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

Area of Science:

• Fluid Dynamics
• Rheology
• Non-Newtonian Flows

Background:

• Kelvin-Helmholtz instability (KHI) typically describes fluid interfaces with velocity and density differences.
• KHI is a fundamental concept in fluid dynamics, observed across various natural and industrial scales.
• Elastic turbulence (ET) is a phenomenon in viscoelastic flows characterized by chaotic behavior without significant inertia.

Purpose of the Study:

• To report the observation of an elastically driven KH-like instability in straight viscoelastic channel flow.
• To challenge the established view that interface perturbations are stable in low-inertia flows.
• To elucidate the novel mechanism behind this instability in elastic turbulence.

Main Methods:

• Experimental observation of viscoelastic channel flow exhibiting elastic turbulence.
• Analysis of coherent structures (CSs) and velocity fluctuations.
• Investigation of the role of elastic waves and vorticity dynamics.

Main Results:

• Observation of KH-like instability in viscoelastic channel flow, contradicting stability assumptions at low inertia.
• Identification of self-organized, cycling coherent structures (streaks) synchronized by elastic waves.
• Demonstration that elastic waves interact with wall-normal vorticity to amplify perturbations, driving the instability.

Conclusions:

• Elastic turbulence exhibits a novel KH-like instability distinct from Newtonian KHI.
• The instability is driven by the interaction of elastic waves with vorticity, overcoming stabilizing hoop stress.
• This finding expands the understanding of instabilities in viscoelastic flows and challenges existing paradigms.