A doublet flow is the circular free jet added the annular suction flow mounted on the same axis by using a double coaxial pipe. This flow was proposed for the purpose of diffusion control of a circular free jet. The doublet flow was examined experimentally with respect to time mean and unsteady characteristics over the full flow field by using PIV. It was found from the time mean analysis that as the suction velocity increases, the area average value of the fluctuating velocity increases. From the unsteady analysis, it was found that the vortex structures, which have the vorticity that is nearly equal to that of a jet, exist when the suction velocity is high. The vortex structure behaves intermittently, and exists randomly to the radial directions. Thus, the cause of the increase of the fluctuating velocity as described above is clarified by showing the behaviors of these vortex structures.
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