Characterizing elastic turbulence in channel flows at low Reynolds number

Abstract

We experimentally investigate the flow of a viscoelastic fluid in a parallel shear geometry at low Reynolds number. As the flow becomes unstable via a nonlinear subcritical instability, velocimetry measurements show non-periodic fluctuations over a broad range of frequencies and wavelengths, consistent with the main features of elastic turbulence. Using the same experimental setup, we compare these features to those in the flow around cylinders, which is upstream to the parallel shear region; we find significant differences in power spectra scaling, intermittency statistics, and flow structures. We propose a simple mechanism to explain the growth of velocity fluctuations in parallel shear flows based on polymer stretching due to fluctuations in streamwise velocity gradients.

Pore‐Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media

“…Once sufficiently perturbed, the flow remains unsteady far downstream of the perturbation; this observation suggests the generation of a new dynamical flow state in which fluctuations are sustained indefinitely . However, the statistics of the flow fluctuations are markedly different near the pillars and far downstream . As the flow progresses downstream, regions of positive or negative streamwise velocity fluctuations coalesce, leading to large rolling advection cell structures (Figure b–d).…”

“…b) Schematic of microfluidic porous media, and kymographs of streamwise velocity fluctuations ( u ′) c) nearby and d)far downstream of circular pillar array. Reproduced with permission . Copyright 2017, American Physical Society.…”

“…Because polymer stretching is hysteretic and can persist over large length scales, polymer elongation may be retained across multiple pores. This “memory” may therefore provide new spatial structure to the flow over macroscopic scales in a porous medium.…”

Vortex generation in a finitely extensible nonlinear elastic Peterlin fluid initially at rest

“…Large amplitude perturbations to the base flow were introduced by an array of cylinders mounted vertically near the entrance to the channel. It was observed that the flow became unstable for a sufficiently high Weissenberg number (defined in References and as , where λ is the fluid relaxation time and is the mean flow strain rate), Wi ≥ 5.2, and for a sufficiently energetic flow disturbance (15 cylinders). In fact, the instability was observed far downstream (200 channel widths) from the channel entrance, where the flow transitioned to a chaotic, turbulent‐like state.…”

“…It was observed that the elongational strain rates were apparently amplified downstream of the cylinders, indicating the possibility of significant stretching of the polymer molecules. Experimental evidence thus far suggests that the observed chaotic flow is due to a nonlinear subcritical instability …”