The unsteady swirling flow of an aqueous polymer solution due to a rotating disc in a cylindrical casing was investigated using flow visualization and particle image velocimetry (PIV) system. As the aqueous polymer solution, polyacrylamide (PAA) solutions whose concentrations were 0.025, 0.1, 0.2, 0.5, 0.8 and 1.0 wt% were used. We found a new phenomenon of vortex shedding in which the ring vortex formed near the center of rotating disc was periodically shed away from the rotating disc in the unsteady flow regime. It was found that the fluid within the ring vortex formed near the rotating disc rotates with semi-rigid rotation, where the angular velocity of ring vortex was about 0.9 times that of the rotating disc. A high shear layer existed at the boundary between the ring vortex and the outer large-scale secondary flow. Just after the ring vortex was shed, the surrounding large-scale secondary flow rushed into near the rotating axis, so that the strong axial flow was observed near the rotating axis.

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