How two-way coupling modifies the multiscale preferential sweeping mechanism

TL;DR

This paper investigates how two-way coupling in turbulent flows affects the multiscale preferential sweeping mechanism, revealing that preferential sweeping remains significant and is further enhanced by particle-fluid interactions even at low particle loading.

Contribution

It demonstrates that two-way coupling modifies flow scale contributions but does not eliminate the importance of preferential sweeping in particle settling.

Findings

Two-way coupling enhances particle settling even at low loading.

Preferential sweeping remains a key mechanism in two-way coupled flows.

Particles in straining regions drag fluid down, boosting settling.

Abstract

For one-way coupled (1WC) flows, Tom & Bragg (J. Fluid Mech., 871, pp. 244-270, 2019) advanced the analysis of Maxey (J. Fluid Mech., 174, pp. 441-465, 1987), which applied to weakly inertial particles, to particles of arbitrary inertia, and the new theoretical result revealed the role that different scales play in the preferential sweeping mechanism that leads to enhanced particle settling in turbulent flows. Monchaux & Dejoan (Phys. Rev. Fluids, 2, 104302, 2017) showed using direct numerical simulations (DNS) that while for low particle loading the effect of two-way coupling (2WC) on the global flow statistics is weak, 2WC enables the particles to drag the fluid in their vicinity down with them, significantly enhancing their settling, and they argued that 2WC suppresses the preferential sweeping mechanism. We explore this further by considering the impact of 2WC on the contribution made by eddies of different sizes on the particle settling. In agreement with Monchaux & Dejoan, we show that even for low loading, 2WC strongly enhances particle settling, and we show how 2WC modifies the contribution from different flow scales. However, contrary to their study, we show that preferential sweeping remains important in 2WC flows. In particular, for both 1WC and 2WC flows, the settling enhancement due to turbulence is dominated by contributions from particles in straining regions of the flow, but for the 2WC case, the particles in these regions also drag the fluid down with them, leading to an enhancement of their settling compared to the 1WC case.