Finite-sized rigid spheres in turbulent Taylor-Couette flow

TL;DR

This study investigates how neutrally buoyant rigid spheres influence drag in turbulent Taylor-Couette flow, finding minimal drag change with particle size but increased drag with higher particle volume fractions, contrasting with bubble effects.

Contribution

It provides new insights into the effects of rigid spheres on drag in turbulent flow, highlighting differences from bubbly flows and the role of particle size and volume fraction.

Findings

Rigid spheres minimally affect drag beyond viscosity effects.

Increasing particle volume fraction raises drag and turbulence.

Bubbles are more effective in reducing drag than rigid spheres.

Abstract

We report on the modification of drag by neutrally buoyant spherical particles in highly turbulent Taylor-Couette flow. These particles can be used to disentangle the effects of size, deformability, and volume fraction on the drag, when contrasted with the drag for bubbly flows. We find that rigid spheres hardly change the drag of the system beyond the trivial viscosity effects caused by replacing the working fluid with particles. The size of the particle has a marginal effect on the drag, with smaller diameter particles showing only slightly lower drag. Increasing the particle volume fraction shows a net drag increase as the effective viscosity of the fluid is also increased. The increase in drag for increasing particle volume fraction is corroborated by performing laser Doppler anemometry where we find that the turbulent velocity fluctuations also increase with increasing volume fraction. In contrast with rigid spheres, for bubbles the effective drag reduction also increases with increasing Reynolds number. Bubbles are also much more effective in reducing the overall drag.