Vortex merging and splitting events in viscoelastic Taylor Couette flow

TL;DR

This paper uses numerical simulations to explore a novel vortex merging and splitting transition in viscoelastic Taylor--Couette flow, revealing the role of elastic vortices called diwhirls in chaotic flow dynamics.

Contribution

The study demonstrates that vortex merging and splitting in viscoelastic Taylor--Couette flow are driven by elastic diwhirls, not classical Taylor vortices, and provides detailed simulation insights into this new transition.

Findings

Vortex merging and splitting are caused by interactions among elastic diwhirls.

The chaotic dynamics persist even at high elasticity levels.

Diwhirls become independent and move axially as elasticity increases.

Abstract

Recent experiments have reported a novel transition to elasto-inertial turbulence in the Taylor--Couette flow of a dilute polymer solution. Unlike previously reported transitions, this newly discovered scenario, dubbed vortex merging and splitting (VMS) transition, occurs in the centrifugally unstable regime and the mechanisms underlying it are two-dimensional: the flow becomes chaotic due to the proliferation of events where axisymmetric vortex pairs may be either created (vortex splitting) or annihilated (vortex merging). In this paper, we present direct numerical simulations, using the FENE-P constitutive equation to model polymer dynamics, which reproduce the experimental observations with great accuracy and elucidate the reasons for the onset of this surprising dynamics. Starting from the Newtonian limit and increasing progressively the fluid's elasticity, we demonstrate that the VMS dynamics is not associated with the well-known Taylor vortices, but with a steady pattern of elastically induced axisymmetric vortex pairs known as diwhirls. The amount of angular momentum carried by these elastic vortices becomes increasingly small as the fluid's elasticity increases and it eventually reaches a marginal level. When this occurs, the diwhirls become dynamically disconnected from the rest of the system and move independently from each other in the axial direction. It is shown that vortex merging and splitting events, along with local transient chaotic dynamics, result from the interactions among diwhirls, and that this complex spatio-temporal dynamics persists even at elasticity levels twice as large as those investigated experimentally.