One mechanism by which laminar flow becomes turbulence is through the instability of vortex interaction. The nonlinear evolution of the interactions between the ∧-vortex and the secondary closed vortex has been investigated.This nonlinear instability process leads to some fundamental changes in the topology of flows. Experimental observations are presented here, showing the formation of a chain of ring-like vortices, which generate high frequency spikes on time traces in a transitional boundary layer.