Drag reduction in turbulent channel flow laden with finite-size oblate spheroids

TL;DR

This study demonstrates that suspensions of oblate spheroids in turbulent channel flow reduce drag and turbulence more effectively than spherical particles, due to their unique orientation and slower rotation near walls.

Contribution

The paper introduces direct numerical simulations of oblate spheroid suspensions, revealing their significant drag reduction and altered turbulence dynamics compared to spherical particles.

Findings

Drag is reduced in flows with oblate spheroids.

Turbulent fluctuations are attenuated by oblate particles.

Oblate particles tend to align parallel to the wall, decreasing Reynolds stresses.

Abstract

We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions. Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated. In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity. To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles. These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.