A doublet flow or a coaxial flow consisting of injection and suction flow, which is proposed for the purpose of diffusion control of a circular jet, was experimentally examined. In order to clarify the behavior of such flow, the correlations of fluctuating velocity in the streamwise and transverse direction were measured and the flow visualization technique was also used. It was clarified from these experiments that as the suction velocity increases, diffusion of the circular jet is promoted because the scale of vortical structure in the shear layer becomes large due to the strong shear stress. It was found from the results of the cross correlations of fluctuating velocity that asymmetrically ejected vortices exist and contribute strongly to diffusion of the jet. Furthermore, this phenomenon of asymmetrically ejected vortices moves gradually upstream as the suction velocity increases.
