Patchy Polymeric Scalar Turbulence

TL;DR

This study compares mixing efficiency in polymeric versus Newtonian turbulence, revealing that polymeric turbulence exhibits less efficient mixing, smaller scalar flux, and smoother patch boundaries, indicating altered turbulence dynamics.

Contribution

It provides new insights into how polymeric turbulence affects scalar mixing, highlighting differences in patch structure, intermittency, and flux compared to Newtonian turbulence.

Findings

Polymeric turbulence is less efficient at mixing than Newtonian turbulence.

PST has smaller scalar flux and more space-filling patch boundaries.

Scalar fluctuations are stronger but less intermittent in PST.

Abstract

Turbulent polymeric flows show strong deviations from Kolomogorov-like behaviour resulting from more complex dynamics compared to Newtonian turbulence. We now study the nature of mixing in polymeric turbulence via Eulerian passive scalar fields of varying molecular diffusivities, given by the Schmidt number Sc. We show that polymeric turbulence is a less efficient mixer than the Newtonian one at small to moderate Sc numbers. Newtonian scalar turbulence (NST) forms large islands of fluctuations with extended, contiguous fronts. In contrast, polymeric scalar turbulence (PST) is marked by small, interspersed patches of strong but less intermittent fluctuations. These patches collectively comprise a larger volume fraction of strong fluctuations, indicating a less efficient mixing, alongwith smaller scalar gradients and therefore smaller average flux across their boundaries. Box counting dimensions reveal a smoother and more space filling nature of patch boundaries in PST compared to NST fronts. Finally, spatial changes of the scalar are stronger in PST, but with a slower self-similar growth and less intermittency as revealed by the kurtosis of scalar differences. Overall, these observations hint at reduced mixing in PST where fluctuations are typically stronger while the average scalar flux is smaller in a stationary state.