Dynamics of two air bubbles rising in a shear-thinning fluid

TL;DR

This study investigates the complex three-dimensional behavior of two air bubbles rising in shear-thinning fluids, utilizing advanced numerical methods to reveal unsteady motions, shape oscillations, and the influence of fluid rheology on bubble dynamics.

Contribution

The paper introduces a combined CLSVOF and SSF numerical approach implemented in CUFLOW, effectively simulating bubble interactions in shear-thinning fluids with improved accuracy and stability.

Findings

Bubbles exhibit unsteady motion with shape oscillations in shear-thinning fluids.

Viscosity modification by the leading bubble influences the trailing bubble's dynamics.

Differences between shear-thinning and Newtonian fluids are highlighted through power-law index variations.

Abstract

In this paper, we have studied the three-dimensional dynamics of two equally sized air bubbles rising in a shear-thinning fluid. We have used the combined level set and volume of fluid (CLSVOF) method to track interface, maintain mass balance and estimate the interface curvature. Additionally, we have incorporated a Sharp Surface Force Method (SSF) for surface tension forces. This method significantly suppressed the spurious velocities commonly observed with the conventional volume of fluid method and the Continuum Surface Force (CSF). The algorithm is implemented in an in-house code called CUFLOW and runs on multiple GPUs platform. We have explored the effects of fluid rheology on the three-dimensional dynamics of two in-line bubbles. Two power-law indices (0.5 and 1) are investigated to highlight differences in shear-thinning and Newtonian fluids. For a range of parameters examined here, bubbles motion in a shear-thinning fluid is seen to be unsteady with significant shape oscillations. Further, we have examined the rise velocity, droplets rise path, transient shapes and found that modification of viscosity by the motion of the leading bubble changes the dynamics of the trailing bubble.