Physical mechanisms governing drag reduction in turbulent Taylor-Couette flow with finite-size deformable bubbles

TL;DR

This study uses 3D simulations to explore how bubble deformability affects drag reduction in turbulent Taylor-Couette flow, revealing that more deformable bubbles lead to greater drag reduction by altering turbulence and dissipation.

Contribution

It provides new insights into the physical mechanisms of drag reduction by deformable bubbles in turbulent flow through detailed numerical analysis.

Findings

Increased bubble deformability enhances drag reduction.

Deformable bubbles reduce dissipation near the inner cylinder.

Turbulence is amplified near the inner cylinder but suppressed in the bulk.

Abstract

The phenomenon of drag reduction induced by injection of bubbles into a turbulent carrier fluid has been known for a long time; the governing control parameters and underlying physics is however not well understood. In this paper, we use three dimensional numerical simulations to uncover the effect of deformability of bubbles injected in a turbulent Taylor-Couette flow on the overall drag experienced by the system. We consider two different Reynolds numbers for the carrier flow, i.e. $Re_i=5\times 10^3$ and $Re_i=2\times 10^4$; the deformability of the bubbles is controlled through the Weber number which is varied in the range $We=0.01 - 2.0$. Our numerical simulations show that increasing the deformability of bubbles i.e., $We$ leads to an increase in drag reduction. We look at the different physical effects contributing to drag reduction and analyse their individual contributions with increasing bubble deformability. Profiles of local angular velocity flux show that in the presence of bubbles, turbulence is enhanced near the inner cylinder while attenuated in the bulk and near the outer cylinder. We connect the increase in drag reduction to the decrease in dissipation in the wake of highly deformed bubbles near the inner cylinder.