Equilibrium circulation and stress distribution in viscoelastic creeping flow

TL;DR

This paper derives an analytical approximation for steady viscoelastic creeping flow, revealing how stress islands form near stagnation points and influence secondary flow, with results validated against numerical simulations.

Contribution

It introduces an asymptotic analytical solution for stress distribution and flow response in viscoelastic creeping flow modeled by Oldroyd-B equations, highlighting stress island formation.

Findings

Stress islands are Gaussian-shaped and aligned with incoming streamlines.

The amplitude of stress islands scales with the Weissenberg number.

Analytic solutions agree well with numerical simulations.

Abstract

An analytic, asymptotic approximation of the nonlinear steady-state equations for viscoelastic creeping flow, modeled by the Oldroyd-B equations with polymer stress diffusion, is derived. Near the extensional stagnation point the flow stretches and aligns polymers along the outgoing streamlines of the stagnation point resulting in a stress-island, or birefringent strand. The polymer stress diffusion coefficient is used, both, as an asymptotic parameter and a regularization parameter. The structure of the singular part of polymer stress tensor is a Gaussian aligned with the incoming streamline of the stagnation point; a smoothed $\delta$-distribution whose width is proportional to the square-root of the diffusion coefficient. The amplitude of the stress island scales with the Wiessenberg number and, although singular in the limit of vanishing diffusion, it is integrable in the cross stream direction due to its vanishing width; this yields a convergent secondary flow. The leading order velocity response to this stress island is constructed and shown to be {\em independent} of the diffusion coefficient in the limit. The secondary circulation counteracts the forced flow and has a vorticity jump at the location of the stress islands, essentially expelling the background vorticity from the location of the birefringent strands. The analytic solutions are shown to be in excellent quantitative agreement with full numerical simulations and, therefore, the analytic solutions elucidate the salient mechanisms of the flow response to viscoelasticity and the mechanism for instability.