Self-sustained elastoinertial Tollmien-Schlichting waves

TL;DR

This paper demonstrates the existence of self-sustained elastoinertial Tollmien-Schlichting waves in viscoelastic channel flow, revealing a new nonlinear solution family that may mediate transition to elastoinertial turbulence.

Contribution

It identifies a novel nonlinear solution family in viscoelastic flow that is not connected to Newtonian TS solutions, highlighting viscoelasticity's role in flow self-sustenance.

Findings

Existence of a family of attractors resembling TS modes with localized stress fluctuations.

The stress-to-velocity fluctuation ratio increases with Weissenberg number, Wi.

Transition to elastoinertial turbulence involves nonlinear amplification of TS-like perturbations.

Abstract

Direct simulations of two-dimensional plane channel flow of a viscoelastic fluid at Reynolds number Re = 3000 reveal the existence of a family of attractors whose structure closely resembles the linear Tollmien-Schlichting (TS) mode, and in particular exhibits strongly localized stress fluctuations at the critical layer position of the TS mode. At the parameter values chosen, this solution branch is not connected to the nonlinear TS solution branch found for Newtonian flow, and thus represents a solution family that is nonlinearly self-sustained by viscoelasticity. The ratio between stress and velocity fluctuations is in quantitative agreement for the attractor and the linear TS mode, and increases strongly with Weissenberg number, Wi. For the latter, there is a transition in the scaling of this ratio as Wi increases, and the Wi at which the nonlinear solution family comes into existence is just above this transition. Finally, evidence indicates that this branch is connected through an unstable solution branch to two-dimensional elastoinertial turbulence (EIT). These results suggest that, in the parameter range considered here, the bypass transition leading to EIT is mediated by nonlinear amplification and self-sustenance of perturbations that excite the Tollmien-Schlichting mode.