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Recent experiments have reported a novel transition to elasto-inertialturbulence in the Taylor--Couette flow of a dilute polymer solution. Unlikepreviously reported transitions, this newly discovered scenario, dubbed vortexmerging and splitting (VMS) transition, occurs in the centrifugally unstableregime and the mechanisms underlying it are two-dimensional: the flow becomeschaotic due to the proliferation of events where axisymmetric vortex pairs maybe either created (vortex splitting) or annihilated (vortex merging). In thispaper, we present direct numerical simulations, using the FENE-P constitutiveequation to model polymer dynamics, which reproduce the experimentalobservations with great accuracy and elucidate the reasons for the onset ofthis surprising dynamics. Starting from the Newtonian limit and increasingprogressively the fluid's elasticity, we demonstrate that the VMS dynamics isnot associated with the well-known Taylor vortices, but with a steady patternof elastically induced axisymmetric vortex pairs known as diwhirls. The amountof angular momentum carried by these elastic vortices becomes increasinglysmall as the fluid's elasticity increases and it eventually reaches a marginallevel. When this occurs, the diwhirls become dynamically disconnected from therest of the system and move independently from each other in the axialdirection. It is shown that vortex merging and splitting events, along withlocal transient chaotic dynamics, result from the interactions among diwhirls,and that this complex spatio-temporal dynamics persists even at elasticitylevels twice as large as those investigated experimentally.

Vortex merging and splitting events in viscoelastic Taylor Couette flow