Purely elastic instabilities in parallel shear flows

Paulo Arratia,

University of Pennsylvania, USA

Abstract

It is a common assumption that, in the absence of inertia, the flow of a viscoelastic fluid in pipes and channels is linearly stable to flow perturbations. Recent evidence, however, has suggests that such flow may be unstable to finite amplitude perturbation. This type of instability is akin to the transition from laminar to turbulent flows in ordinary Newtonian fluids where the control parameter is the Reynolds number (Re). Here, on the hand, the control parameter is the Weissenberg number (Wi). In this talk, we present evidence of a subcritical nonlinear instability for the flow of a dilute polymeric solution in a parallel channel flows using a microfluidic device. The flow is investigated using dye advection, particle tracking velocimetry, and pressure sensors. Results show sustained velocity fluctuations far downstream away from the initial perturbation for strong enough disturbances; small disturbances decay quickly under the same flow conditions. A hysteresis loop, characteristic of subcritical instabilities, is observed. The flow is characterized by non-periodic velocity fluctuation showing common signatures of elastic turbulence. Pressure measurements show rapid increase in drag as Wi is increased followed by and a turbulent-like regime characterized by a sudden decrease in drag and a weak dependence on Wi. Finally, we explore the mechanisms for these instabilities using 3D particle tracking.

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