Starting jets in non-Newtonian viscoelastic fluids: on vortex ring generation and behavior

The generation process and behavior of non-Newtonian viscoelastic vortex rings generated with a piston cylinder apparatus are studied through fluorescent dye visualizations. The generalized Reynolds numbers targeted by this study are \({\varvec{Re}}\in \mathbf{[10,600]}\) and allow the identification of different regimes leading gradually from the generation of a blob that remains attached to the cylinder to that of a starting jet and then to a self-propagating vortex ring. Experiments are performed in eight different viscoelastic solutions and allow to evaluate the influence of the rheological properties of the fluid on the dynamics of the coherent structure. Regardless of the viscoelastic fluid used, the kinematics of the structure exhibits two steps: an initial propagation until reaching a maximum penetration position which depends on the parameters of the experiment (Reynolds number and elasticity number) and then a backward movement over a distance which also depends on these same parameters. The visualizations highlight important deformations of the structure envelope, in particular a spanwise flattening just before reaching the maximum position and a refocusing around the propagation axis during the backward movement. Results are interpreted in terms of Reynolds number and elasticity number.