A nonlinear elastic instability in channel flows at low Reynolds numbers

TL;DR

This study provides experimental evidence that viscoelastic polymer flows in straight channels can become nonlinearly unstable and exhibit subcritical bifurcations at high flow rates, challenging the belief of their linear stability.

Contribution

The paper demonstrates nonlinear instability and subcritical bifurcation in polymer flows within straight channels through experimental velocimetry measurements.

Findings

Flow disturbances lead to large velocity fluctuations above critical conditions.

Flow becomes unstable at sufficiently high flow rates.

Instability occurs even in geometries previously thought stable.

Abstract

It is presently believed that flows of viscoelastic polymer solutions in geometries such as a straight pipe or channel are linearly stable. Here we present experimental evidence that such flows can be nonlinearly unstable and can exhibit a subcritical bifurcation. Velocimetry measurements are performed in a long, straight micro-channel; flow disturbances are introduced at the entrance of the channel system by placing a variable number of obstacles. Above a critical flow rate and a critical size of the perturbation, a sudden onset of large velocity fluctuations indicates presence of a nonlinear subcritical instability. Together with the previous observations of hydrodynamic instabilities in curved geometries, our results suggest that any flow of polymer solutions becomes unstable at sufficiently high flow rates.