Viscoelastic fluid flow in a slowly varying planar contraction: the role of finite extensibility on the pressure drop

TL;DR

This paper develops a lubrication-based theory for steady viscoelastic flow in slowly varying contractions, highlighting how finite polymer extensibility influences pressure drops across Deborah numbers, validated by numerical simulations.

Contribution

It introduces a FENE-CR model-based analytical framework for viscoelastic flow in contractions, capturing finite extensibility effects on pressure drop at various Deborah numbers.

Findings

Pressure drop decreases monotonically at low De.

At high De, pressure drop shows non-monotonic behavior due to finite extensibility.

FENE-CR fluid pressure drop remains lower than Newtonian in high De limits.

Abstract

We analyze the steady viscoelastic fluid flow in slowly varying contracting channels of arbitrary shape and present a theory based on the lubrication approximation for calculating the flow rate-pressure drop relation at low and high Deborah ($De$) numbers. Unlike most prior theoretical studies leveraging the Oldroyd-B model, we describe the fluid viscoelasticity using a FENE-CR model and examine how the polymer chains' finite extensibility impacts the pressure drop. We employ the low-Deborah-number lubrication analysis to provide analytical expressions for the pressure drop up to $O(De^4)$. We further consider the ultra-dilute limit and exploit a one-way coupling between the parabolic velocity and elastic stresses to calculate the pressure drop of the FENE-CR fluid for arbitrary values of the Deborah number. Such an approach allows us to elucidate elastic stress contributions governing the pressure drop variations and the effect of finite extensibility for all $De$. We validate our theoretical predictions with two-dimensional numerical simulations and find excellent agreement. We show that, at low Deborah numbers, the pressure drop of the FENE-CR fluid monotonically decreases with $De$, similar to the previous results for the Oldroyd-B and FENE-P fluids. However, at high Deborah numbers, in contrast to a linear decrease for the Oldroyd-B fluid, the pressure drop of the FENE-CR fluid exhibits a non-monotonic variation due to finite extensibility, first decreasing and then increasing with $De$. Nevertheless, even at sufficiently high Deborah numbers, the pressure drop of the FENE-CR fluid in the ultra-dilute and lubrication limits is lower than the corresponding Newtonian pressure drop.