Dynamics of vortex rings in viscoelastic fluids

Vortex rings are coherent vortical structures widely encountered in geophysical flows and engineering applications. They are found in industrial systems, for instance during injection processes or in the flow in the vicinity of blades in mixing systems. Numerous studies are concerned by vortex rings. But, to the best of our knowledge, only few of them address vortex dynamics in non-Newtonian shear-thinning fluids, and none in viscoelastic ones while such fluids are widely involved in industrial processes. The purpose is here to study the dynamics of vortex rings in viscoelastic fluids. In the present experiments these structures are generated through a piston-cylinder system and the mechanical parameters for injection are the piston velocity and stroke, Vp and L. Three different viscoelastic fluids are used: two aqueous solutions of Zetag 7587 (0.04% and 0.1%) and a solution of PAM (0.1%). The Newtonian reference fluid is water. A fluorescent dye visualization technique is used and images are recorded using a HD camera (2160x2560 pixels, 50Hz). In addition to the video sequences obtained, image processing is used to determine the two main characteristics of the vortex ring: its motion and its size. As expected vortex ring in Newtonian fluid furls, progresses downstream by auto-induced effects and diffuses (as materialized by its diameter increase). The behaviour strongly differs for Non-Newtonian viscoelastic fluids: first, the rolling up phase is delayed and occurs further downstream. Then the vortex ring actually furls and expends during its way downstream. Contrarily to non-elastic fluids and unexpectedly, the viscoelastic ring afterwards stops, unfurls and goes back, inducing a contraction in the radial direction. This new dynamics is studied through time evolution of vortex position and diameter for different fluids and flow configurations. The competitive influences of the fluid nature and elasticity and of inertia are emphasized