Velocity measurements in confined swirling flow of polymer solutions with vortex shedding

A particle image velocimetry system is used to examine the velocity field in the unsteady swirling flow of polyacrylamide (PAA) solutions (PAA 0.5 wt. % and 1.0 wt. % ) with the vortex shedding due to a rotating disk in a cylindrical casing. In our earlier work [Tamano et al. , Phys. Fluids 19, 023103 (2007)], the vortex shedding is investigated by the flow visualization technique. In this work, we investigate the velocity field for better understanding of the vortex shedding mechanism and the nonaxisymmetric ring vortex which was observed for the higher Reynolds number compared to that of the axisymmetric ring vortex. It was found that the fluid within the ring vortex formed near the rotating disk rotates with semirigid rotation, where the angular velocity of the ring vortex was about 0.9 times that of the rotating disk. A high shear layer existed at the boundary between the ring vortex and the outer large-scale secondary flow. When the ring vortex was shed, the surrounding large-scale secondary flow rushed into near the rotating axis, so that strong axial flow was observed near the rotating axis. For the nonaxisymmetric ring vortex, azimuthal and radial velocities oscillate periodically with considerably large amplitude. In addition, the joint probability density function showed that the correlation between azimuthal and radial velocity fluctuations varied with the radial locations.