Two-layer channel flow involving a fluid with time-dependent viscosity

TL;DR

This study numerically investigates a two-layer channel flow with a Newtonian top layer and a bottom layer whose viscosity increases over time due to aging, revealing how structuration affects flow stability and interfacial instabilities.

Contribution

It introduces a numerical analysis of flow involving a time-dependent viscosity fluid with a Coussot-type relationship, highlighting the impact of aging on flow dynamics and stability.

Findings

Viscosity increase suppresses Kelvin-Helmholtz instabilities.

Bottom layer behaves like a Newtonian fluid when tau > 10 and beta > 1.

Lower Froude number stabilizes interfacial instabilities.

Abstract

A pressure-driven two-layer channel flow of a Newtonian fluid with constant viscosity (top layer) and a fluid with a time-dependent viscosity (bottom layer) is numerically investigated. The bottom layer goes through an aging process in which its viscosity increases due to the formation of internal structure, which is represented by a Coussot-type relationship. The resultant flow dynamics is the consequence of the competition between structuration and destructuration, as characterized by the dimensionless timescale for structuration (tau) and the dimensionless material property (beta) of the bottom fluid. The development of Kelvin-Helmholtz type instabilities (roll-up structures) observed in the Newtonian constant viscosity case was found to be suppressed as the viscosity of the bottom layer increased over time. It is found that, for the set of parameters considered in the present study, the bottom layer almost behaves like a Newtonian fluid with constant viscosity for tau greater than 10 and beta greater than 1. It is also shown that decreasing the value of the Froude number stabilizes the interfacial instabilities. The wavelength of the interfacial wave increases as the capillary number increases.