Numerical study of the formation of vortex rings with reversed rotation in viscoelastic liquids

Laminar vortex rings in viscoelastic liquids display distinct characteristics compared to the Newtonian counterparts. One interesting scenario is that the core of a vortex ring rotates reversely (in contrast to the Newtonian counterpart), termed as the “negative vortex ring.” This phenomenon was initially observed experimentally by Palacios-Morales et al. [“Negative vortices: The formation of vortex rings with reversed rotation in viscoelastic liquids,” Phys. Fluids 27, 051703 (2015)]. In this study, we numerically investigate the formation of the negative vortex rings through controlling the polymer viscosity ratios, Weissenberg numbers, and Reynolds numbers. In the simulations, we indeed observe the formation of negative vortex rings. In the examination of the effects of flow parameters, both the shear thinning and viscoelastic effects of the fluids are found to be necessary for the formation, while the flow inertia suppresses the formation. The concurrent existence of high shear and low extensional viscosities in flow fields seems to be a critical condition for the formation.