vortex associated with vortex dislocation.
