Elastically driven Kelvin-Helmholtz-like instability in planar channel flow

TL;DR

This paper reports the discovery of a purely elastic Kelvin-Helmholtz-like instability in planar channel flow of viscoelastic fluids, driven by elastic waves and differing fundamentally from classical Newtonian KHI.

Contribution

It introduces a novel elastic instability mechanism in viscoelastic flows, distinct from traditional shear-driven instabilities, involving elastic wave energy pumping.

Findings

Elastic KHI occurs in flows previously considered stable.

Elastic waves significantly influence interface dynamics.

The instability results from competition between elastic wave energy and elastic stress differences.

Abstract

Kelvin-Helmholtz instability (KHI) is widely spread in nature on scales from micrometer up to Galactic one. This instability refers to the growth of perturbation of an interface between two parallel streams of Newtonian fluids with different velocities and densities, destabilized by shear strain and stabilized by density stratification with the heavier fluid at the bottom. Here, we report the discovery of the purely elastic KH-like instability in planar straight channel flow of viscoelastic fluid, which is theoretically considered to be stable. However, despite the remarkable similarity to the Newtonian KHI temporal interface dynamics, the elastic KHI reveals qualitatively different instability mechanism. Indeed, the velocity difference across the interface strongly fluctuates and non-monotonically varies in time due to energy pumping by elastic waves, detected in the flow. A correlation of the elastic wave intensity and efficiency of the elastic KHI in different regimes suggests that the competition between the destabilizing factor of the elastic waves and the stabilizing effect of the elastic stress difference generated by the velocity difference at the interface is the novel instability mechanism of the elastic KHI.